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Here's a schem for a generic envelope follower. I Made this by adding the GGG buffer in front of the envelope from the Dr. Q. I think I used the Dr. Q drawing from Jack Orman's site so it probably has some his mods in it.

I breadboarded this and it seems to be working well. At this time I've just been lighting an LED by playing notes ont eh guitar. It seems to be working pretty well.

I'm hoping this could be a simple envelope follower that could be added to any circuit that might need one. My own plans are to add to the Rate control of the Frobnicator.

I'm looking for suggestions or thoughts for improvements. I'm a novice I'm sure I'm not seeing something that could be done.

(http://www.webdevils.com/stompbox/images/Envelope_Follower.jpg)

The one I'd like to see a board for is Harry Bisell's envelope follower that he designed expressly for guitar (well, for his guitar synth) and published in EDN.  I've misplaced the link, so someone with better research skills (or the link itself) will have to help out.

The one you've drawn (and thanks) from Jack's chip-adaptable Dr. Quack is certaily simple and cheap.  Harry's is just plain responsive.  I can vouch for it because I've played it. He nails the "holy grail" of envelope followers - ultra-fast response time with little detectable ripple.  If you can imagine and play a dynamic difference, this thing will pick it up.  Usually, if you want a fast attack, you end up with more envelope ripple, and if you try and smooth out the ripple you have to forfeit the fast attack.  Harry's idea uses a round-robin combo of what are essentially 3 envelope followers combined into one.  Sort of a big build.  In fact more components than a great many effects pedals themselves.  But if the goal is a nondedicated envelope follower that can be assigned to different things, this is the shiznaz.

Quote from: Mark Hammer on September 27, 2005, 12:51:20 PM
The one I'd like to see a board for is Harry Bisell's envelope follower that he designed expressly for guitar (well, for his guitar synth) and published in EDN.  I've misplaced the link, so someone with better research skills (or the link itself) will have to help out.

Mark, is it this one? http://go.cadwire.net/?3572,3,1 (click on the links for the schem)

That's the one.  A real keeper.

How difficult would it be to implement Harry's envelope follower idea in a PIC micro?

A very good weekend to you Mark, this is the link I have to Harry Bissell's full envelope circuit and there seems to be a PCB, but I think it needs a bit of unravelling! Do try my Spacedrum adapted Envelope Generator which is great with the Bi Filter Follower filter, albeit a little fuzzy, for reasons better known by greater minds than mine!

QuoteHow difficult would it be to implement Harry's envelope follower idea in a PIC micro?

Difficult. The analog portion of sensing the peaks is not something that the PIC does well - yet. The digital part is trivial, of course. A 16F629 in an 8 pin DIP would do nicely, and is smaller than an oscillator and a 4017.

QuoteThe one I'd like to see a board for is Harry Bisell's envelope follower

I did one of those. It turns out not to be a nicely behaved circuit for effects. The board is big, even trying to stuff it down. There are four dual opamps, three transistors, and two other chips, either 14 pin or one 14, one 8. You can replace the 14 and 14/8 with an 8 pin PIC, but you still have enough junk in the peak/holds to make an entire phaser. This is one circuit that begs to be made into an SMD module that you could graft onto another board. I've done that as well, but most people are afraid of SMD.

RG that sounds a bit pessimistic, I would expect to use a considerable number of chips if the end product warranted the effort, and Mark seemed very impressed with it.
Stephen

To me, circuits have a feeling attached when I do layout. That's my own personal interpretation, of course.

The three-peaks envelope follower does not produce a satisfying layout, either of the several times I've done it. It's somehow resistant to tight, efficient layout, and that irritates me. I've messed with the circuit, recast it with other parts, etc. I've even looked for oddball ICs that could be pressed into service for it to do the layout better, with no luck.

I have no doubt at all that it's a superior envelope follower. The simulation runs show really good results.

There are a few semi-natural-laws with effects, a result of the history and what guitarists expect. You can violate one or two of them, but things get unwieldy after that. One of those is a smallish box. An envelope follower that takes up as much circuitry as the effect itself is a problem, it feels like IMHO.

So, yeah, I'm pessimistic about the practical aspects of layout and inclusion in a box, but not at all about the performance. That's why I suggested encapsulating it in an SMD self-standing circuit.

I feel there must be simple effective envelope that can be used in many circuits. Something that can be made with a single op-amp and a few more parts. A complex circuit is not as useful, and causes problems for amateurs like myself. I had the idea of coming up with something that could be grafted onto any circuit where you felt an envelope might enhace or create new sounds.

Ah RG that's the perfectionist in you springing forth! I'm just happy if I get something to work, and as 95% of what I make is on veroboard (that's gone quiet hasn't it!) .......need I say more? I must confess that when I gave the Bissell Envelope Follower a try on breadboard I couldn't get it to work but I certainly intend to try again.
Stephen

Quote from: R.G. on October 01, 2005, 10:35:40 AM

QuoteHow difficult would it be to implement Harry's envelope follower idea in a PIC micro?

Difficult. The analog portion of sensing the peaks is not something that the PIC does well - yet.

Is there a reason for that?  In my mind you're just measuring the first peak off the A/D converter.  From there it would follow the waveform until it reached the noisefloor, whatever that might be.  As it rides the waveform down, you'd be smoothing the waveform in software  rather than relying on caps, so it seems like it could be pulled off. 

/scared to attach my homemade PIC programmer to the serial port so I may never know...   :)

I'm not convinced we want to follow the waveform because that is probably asking for ripple. Surely it is better to read the initial peak of the note, then have the decay decided by separate circuitry independent of the decay of the guitar note, which is just what this does - although there should be a 10k resister out of pin 3 of the 13600 to the wiper of the start frequency pot.
(http://img.photobucket.com/albums/v517/uncle_boko/Workingenvelopegenerator.gif)
Stephen

Quote from: Paul Perry (Frostwave) on October 01, 2005, 04:47:46 AM
How difficult would it be to implement Harry's envelope follower idea in a PIC micro?

I am actually not very skeptical of that at all.  The first derivative of the waveform always crosses 0 at each peak on the waveform.  The second derivative tells you if it's concave up or concave down (so you can determine if it's a maximum peak or minimum).  The "first difference" is very easy to implement, and you just set a test bit to determine when x[n] - x[n-1] is reasonably close to 0, then you store the sample at that point in time in a registor, or even a RAM location until
1) Something with a higher amplitude comes along, then store the new maximum
2) you reset it to 0, then take the next series of maximums into the buffer and hold until you reset it again.

You could do this with 3 registers or memory locations in a round robin fashion--you're just choosing a different time to reset the register (or memory location) at different times.

In the end of it, you can add the outputs all together, drop it into the A/D converter and throw a simple RC filter on the output of the PIC chip like on the output of Harry's circuit to avoid having to pull any crafty programming to implement a digital filter in between the rest of the processing.

I think I'm with Stephen, an envelope generator might be a better choice. A PIC has great potential but you need special equipment to create and program the PIC, not to mention the know how.

If there were an envelope generator that could be made from a dual op-amp this would be perfect.

Why do you want to use a just one dual opamp soggybag?
Stephen

I got started on the idea when I was playing with the Ring Fronnicator, I thought it might be interesting to add an envelope to control the frequency. This could be interesting in the Ringing mode. I made up the Dr Q envelope on a breadboard. I used an LED with an LDR across the frequnecy pot. It worked pretty well after I added the buffer.

After working on this I began to think that a simple envelope follower might be a good circuit block to have around to add to anything. And it would be especially handy if it was compact. A dual op-amp and some other parts makes a nice easy to work with package.

The schematic above is what I came up with. It's sort of a kluge of the op-amp buffer and the envelope from the Dr Q. The control is simple, one knob determining how strong the effect. It works with an LED/LDR which is pretty easy to fit into most circuits.

Having only one control makes it easy to add to a project. If you have an effect with two knobs, you don't want to add three more. But you might want to add an envelope to your Phase 45 or easyvibe with a knob to determine how much the envelope is effecting the sound. If the whole thing is just a dual op-amp, a few extra parts and one knob it could be added to a lot of projects that are already boxed up with minimal trouble.

Ah that explains it.
Stephen

It's easy to get sucked in, with envelope follower designs.... I find myself working wiht an "ideal" guitar signal in mind, rather than the horibble spiky mess full of crossovers that you actually see on a scope. I think that is why there are so many disappointments in envelope follower design. I have a PIC based product (the Sonic Alienator wiht variable sample and bit number, also distortion algorithms) that works on a PIC. (But, someone else did the programming!). So I'm sure it is possible to use a PIC.
Maybe Steve or someone would be happy to sell an envelope follower PIC as a product, with a couple of bucks royalty to the programmer??

Why couldnt you have the envelope detector as a separate box with a CV out, like analogue synths?
then just mod your pedals so that they could take a CV in, and have some sortof switching to switch between which values are controlled by the envelope of your signal.

Here's something really, really simple...

(http://www.moosapotamus.com/EnvControl2.gif)

... from an older version of Tim Escobedo's Ugly Face.
An LDR opposite the LED can go across the outside lugs of a pot that is wired as a variable resistor.
That's what I used in my CrashSync. :icon_cool:

~ Charlie

I think I tried this and couldn't get it to work. But then again I don't really know what I'm doing either. Maybe I'll give it another try.

would you need a buffer to drive it if your input was split between this and the effect circuit?

Quote from: StephenGiles on October 01, 2005, 01:27:38 PM
I'm not convinced we want to follow the waveform because that is probably asking for ripple.

You're right, I should clarify that.  What I meant to say was, "Sample the initial peak, then follow the peaks down (not the waveform)."  In theory, that's easy.  In reality, there must be a reason it isn't commonplace.

Quote from: soggybag on October 01, 2005, 04:51:08 PM
I think I'm with Stephen, an envelope generator might be a better choice. A PIC has great potential but you need special equipment to create and program the PIC, not to mention the know how.

For what it's worth, I built my programmer for $3.66, a broken mouse, and some spare diodes and caps.  The software to program them is free.

Here's the schemo: http://www.olimex.com/dev/images/pic-pg1-sch.gif

As I mentioned earlier today, I haven't had the courage to plug it into the computer yet, but I expect to before the weekend is over.  It's good to hang on to old P-II motherboards for  events like this.    :icon_biggrin:

QuoteI feel there must be simple effective envelope that can be used in many circuits. Something that can be made with a single op-amp and a few more parts. A complex circuit is not as useful, and causes problems for amateurs like myself. I had the idea of coming up with something that could be grafted onto any circuit where you felt an envelope might enhace or create new sounds.

Yes, right! Good idea. I love it.

The problem is, I don't know of anyone that has been able to do that, and I've looked for a long time. The closest to a single chip (not single opamp) envelope follower is the old double full wave rectifier done with an NE570/571. It's pretty good because double-full-wave quadruples the frequency of the ripple and makes the filtering easier. I guess you could do something similar with a quad opamp or three rail-to-rail opamps that had input common mode ranges below the minus supply.

Einstein said "Everything should be as simple as possible, but no simpler." I hope you are successful, but don't be hard on yourself if you're not. A lot of people have looked for that kind of circuit for a lot of years. I banged my head on it quite a few times, and the complexity to do a good envelope extractor just grew. Frankly, Bissell's is the highest performance per circuit cost I know of. There may well be better ones at lower circuit cost, but I haven't seen them.

A good envelope extracter has conflicting requirements, always a good hill to climb. It has to find peaks, no matter what time difference there is between them for frequencies of interest, and then smooth them. Problem is, the smoothing you need for the lowest guitar frequencies is too much for the high frequencyies. Get the highs right and you have ripple on the lows. I think that some commercial units may use dual (or triple or fourple) frequency bands for finding the envelope.

QuoteYou're right, I should clarify that.  What I meant to say was, "Sample the initial peak, then follow the peaks down (not the waveform)."  In theory, that's easy.  In reality, there must be a reason it isn't commonplace.

Yep. The reason I personally haven't done it is because I can't get today's PICs to digitize audio fast enough to digitize audio and catch the peaks, then compute something to do with them in real time. In theory it's easy. A good high audio-capable DSP should be able to do it in real time, but cheap and available PICs can't as far as I can tell. To catch guitar peaks, you have to be able to digitize well enough to find peaks on a 7kHz waveform. That means that you have to sample a minimum of three times per half cycle, probably more like five - I haven't done the math to figure out the necessary oversampling rate. Note that finding a peak on a waveform is a more stringent criteria than reproducing the waveform by sampling. At least as I dimly understand it.

Maybe there's a combination of analog and PIC processing that can do it.

I can postulate an oversampling sigma-delta modulator that could find peaks and then limit its up/down rate based on the time between peaks, but I haven't tried to design that.

PIC programming is trivially easy, needing no really fancy equipment that can't be made easier than an effects box.

QuoteWhy couldnt you have the envelope detector as a separate box with a CV out, like analogue synths?
then just mod your pedals so that they could take a CV in, and have some sortof switching to switch between which values are controlled by the envelope of your signal.

Good idea. I thought that a good envelope detector would make a good module for the FxBus. However, it does violate one of what I fuzzily perceive the rules are for stompboxes - make it all fit in one box so the guitar player can just think about what that one box does. I could be wrong, but that's what I think.

I really don't want to discourage anyone, because I'd like to see a simple one opamp high performance envelope detector too.

everybody having a party and they didn't invite me ???    :^P

Some comments from horse's mouth   (not the other end...)

My "Envelope follower combines feat response, lowest ripple" from EDN
has some real advantages ~and~ disadvantages.  It was designed for guitar
in specific. Guitar waveforms can have multiple zero crossings in a cycle...
and can have multiple peaks of essentially the same amplitude. Simple circuits
that look for the highest peak between zero crossings don't work well for guitar
(an example would be the circuit posted on Rene Schmitz's website, from the
synth-diy list).

I decided to trade having a really accurate attack for having the decay phase
delayed by about 1-1/2 cycles.  You don't notice this easily. If you mute a guitar string
it will probably ring longer than this anyway.

The down side of this circuit is you have to know the lowest fundamental frequency
in the input. Iused this in a polyphonic envelope follwer, so I do know the lowest possible note. If you use this circuit on a general audio signal (radio ?) you will need to set the clock REALLY slow...
in which case its a really complex circuit to get crappy performance with   :^P

There are other ways to do this circuit.   I did a version using a 4053 CMOS switch to run the
sample holds AND the resets.  Its a little more compact.

The full cirucit for the hex version of my guitar processor is at Larry Hendry's
www.wiseguysynth.com. Look for "muffy"

This is my first post here.  Does anyone know how I could kill those animated emoticons on
the post composition screen.  They make my computer run REALLY dog slow.  I can barely
type.

H^) harry  (bissell)

It's certainly possible to sample beyond 20KHz with a PIC! My Sonic alienator does that, plus a tiny bit of elementary processing as well, then sends the output to a serial D to A.
but it hardly has to do that in the case of an envelope follower, because in an envelope follower... well, what do we want? Do we REALLY want the absolute envelope? If you get into a sound editing program & expand the waveform right out, you will get some insight into what might work int eh real world.. especially if you print out the waveform, and cover it with a card, which you slide away so that the waveform is progressively revealed. You quickly find that you can't easily generate a 'perfect' envelope even with your brain & a pencil! (yeah, it's easy if you can see the WHOLE waveform, including what is coming, but that is cheating).
So the question is, how WELL can one do?
Personally, I'd try a tradional follower, with a very small cap, then sample the output at about 1khz, and choose the maximum reading during the last .05 seconds.

I'm far from discouraged! I think sometimes the idea of performance dominates design too much. I don't feel the envelope needs to be so high performance or complex for many uses. I think you just need to hear the effect in time with the instrument.

Take the Ugly Face for example. I think this uses a single 2u2 cap and vactrol for the envelope. It sounds good, I don't think I notice any ripple (of course it's probably masked pretty well by raucous square wave produced by the Ugly Face). I think this is simple and effective.

I keep thinking that a guitar signal through an LED/LDR could make a pretty simple envelope with the righ amount of low pass filtering, or... something else that I haven't thought of yet!

The Dr. Q envelope works pretty well with a minimum of parts. I suspect it can still be smoothed out some. I added a buffer on the front, I suspect you could drop this into a lot of projects with little effort. It probably  could use some improvement.

There is an issue of Stompboxology all about envelopes. On the second page Mr. Boscorelli shows about 8 different envelope circuits and none of them are that complex. He also talks about the types of envelopes generated. I'm not sure how people feel about this but I would be willing to post this page for discussion?

Reply to Paul Perry (Hi Paul)

The PIC should be pretty easy... I'd use one resettable
peak detector.  Have the PIC read the value stored, then
save that in a buffer.  Do that three times and output the
highest value you saw.  After each read, dump the peak
detector. The peak detector could be a simple opamp
circuit and dump it with a transistor.

What you really WANT depends on the use for the follower.
I don't like IM distortion in my VCA or VCF so the extra effort
is worth it.   If I was trying to track drums, I'd use the peak
detector.  The guitar case is unique because of the ability of
MUTING the strings suddently....

H^) harry

Hi Harry - welcome to the forum, we had a brief exchange of emails a couple of years ago regarding your envelope follower. Now, I didn't appreciate at the time that a fundamental extractor is required in order to get maximum performance. Well, we have one from the EH Guitar Synth which I know works, albeit with a high component count here:
http://ampage.org/hammer/files/EH-Guitar-Synth.ZIP
.so that's a possibility, but what does your fundamental generator consist of?

Harry -

QuoteThe down side of this circuit is you have to know the lowest fundamental frequency in the input. Iused this in a polyphonic envelope follwer, so I do know the lowest possible note. If you use this circuit on a general audio signal (radio ?) you will need to set the clock REALLY slow...

This is the ripple problem in another guise. Or, more properly, the problem behind the ripple problem. Your diode-OR in the output offers a solution in that you can create a lowest-couple-of-octaves envelope detector running from a lowpass filter and detect them the same way, then OR them in.

I was curious at the loss of envelope level with the diodes in the diode OR. Are you working only with a preamplified signal, not raw pickup level? It seems like losing half a volt of envelope would be a disaster with a 100mv signal. Or did I set up my simulation wrong? I used active diodes to get rid of the forward voltage and it simmed well with pickup level signals then. The second diode in series with the 100 ohm resistor isn't the same kind of problem, paralleled with a 100K. Can you expound a bit on the reasoning in that area?

QuoteThere are other ways to do this circuit.   I did a version using a 4053 CMOS switch to run the sample holds AND the resets.  Its a little more compact.

Do you have a schematic for that that you're willing to share?

I really like the stepping-sideways involved with your envelope generator. Clever.

Here's a circuit I've been toying with in PSPICE:

http://putfile.com/pic.php?pic=10/27413130337.jpg&s=x2

It's a self-clocking sample & hold cell.   The first stage (into the opamp inverting side through a capacitor) is a differentiator amplifier, thus the output is a first derivative.  The next part with the comparator is a 0 crossing detector, which also serves as a nonlinear second derivative, in that the output reveals whether the peak is a positive or negative peak.

The final stage is a quick and dirty one-shot to generate a short pulse for sampling.  The switch model shown could be a 4053 or 4066 analog switch.

I'm operating off the student version of PSPICE, so I'm lacking the ability to try this, but I suspect that if you put two of these in series, it would detect the envelope of almost any audio signal.  The other problem I would have with a simulation even with a full version of SPICE is that I would have to apply a realistic audio signal over a much longer simulation run.

It worked very well using the ideal switch for the S/H on a "nice" signal that had been generated by summing 5 AC sources of different amplitudes.  The main thing I wanted to see was that it faithfully set a short pulse on each concave down peak, but nowhere else, according the the theory.  It did quite well.  There was a more noticeable lag near 1kHz, but not so much that it was sampling the waveform at illogical places.  It was about 10% error in the 1kHz range, but more like 1% error at 400 Hz.   I think my next step is to build it and try it.  It's more simple than my current envelope follower that uses a full-wave rectifier and a 5th order low pass filter (who's 3dB frequency depends on the lowest expected audio frequency)

Quote from: moosapotamus... from an older version of Tim Escobedo's Ugly Face.

Quote from: soggybagI think I tried this and couldn't get it to work. But then again I don't really know what I'm doing either. Maybe I'll give it another try.

I'd definately recommend giving it another try. I think it fits your initial design objective quite well...

Quote from: soggybagI feel there must be simple effective envelope that can be used in many circuits. Something that can be made with a single op-amp and a few more parts. A complex circuit is not as useful, and causes problems for amateurs like myself. I had the idea of coming up with something that could be grafted onto any circuit where you felt an envelope might enhace or create new sounds.

It worked great for me in my AutoCrash which is a Crash Sync with the envelope follower modulating the frequency. And IIRC, this little follower circuit (or the LM386 version) was also used by someone else here to modulate the speed in a tremolo circuit. So, it works pretty well as a simple drop-in envelope follower. :icon_cool:

Quote from: soggybag...would you need a buffer to drive it if your input was split between this and the effect circuit?

Possibly... input ties to Q1 collector in the Uglyface. In my Autocrash the input tied to the TL072 pin 1.

~ Charlie

Hello All

(Steven Giles)  When I say 'know' the fundamental frequency, I mean what the lowest frequency
might be. For instance, Low E of guitar is 80Hz.   If you actually KNOW the fundamental (a'la
the EH Guitar Synth or other) you can just use one peak detector and sample once every cycle.
You need a circuit to determine when a note has stopped, then force one more sample of 'zero'
volts to end the note.  In my guitar synth I use this method rather than the envelope follower from EDN.

(RG)  I use the circuit after a preamp.  The diode drops actually help to establish a 'full off'
level for the follower.  I have a circuit that sum the S/Hs actively... no diode drop.  It did not improve
this application IMHO so I did not use the more complex circuit.

The envelope follower question is really interesting... your eye can 'see' the proper envelope in a scope
but is is really hard to detect it electronically. 

I follow the fast envelope with a circuit similar to the "morphlag' over at Larry Hendry's site
www.wiseguysynth.com

REQUEST : Please, can anyone tell me how to abort the animated emoticons on the posting
page. Its costing me ONE SECOND per letter... I can't get much done. Is there a text-only page available ???

H^) harry

Hello Transmogrifox

I saw your simulation, I'll check it more carefully. I'd suggest you use
thee sine wave generators with the following settigns

1: 1V @ 80Hz
2: 0.5V @ 161Hz
3: 0.33V @ 243Hz

Sum these together and  scale them to suit your test. This will be the first three
harmonics of a 'sawtooth' wave... with the upper harmonics made sharp for guitar
string 'stiffness'.   You will get the traditional guitar wave this way, with multiple peaks
and zero crossings.

I test my circuits with this stimulus. I even use a multiplier as a VCA to fake an exponential
decay.

Using a single sine wave source will make (almost) ANY circuit seem to work.

H^) harry

Quote from: H^)harry on October 02, 2005, 05:29:07 PM
Hello Transmogrifox

I saw your simulation, I'll check it more carefully. I'd suggest you use
thee sine wave generators with the following settigns

1: 1V @ 80Hz
2: 0.5V @ 161Hz
3: 0.33V @ 243Hz

Sum these together and  scale them to suit your test. This will be the first three
harmonics of a 'sawtooth' wave... with the upper harmonics made sharp for guitar
string 'stiffness'.   You will get the traditional guitar wave this way, with multiple peaks
and zero crossings.

I test my circuits with this stimulus. I even use a multiplier as a VCA to fake an exponential
decay.

Using a single sine wave source will make (almost) ANY circuit seem to work.

H^) harry

I, in fact have done that with that pattern of relative amplitudes up to the 5th harmonic (starting with 80 Hz).  I actually did a few more using the same pattern by shifting it all up to the A string relative harmonics and a few extras.

What I was missing, and appreciate your input for is using a multiplier as a VCA to fake an exponential decay.  This would give me a much better idea of how this thing responds to envelope changes.

From the simulations I did, I could immediately tell that a single stage of this would be useless since it also samples at positive peaks that occur below 0 on the Y axis.  If that output was filtered, then put through a second stage of this, then it would work very well on a well-behaved signal.  That's why I am more interested in actual audio signals--so at this point it's easiest for me to breadboard it and see how it works in real life.  The theory seems good.

I appreciate your input.

Soggybag:  sorry for steering this off topic.  This is not a generic, nor simple envelope follower concept.

Quote from: R.G. on October 01, 2005, 10:21:51 PM
The closest to a single chip (not single opamp) envelope follower is the old double full wave rectifier done with an NE570/571. It's pretty good because double-full-wave quadruples the frequency of the ripple and makes the filtering easier.

I've seen this circuit also, and although I haven't tried it yet, it struck me as possibly having a high performance/simplicity ratio (as per Einstein), considering it only takes about 10-12 parts including the 570/571.  Seems like a very tight layout would be possible, plus it's single supply. 

RG, you say it's pretty good.  Better than?  Not quite as good as?  Thanks.

Joe

Transmogrifox: No worries, this is a good subject and needs discusison.

I would like to see the NE570/571 envelope can anyone point me to a schematic?

I keep thinking there is a simple solution that would work for may guitar applications and built around an LED/LDR driven by the guitar signal and smoothed by some filtering and the lag of the LDR.

The 'double rectification' technique really does not help with a guitar input.
The typical guitar wave has one large peak, and two smaller peaks of opposite
polarity.  Rectify that twice and the extra peaks you WISH would fill in the gaps
simply won't. They are sure to be below the main peak and won't help at all with
the ripple. Another good-idea-on-paper.  It seems to work on a sine wave, but even then
the extra peaks are way too thin to deliver much extra charge to the filter cap.

What 'could' work is the roland quad-rectifier idea from their is it SPV-355.  It only works
with an infinitely compressed signal.  You might do something like it with tapped BBDs
(yes... I considered that :^).  That is what led me to the triple peak detector method.

BTW before someone TRIES this... I made an 8 peak detector model, thinking it would
be an improvement. Its not. Three is the ~magic~ number to be sure of having the peak in any full cycle.

If you want the other features of the 570/571 etc... its an OK chip.  Hit it with 80Hz
or even 40Hz and you are hosed, ripple wise vs response time.  Note most compander
systems work in the high frequency range where you get some benefits and response vs.
ripple is less of an issue.

I really wish this didn't suck the way it does, but thats the nature of the beast.


H^) harry

Quote from: H^)harry on October 02, 2005, 05:07:02 PM
REQUEST : Please, can anyone tell me how to abort the animated emoticons on the posting
page. Its costing me ONE SECOND per letter... I can't get much done. Is there a text-only page available ???

Best thing would be to report it in the problems thread at the top of the main index.  Perhaps if you type your reply in Works/Word/Notepad then paste it into the reply box it would be easier?

QuoteI really wish this didn't suck the way it does, but thats the nature of the beast

No kidding.  I think we need to start working on the non-causal system.  A device that can determine what you're about to play before it even comes through your fingers could sell for a lot of money in the psychic/fortune telling circles.  A lot of DSP enthusiasts would be writing home about it, too.

The closest I know of to 'precognition' on guitar controllers, is the use of neural networks to predict what note a person is playing, while the initial transients are still there.

"Yamaha Corp. has developed a more versatile sound-synthesis system by tapping neural network technology. Unlike conventional digital signal processing methods, the new system can predict a signal's waveform even before its full cycle of data is known. This capability is expected to accelerate the information-processing speed of guitar waveform-synthesis components. Yamaha's G50 pitch detector for electric guitars uses neural learning algorithms to reliably predict the pitch of a signal from the characteristic shape of its beginning transient. When the neural network was trained on thousands of guitar sound samples, it learned to detect frequency within a half cycle four times faster than conventional systems, which need two full cycles to reliably detect pitch"

QuoteThe 'double rectification' technique really does not help with a guitar input. The typical guitar wave has one large peak, and two smaller peaks of opposite polarity. Rectify that twice and the extra peaks you WISH would fill in the gaps simply won't. They are sure to be below the main peak and won't help at all with the ripple. Another  good-idea-on-paper.  It seems to work on a sine wave, but even then the extra peaks are way too thin to deliver much extra charge to the filter cap.

I suspect that may be true with single-string signals. But the double rectification does seem to work at least to my ear on more general signals - like two or more strings at a time. And it's a notable help on generic audio. I guess like all signal processing, it depends on what signal you're processing.

On the icons:
Have you tried turning off display of images in your browser?

Quote from: H^)harry on October 02, 2005, 05:07:02 PM
REQUEST : Please, can anyone tell me how to abort the animated emoticons on the posting
page. Its costing me ONE SECOND per letter... I can't get much done. Is there a text-only page available ???

Harry,

This works for me: If you're using MS Internet Explorer, go to 'Internet options' => 'Advanced' => look for the 'Multimedia' header and uncheck 'Play animations in web pages'.

Good luck!!

Just a thought before coffee: What if you used a fast noise gate on the guitar signal that actually gated the EF?  Would that get you there?

Here's a bit of simplification - I think I figured out how to make this thing self clocking. More later.

Harry,

What a pleasure to see you here!  Warmest regards to both you and Dana.

Mark

Quote from: soggybag on October 02, 2005, 11:12:20 PM
I would like to see the NE570/571 envelope can anyone point me to a schematic?

Soggy: If no one else here can post this circuit, I can take a digital pic of what I have and email it to you (sorry, got no personal webspace).  Let me know. 

BTW, on simpler rectification-type envelope followers for guitar (similar to Soggy's first draft), I've found germanium diodes help a lot with initial attack sensitivity over silicon.  My 2 cents.

Joe

Quote from: H^)harry on October 02, 2005, 05:07:02 PM
REQUEST : Please, can anyone tell me how to abort the animated emoticons on the posting
page. Its costing me ONE SECOND per letter... I can't get much done. Is there a text-only page available ???

if you press Esc after the page has loaded it should stop the animations

Hello All.

Thanks for the tips on the smiley faces. I have them stopped now. its still pretty slow but I
can live with it.

Greetings (returned) to Mark Hammer :^P

(RE: RG)  The double rectification technique has 'some' benefit on a generic signal. The problem I'm describing
is documented in some Electronotes issue, where they did some work on DC restoration to make sure that the
peaks do match in amplitude. It gets pretty complex as well...

(RE: Paul Perry)   The Axon converter is siad to use 'neural network' techniques as well. In reality it tries
to match the shape of the attack transient as it reflects from the bridge and the fret. It can give a 'so-so'
guess of the pitch in the first half cycle, followed with a traditional pitch ocnversion for accuracy.

I'm using the actual string to generate waveforms. I get some horrible distortion for a single cycle... but you cannot hear
that easily. At least it does something in 'real time' so the perception of delay is not bad.  Presumable even if the Axon
or Yamaha guesses wrong for 1/2 cycle you will not be able to tell exactly that anything was wrong.

H^) harry

Actually, to stop the animations just hit the 'stop' button on the explorer bar. That way you don't have to permanently disable them if you don't want to (for other sites).

--john

A couple of questions

I have been trying to get this work with no luck. Can anyone give me any pointers? I hooked my guitar directly to input, should I add a buffer? It looks like the Base is at about 1.9v. I have an LED connected as shown and I'm expecting it should light up as a I play?
(http://www.moosapotamus.com/EnvControl2.gif)


Also I have been working with the enevelope follower I posted originally. It seems to work but it is not sensitive enough. I have the output connected to an LED. As a play the LED lights up, but only when I pick hard on bass notes or hit a chord hard.
(http://www.webdevils.com/stompbox/images/Envelope_Follower.jpg)

Now if you extract the envelope with a PIC, why not using the PIC to output a PWM'ed envelope to make a controlled filter, or phaser, or a compressor, or ...

Okay, here's an idea.

One of the problems inherent to envelope-tracking a guitar signal is the sub-audio, or almost sub-audio, volume fluctuations, that can come from a variety of sources like beats between strings, beats within strings (when their worn), finger vibrato, and so on.

I'm wondering if something akin to how Joe Gagan uses an alternate cap and variable resistor to adjust how much bottom comes in his various Fuzz Face derivatives (well, their more than that, but you know what I mean) might be of use in nailing a more ripple-free envelope voltage. Smoothing at the other end certainly helps, but it is really lowpass filtering of the envelope.  I'm suggesting here that both highpass AND lowpass can help to complement each other and achieve an envelope output that ignores some of th usual sources of error.

Quote from: soggybag on October 07, 2005, 12:09:33 PMI have been trying to get this work with no luck. Can anyone give me any pointers? I hooked my guitar directly to input, should I add a buffer? It looks like the Base is at about 1.9v. I have an LED connected as shown and I'm expecting it should light up as a I play?

I would try adding a buffer in front. In the circuits where I have seen this follower work, it's input gets fed by the circuit's input buffer, i.e. uglyface.

Quote from: soggybag on October 07, 2005, 12:09:33 PMAlso I have been working with the enevelope follower I posted originally. It seems to work but it is not sensitive enough. I have the output connected to an LED. As a play the LED lights up, but only when I pick hard on bass notes or hit a chord hard.

Sounds like you just need more gain. Take a look at Tim's most recent version of the uglyface... LM386. :icon_wink:

~ Charlie

Soggy,

I could be seeing this wrong, but the diodes on your output (op amp version) look backwards.  I have a similar circuit that works okay, but with those diodes reversed and with +V connected to where you are showing "CV."  FWIW.

Joe

   

Thanks for the reply Moose (can we call ya Moose?) I will try a few things with this this weekend. I'm not the most electronically inclined, it's all like alchemy to me. One day I may see the light. I hope at that point that it doesn't loose the fun.

I keep thinking that you should be able to light an LED with a simple transitor fed by the signal from your instrument. I had been thinking to use a FET. It seems that if the LDR had a little lag that might smooth out the on off of the audio going through the LED. Maybe a cap in there would help?

Joe: There may be an error in there. I took the envelope section from Jack Orman's Dr Quack and added a non inverting op-amp buffer to the front of it. I figured that would make it self contained and able to drop into a wider variety of situations.
(http://www.muzique.com/schem/quack.gif)

No one seemed to express any sentiment either way about posting a page from Stompboxology. So here's a link. (http://www.webdevils.com/stompbox/images/Page_02.gif) The image was too large to post in the frame here. This page shows several envelope followers, the wave forms they create and some notes about the use.

Number 3 looks promissing. I tried to build it with a TL082 and it didn't work. But the notes say it must be 358 or 324 type op-amp. I didn't have a 358, which is why tried the 082, but I do have a 324 so I will give this a try next. I'm clear on what the difference is here?

Stompboxology was a really nifty publication with a lot of good stompbox ideas. Too bad it isn't around any more. Does anyway know how many issues are out there? I have a few, but I'm guess there were a lot more.

I have been trying this design and can't get it to work. There is a < at the end of R1 I assumed this should go to ground? But I'm not sure. Any suggestions? I used an LM324 as the chip and 1K for both R3 and 4.
(http://webdevils.com/stompbox/images/Stomboxollogy_3.jpg)

Soggybag, where did that circuit come from? I don't like the look of it!

This came from Stompboxology 13, 4 Envelope effects. The Author Nicolas Boscorelli has a fair amount of recognition, I know he's written at least one book on the subject of stompbox circuitry and apparently 13 issues of the Stompboxology publication. I think Stompbology came out quarterly, so thats potentially 52 issuses. The issues are pretty packed with stuff and it all looks pretty well thought out.

This is from the second page of Vol 13, issue 4. I posted a link to the whole page in an earlier post to this thread. The caption for this example says:

Integration smooths the control envelope; choice of R3, R4 & C2 determine relative attack and decay times.

I tried to get this to work with no luck. The < off of R1 was a little perplexing. I couldn't decide if this should go to ground or to V+ or Vb or ? I used an LM 324 and 1n34 diodes. I used 1k for both R3 and 4.

Try putting a 100K or 220K resistor across D1, the op amp (+) input needs a path to ground for the input bias current - as shown, this depends on the diodes' reverse leakage current (rather tenuous) - also the resistor provides a discharge path for the capacitor and  sets the cutoff freq for the RC combination. The 10K going to the op amp (-) input probably goes to ground if this is a single-supply env follower, rather than V/2.
Cheers, Mike

another (old) approach, here: ripple free by filament-lag (http://www.diystompboxes.com/smfforum/index.php?topic=37973.msg268541#msg268541)...

Thanks Vsat & soggybag, I'm happier now!

LM358 LM324 should be fine in the circuit (with added resistor across the diode). If you assume a typical input bias current of 100 nA for the  op amp, the 0.1 uF cap will pull up towards  (+) 9V at a rate of one volt/sec, when the other side of the cap is connected to the output of another op amp, or buffer, etc, in the absence of any input signal.... making the env follower put out full voltage. The resistor should cure this. TLC272 would be another good one to use. None of these single-supply op amps like to see input signals go more than a few hundred mV below B(-) or ground, particularly if the signal comes from a low-impedance source. Schottky diodes would work well here throughout the circuit.
Cheers, Mike

Also good idea to make R3 bigger than R4, so the  capacitor C2 is able to charge up properly, without completely discharging between the (+) half-cycles of the input signal.
Mike

Thanks for the help vsat. Can I run a guitar signal directly into this or should I put a buffer in front of it?

The circuit as shown has a gain of two for the positive half-cycle (the two 10K resistors on the first op amp define this)... minus one diode forward drop. To minimize the envelope "error" from the diode drop make sure the first op amp is capable of putting out lots of voltage, and that either Schottky or germanium diodes are used.  It would like to see a large input signal (say 8V peak-peak max or 4V peak max ) with a 9V supply, alternatively you can boost the gain of the first op amp by reducing R1 to around 470 ohms to 1000 ohms, or by increasing R2. There has to be enough signal coming out of the first op amp else D2 won't even conduct. With a big signal coming out of A2 now, put a potentiometer to ground on the output of A2 and you will have a variable envelope amount control for setting the level to a VCF or flanger  etc. And don't forget to put a 100K resistor across D1.

Can't really hook it directly to a guitar because D1 will short out the pickup on the negative half-cycles. No reason not to try it though. A buffer in front would be better. It would also be a good idea to put a resistor in series with C1 (say around 10K) to limit the current flow through D1 when you use another op amp or other low-impedance source to drive this follower.
Mike

Thanks Mike that was very helpful and informative. I will try all of these ideas out this afternoon and report back with the results...

Quote from: soggybag on October 08, 2005, 12:56:19 AM
Stompboxology was a really nifty publication with a lot of good stompbox ideas. Too bad it isn't around any more. Does anyway know how many issues are out there? I have a few, but I'm guess there were a lot more.

Soggy,

Thanks for posting that STMPBXLGY page.  I agree it was a great publication, sorry to see it go.  I have a couple of issues and would like to trade with you or anybody else that has them.  I wonder what the ethics are concerning that?  Seems like it would be safe as long as it didn't involve posting the pages or selling them.  Anyway, maybe "Trading Issues of Stompboxoglogy" should be a separate thread.  What do you think?

Joe

Here's an envelope follower from a book called "Electronic Projects for Guitar" by RA Penfold. I extracted this from a project title Dynamic Tremolo. This one seems to work pretty well with a minimum of parts. I tested with an LED hooked to the CV out only so far.
The range on R7 could use some improvement. Seems to be useful only in a certain part of the range, and off for the first 25% of the travel.
(http://www.webdevils.com/stompbox/images/envelope_02.jpg)

I'm happy with this last design, but! I keep thinking that the op-amp could be replaced by a transistor. Does anyone have any suggestions on this? It looks like I would heading in the direction of Snippet posted by Moosapoamus earlier in the thread...

There is likely a very bad problem with that Penfold circuit, driving the LED directly from the wiper of the 100K pot not a good idea. The wiper should be buffered with an op amp or transistor before doing anything with the LED. The LED could be operated with current drive by placing it in the feedback path of the op amp, or could be simply connected to ground and driven by the op amp output through a resistor. And if the LED is used to illuminate an LDR, you could use the LED for the rectifier diode and simplify the circuit.

BTW tried the Boscorelli circuit on my breadboard, does indeed work when resistor is added across input clamp diode. I used 1N6263 Schottky diodes, it is possible that 1N34A would be leaky enough that resistor would not be required - but Schottky's are cheaper and readily available nowadays. Keep in mind that both of these circuits use half-wave rectifiers, full-wave would give less ripple.
Mike

Thanks for the comments Mike. I have to take credit for the arrangement of the pot and LED/LDR arrangement since I just tacked that on to a snippet lifted from the work of Mr Penfold. The R7 pot did not seem to work that well for adjusting the CV range. The circuit seemed to work better with out the pot.

Since a single op-amp is the same size as a double maybe I'll add an op-amp buffer on the end.

I'll give another try with the Boscorelli thing. I breadboarded it twice with no luck so far. Maybe third times the charm? I had connected the output to an LED to ground as a visual indicator to see if it was working. But couldn't get it to light up.

I have been using the (Penfold) envelope to control the speed of a tremolo. It seems to work well. I can hear the change in speed. I feel it would sound better if the decay time were shorter. The Boscorelli envelope might be a better choice since the attack and decay times can be set with R3 and R4.

What did you use for R3 and R4? I used 1K for both and then tried 10K for both.

Can you point me to an example of a full wave rectifier in a 9v stompbox? I've seen this in power supplies but I don't think I remember seeing one made with op-amps.

http://sound.westhost.com/appnotes/an001.htm
may make things clearer.
The keyword is "precision full-wave rectfier" for anyone googling.
Personally, i havn't found full wave (as opposed to half wave) making
all that much difference, in a lot of applications.
There is a single transistor full wave restifier circuit too, but it's VERY tricky to
tweak into operation!

Soggy,
I used 5k6 for both R3 and R4 during the testing, and looked at the outputs with an oscilloscope. R1 was reduced to 1K. Remember that an LED needs at least 1.6V to turn on, so you need at least 1.6V at the output.

Paul - there is also that "non-precision full-wave rectifier" circuit, using two op amps and two diodes, one op amp inverts the output of the first op amp, anode of each diode goes to respective op amp output, cathodes go to series resistor which feeds capacitor. As in ARP Quadra compander. Making a single-supply version is a bit trickier.
Mike

2 things - firstly the name Penfold always tended to appear in the same sentence as the word "simple" which I think speaks volumes, and secondly - in many cases a dual power supply (2 batteries providing + and -9v) would save a great deal of the design problems involved with a single supply ............. :icon_rolleyes: :icon_rolleyes: :icon_rolleyes: :icon_rolleyes: probably to enable the use of a tiny enclosure...........am I right or am I right???

I am having just this problem in playing about with the EH Space Drum and Crashpad envelope generators, (not the small enclosure as I don't care what size I use) both of which use an LM 324, so I anticipate fewer problems using a 4558 or similar powered by +&-9v.
Stephen

after I just had a phenomenal idea for the perfect env-fol, (whose patentworthiness has yet to be checked...),
maybe it would be interesting to see what Mr. Bissell thinks of :this (older)  quadrature approach (http://www.diystompboxes.com/smfforum/index.php?topic=25313.0).

Btw: you`ve been invited to the party already a long time ago (http://www.diystompboxes.com/smfforum/index.php?topic=21783.0)...

It's pretty embarassing when one can't remember one's own posts, but looking at the dome filter approach, it occurs to me that one doesn't in practice really need a dome filter. Feeding a signal to each of a bunch of phase shift circuits (non identical) and taking the maximum of the outputs, would likely do just as well.

"univibing"...

Thanks again Mike those are really good ideas. I will have to find some time to try them out. This morning I'm liking the sound of replacing one or both of the diodes with an LED.

Perry, thanks for pointing out the ESP article on rectifiers. The last one on the page seems pretty simple I may have to give it try. Speaking of transistor rectifiers, when I woke this morning I was thinking of the Green Ringer, I had the idea that Q2 and following parts might make the start of transistor based envelope.

hmmm... the phase shift only has to be used for frequencies below the ones which cause objectionable ripple. Frequencies above that are OK as is.


By the way, full wave rectifying 3-phase AC needs no filter caps for most power supply work. 3-phase with FWR is less than 5% ripple as is, no filter caps.

I gave the Boscorelli envelope a second try. It seems to be working, this time I tested with the scope, as suggested. Mike was right on, the output is only about 1v peak not enough light the LED. It might have been working the first couple tries but I just didn't see the out output.

I figure the easiest way to interface something like this with a broad range of effects is to use an LED/LDR so it needs to be able to drive an LED. It seems this one would need a higher output to make it useful. I would guess the gain of the first or the second op-amp could be increased to acheive this. I'll have to break out the Forest Mims Engineers Notebook on op-am try some experiments...

It seems like you could put the LED in the feedback path of an opamp, and use a resistor on the (-) input.

That way, the current into the opamp is determined by the input voltage difference from the (-) input DC level, which we know is always equal to the (+) input DC level. The opamp will do whatever voltage on the output is needed to make an equal and opposite current cancel the current coming into the (-) input, so a current flows in the LED equal and opposite to the current in the input resistor.

The voltage at the input now doesn't matter; it's always scaled by the input resistor, and the LED conducts that much current.

If have pictured what you suggested correctly the arrangement would look like this:
(http://webdevils.com/stompbox/images/Stomboxollogy_31.jpg)

Soggy,
The modified circuit  you just posted could be made to work if you do the following - connect the LED directly between the op amp output and (-) input, anode goes to output and cathode goes to (-) input.... and connect a resistor from the (-) input to ground. Make the resistor about 4K7 if you want about 5 mA through the LED when the voltage at the (+) input is 5V (aim for several volts on the capacitor when the pickup is putting out a strong signal, to maximize useful range).

The circuit can be simplified and enhanced if you choose to use a dual supply: instead of one LED, connect a 2nd LED in reverse across the first LED - this then becomes a full-wave rectifier. Connect the pickup directly between the op amp (+) input and ground. Note: for op amp protection, good idea to put a series resistor (say 10K -100K) between the (+) input and pickup. Also reduce the resistor at the (-) input from 4K7 to about 100 ohms to increase the sensitivity. D1 and C1 (and the previously mentioned 100K) are no longer needed. You can shine both LEDs onto an LDR or pair of LDRs to provide a variable resistance  proportional to picking level. The sluggish response of the LDRs provide envelope filtering without need for a capacitor. Only half of an LM358 or 1458 or TL072 is used. This technique might already have been used in a pedal, I'm just mentioning these ideas.
Mike

Thanks Mike and R.G. If I understand correctly this is what you are describing:
(http://webdevils.com/stompbox/images/Stomboxollogy_32.jpg)
The second suggestion sounds good. But for now I think I will get single 9V version working. I already have a tremolo that I've built I want to add the envelope to make it a little more dynamic.

Here's another ultra simple idea...
http://aronnelson.com/gallery/johan/LA_Light_last_one?full=1

~ Charlie

Soggy,
That's the idea - but connect the LED the other way around (anode should be on the  op amp output).

An aside - two main problems with env followers  a) ripple b) intermodulation when chords are played, creating lots of low-freq ripple.
Using a bank of envelope followers, each optimized to a different portion of the audio spectrum,  with the individual outputs summed to produce a single main output would minimize both of these problems. Lots of parts, yes... the good news is that some of you may already have one of these sitting around unused - an analog vocoder. Take the outputs from the analysis bank, sum them, and use that to control your VCF, flanger etc. Good use for an old VC-10 etc. Should provide excellent results.

Another possibility is a "square root  of sum of squares" circuit fed from a 90 degree phase difference network - this eliminates the need for rectifiers to determine the magnitude of a sine input... but uses a lot of parts, but a bank of these would be needed.

Envelope follower DSP chip another possibility - spectrum analyzer with a single voltage output. If they were cheap, people would use them.
Mike

Then again, you could feed the peak detector envelope detector from a full wave rectified signal...

Thanks for being so patient. I'm little dense sometimes. I've updated the image. I'll give this a try tomorrow when I the time. This looks pretty promissing. Of course this won't be perfect but I think it should be close enough for rock and roll as they say.

.......and then again a combo EH Space Drum/Crash Pad circuit......watch this space......
Stephen

I tried out the last version inlcuding the suggestions by R.G. and Mike and it seems to work well. The LED is not lighting up bright enough but I think this can be improved by increasing the gain on the first op-amp stage. I will have to try this after work today or tomorrow morning.

According to Forest Mims the gain of the Non-Inverting amplifier is 1 + ( R2 / R1 ). The gain is now 2. I measure about 1v max across C2.

Quote from: A.S.P. on October 13, 2005, 12:15:24 PM
Stephen: a little bit OT, but do you know  this Fund.-Ex.  (http://www.pat2pdf.org/patents/pat4377961.pdf)yet?

Thanks, it is in my "to read" pile no. 6 which I'll get to at some point. At the moment there is so much to read in the Saturday/Sunday newspapers that they take a whole week of reading time!
Stephen

After swapping out parts and testing somethings I find R3 sets Decay time, R4 sets Atrtack, The output seems low (the LED is not so bright) with R1 and 2 at 10K. I changed R2 to 1M and I get a good brightness on the LED, but it never goes fully off. With the 1M in R2 I get a constant 2v at the output of the first op-amp.

I think it may be necessary to reduce R2 to something between 10K and 1M for a better range. It might also be useful to increase the gain of the second op-amp.

Thinking about using this with an LDR it might be best to put a pot in series with the LDR to set the range.

Hello PureTube...

Sorry for late reply, I don't get out enough... :^P

The "Dome Filter" approach has some merit, but if you want to include guitar fundamental
(80Hz) you will need a dome filter with probably 18ms of group delay (the time it takes
for a signal at the input to reach the output).  You are slow already...

The dome filter has outputs that are 90 degrees apart, but the overall phase shift varies
1000s of degrees...

I wanted to use the method to pitch shift guitar up to ultrasonic frequencies, do a P/V
conversion, and elimiate the delay.  Does not work for reason above...

Roland made a unit called the SPV-300 (or 3000 or something).  There is a 'quad rectifier'
in the circuit... that is done with phase shift techniques. The down side is that it only
works because the signal (by that time) is compressed and filtered... essentially a sine
wave. Put a normal guitar input there and it does not work (I tried :^)

H^) harry

hmmmh...

so: petemoore wins the first price with his re-verb idea in the other thread: (http://www.diystompboxes.com/smfforum/index.php?topic=38222.20)

Quote

Quote from: petemoore on October 18, 2005, 02:09:04 AM
  How about...
  A series of delays [like reverb] into the EF, would smooth out 'two note ripple', you could still get good changes in amplitude [of course slowed by however many milliseconds the delay extends input to the EF]...I'think delays made short enough to allow the follower to still 'follow', and long enough to fill in the 'dips between the swells ...
  Because the echo need not be in the signal path, a relatively low grade [inexpensive] chip [I think there are premade ones around for fairly cheep?] could be used without any degredation of audio signal, and improvement of sweep steadiness...also sweep slowness of course...DPDT...
  Am I rambling yet?

just delay the signal (in the sidechain - with a BBD) by, let`s say (assuming the peaks of the lowest frequency= 80Hz = 12.5ms apart) 2.5ms, and repeat them (by feedback-loop) 5 times in between that interval: then you get a couple of peaks you could rectify with less ripple than before...

ohh: but then you need either a shorter delay for higher signal-frequencies (keeping the same number of repeats),
or increase the number of repeats towards lower signal frequencies,
and therefore decrease the general delay-time....

err???, hummhh...

should I engage the "modify/edit"-button immediately right now,
or leave this up there...???


Yes, my other idea was: heterodyning/multiplying the signal into R.F. territory,
demodulate that,
while only filtering out "some" of the H.F.,
to keep some "chopping", but over-all relatively steady averaging "filling texture"
underneath the envelope, which wouldn`t allow the derived control-voltage
to go "down to zero" within those mentioned 12.5milliseconds,
before the 2nd peak of the 80Hz comes back into life...

All good stuff eh? Somewhere (how many times have I said that?) I have the circuits for the Top Gear Guitar synth, of which they only made about 20. So no doubt it will turn up at some point so I can see what they did about an envelopr follower. I remember trying one and it was pretty good.
Stephen

OK - spinning an idea (or even 2) further:
how to make a pair of quadrature signals (and then invert both, to obtain a quadruple...), that lack the group delay caused by a Dome-filter?
The  third method ?  (http://www.pat2pdf.org/patents/pat2173145.pdf)comes to mind...

Gives you both: quadrature and (an intermediate) higher "pitch"...

speaking of pitch: once you have an audio-quadrature pair, and you`re already modulating anyways, you can go Bode/Moog, and introduce an envelope-controlled Frequency-Shifter, that uses the quadrature-follower...

"Pitch-Hoax" ?   :icon_razz:

These exotic rectifiers are all very well, but they dont (AFAIK) overcome the problem of low frequency 'trem' created by beating between different notes.

so first send the signal thru a hi-pass?

Quote from: puretube on October 20, 2005, 11:23:22 AM
so first send the signal thru a hi-pass?

HEY, Barmentloo!  That's MY question!!  You're cutting in on my territory.   :icon_lol: :icon_lol: ;)

ok - bandpass it then!

and, err: I thought You were here for the answers,
and not for the questions...

:icon_wink:

Had a chat with Harry yesterday who indicated that the problem may be even worse... low-freq ripple can be generated by feeding a single string into an env flwr. The ripple can be very low-freq (well below the pitch of the string) and is a consequence of the fact that the overtones in a vibrating guitar string are not perfectly harmonic... successively higher  overtones become increasingly  sharp with respect to the fundamental. eg  80 Hz fundamental, 161 Hz 2nd overtone (rather than ideal 160 Hz), 243 Hz 3rd.... feed this into rectifier and sum and difference will be produced betwen all the original frequencies, giving a real mess including 1 Hz 2 Hz and 3 Hz difference signals.
Mike

Puretube,
Took a look at the SSB patent you ref'd... that is an interesting variant of the standard SSB stuff. Typically around 500 Khz for the "LFO" aka local oscillator... the bandpass filter is the most critical component for ensuring the opposite sideband is suppressed - a  "crystal filter" aka "quartz lattice filter" is normally used with very sharp attenuation at the bandpass response skirts.  That certainly would generate quadrature audio without the huge amount of group delay imparted by the Dome filter-phasing method.

The bandpass filter is the critical part...  300 Hz - 3 KHz audio is normally used for communications. The low freq response would be limited by how selective the bandpass filter is... we don't need 20 Hz-20 KHz for the env flwr though... 80 Hz for lowest string up to 5 Khz should be sufficient for envelope extraction. Unfortunately processing 80 Hz is even more demanding than the communications 300 Hz lowest freq. Heterodyne 100 Hz low note (for instance) with 250 KHz L.O. and get  250.1 Khz  and 249.9 KHz sum and diff freqs. This is only a 200 Hz spread centered about 250 KHz.... a very sharp filter is needed to remove the unwanted sideband... thus crystal filter is used. The crystal also has to pass a bandwidth wide enough to accommodate the original audio bandwidth.  Heterodyne this back down to baseband using quadrature version of the same local oscillator signal, low pass filter to remove new upper sideband (as patent shows)  and original signal is recovered in quadrature form. Rectify this separately from  original audio and combine rectifier outputs and ripple is reduced.

Three multipliers are needed (the "detectors" in the patent are also a pair of multipliers)... these could be CA3080 or perhaps even analog switches, no need for AD633 here.  Interesting, should give an improvement  but a lot of parts.
Cheers, Mike

Thought about this some more.... the SSB quadrature method will give an improvement for strictly monophonic signals, but the "two-note beat" problem will remain. Could feed the recovered two-note beat signal into a 2nd quadrature generator for ripple reduction, but the beat frequencies will be so low that the quartz crystal filter will not be able to separate the sidebands properly  :(

Yikes, Mike

wait... I had a brickwall bandpass-filter somewhere,
that goes down to zero Hertz...
ahh, but that one again is a part-intensive modulation/heterodyning thingy...

so: "neck-pickup/tone rolled down", and then hipass the sidechain above 60Hz...
(and artificially "recover" the missing overtones by distortion in the signal path),
or first have a quick-acting fundamental-extractor in the side chain... Stephen are you still with us?).

Or: forget about the (lousy group-) delays, and process the side-chain the best way possible,
and (full-bandwidth) delay the whole signal-path by those ms it needs, to have the control-voltage right - can add some nice flanging... - or lead to timing problems in a band-context... (anybody said: "latency"... ?)


btw: that "3rd method" for SSB-ing can be interesting for obtaining 90° audio signals
for other purposes...  :icon_wink:

Is it just me or do the requirements for a so-called generic envelope follower (at least in the analog domain) change depending upon what the heck it is you want to play?

I certainly trust Harry's chops and judgment (as I do Mike's and Ton's), but I ask myself "What does a 1hz or even 3hz subaudio component to the envelope matter?"  If I was going to hang onto a note for a while, probably.  The majority of people who seek out envelope-control, though, intend to use it for rhythmic emphasis.  Which is to say that unless it's a funeral dirge, or some experiment in which alternate occurrences of some signal are used to trigger/drive the envelope follower, and are held onto (deliberately lagged) over a bar or more, few people would want to assure ripple immunity for those sorts of durations.  I'm not saying that perfect ripple rejection is undesirable or unattainable, just that most anticipated uses do not require the sort of worst-case scenario preparation being discussed.

All of that being said, I'd be the first to admit that "pushing the envelope" in threads like this one can sometimes leads to insights and simple-but-elegant solutions that advance us in other areas by leaps and bounds.  So ignore my harumphing, and carry on.

naw, Mark: my "neck-pickup & rolldown tone" was meant exactly to express the same thoughts!

but: it is nice to congregate with the elusive thoughts of those,
who spent more thoughts on this kind of stuff in the synth-/studio surrounding,
than "us" here down at the floor- boxes boot-level...
:icon_smile:

Any DSP people out there who can provide a ballpark figure for latency or frame update rate for an FFT with 1 Hz resolution good over the range of 50 Hz to 5 KHz? (12-bit amplitude resolution probably more than sufficient). Probably just looking at the first few lowest frequency components and summing them would be adequate for covering all cases from single note to 6 strings at once, no need to track individual notes or upper harmonics (unless you want to).
Cheers, Mike

FFT frequency resolution (Hz) = sampling rate fs (Hz) / FFT size
This would also be the frame rate if you did FFTs one after another but in many systems the FFTs are overlapped in time so the frame rate can be whatever you desire/can achieve.

So, to achieve a 1Hz resolution your FFT length would have = the sample rate.  To achieve a 5KHz bandwidth you need an absolute minimum fs of 10KHz (although this isn't a practical minimum) so an FFT size of 10K would be required.  A 10K pont FFT is too memory hungry for most basic DSPs without some form of external memory (easy on a PC though)

I seem to remember that a 1K point FFT on a SHARC running at 40MOPS was about 1 or 2 mS or thereabouts, but it really depends on your DSP hardware.

Thanks Dave!
That makes sense. Hmm..looking at your eqn's.... maybe do an FFT for the lower frequencies only (up to 1 or 2 KHz?)  and process the higher frequencies directly with a (hopefully simple) "absolute-value and average" routine, combine the two sets of results and send to D/A. Or use DSP for the lower freqs, and process the higher freqs in the analog realm  with the typical env follower circuit ( and the high freqs should cause less trouble than the lower freqs which are handled by the DSP, but this is getting a bit klugey).
Regards, Mike

There's a "gotcha" in that idea as well.... if you have the FFT set up for 1 Hz resolution (to adequately separate low notes), it will take one second for an event occurring  at time "x" to be pumped out of the DSP chip as env information. The DSP maybe be computing the entire spectrum and sending out new info every millisecond, but there will still be a one-second latency, which will make it unuseable.

Maybe just look at the higher freq spectrum components, filter out the low freq stuff beforehand,  assign the harmonics to different notes based on a largest-common-divisor routine, and calculate the current amplitude for all notes present.???
Mike

Firstly, sinAsinB = 1/2[sin(a+b) + sin(a-b)] i.e. if you multiply two sines together, you get a sum and difference frequency.  If the two frequencies are close together, then the difference "component" is low frequency.  You may ask why two signals would be multiplied in the first place; well this comes from the fact that a non-linear system (like say a diode I vs V curve will generally have terms which are powers of 2, 3, 4 etc. in varying amounts e.g. y=x^2.  When you apply two or more completely separate sine waves to this sort of system they intermodulate i.e. as per the equation above e.g.

y = x * x (or x^2) so if x = sinA+sinB you would get y = sinAsinA + 2sinAsinB + sinBsinB and you can substitute the above equation into this one to obtain the sum and difference frequencies.

In reality, this happens on a grand scale with a continuous spectrum of frequencies from a guitar or whatever all mixing together to produce all sorts of beat frequencies just as you get playing notes on diferent strings which are fairly close together through an overdriven amplifier.

Probably not a great explanation but that's how I see it.

Dave and A.S.P.,
Yup... that's what I think is happening within the env follower. But with two strings playing nearly the same note you also have  beating taking place in a linear environment... the oscilloscope (and my ears) clearly show a periodic beat envelope before the signals are applied to the follower... simply a result of two nearly identical waves slowly going in and out of phase with each other, producing periodic reinforcement and cancellation at the difference freq. Since the two freqs are not the same (even if the amplitudes are), the null will not be perfect at the time of maximum cancellation, and the null becomes even less intense as the freq difference becomes larger. But the null can be nearly perfect when the two frequencies and amplitudes are very closely matched, giving a large change in overall level at the beat rate, which the follower does a nice job of "following". No need to invoke non-linearity of the air or non-linearity of the ear to explain acoustic beating. It is additive, and will behave the same at low or high sound pressure levels (or by simple summing in an op amp circuit). The rectifier in the env follower is non-linear however, and multiplication takes place and additional terms are generated. as you mention. So the beating has two separate origins in the follower circuit.
Regards, Mike

A.S.P.,
Looks like I sent the previous message  almost the same time as you!
As Harry related to me, even for a single string the overtones are not perfectly harmonic. This is the case for many mechanical systems and has to do with the physics of the string/plate/bell etc vibration, different vibrational modes and non-linear behavior of the vibrating medium.  Waveforms/spectra  from electrical oscillators are usually quite simple in comparison, since all the overtones are perfectly harmonic... not what usually happens in nature.
Regards, Mike

Couldn't you just set a frequency range depending on your instrument, detect which note is loudest, and follow its envelope? I'm a n00b and don't know much, so I'm more than likely wrong. I don't know how you detect which is loudest, but there are some boxes that do that right? You know, the ones that get reviews for their "tracking"? Or is that just how it's sonically perceived, while electronically doing something much different?

Set me straight

Quote from: Vsat on October 21, 2005, 11:54:21 AM
Dave and A.S.P.,
Yup... that's what I think is happening within the env follower. But with two strings playing nearly the same note you also have  beating taking place in a linear environment... the oscilloscope (and my ears) clearly show a periodic beat envelope before the signals are applied to the follower... simply a result of two nearly identical waves slowly going in and out of phase with each other, producing periodic reinforcement and cancellation at the difference freq. Since the two freqs are not the same (even if the amplitudes are), the null will not be perfect at the time of maximum cancellation, and the null becomes even less intense as the freq difference becomes larger. But the null can be nearly perfect when the two frequencies and amplitudes are very closely matched, giving a large change in overall level at the beat rate, which the follower does a nice job of "following". No need to invoke non-linearity of the air or non-linearity of the ear to explain acoustic beating. It is additive, and will behave the same at low or high sound pressure levels (or by simple summing in an op amp circuit). The rectifier in the env follower is non-linear however, and multiplication takes place and additional terms are generated. as you mention. So the beating has two separate origins in the follower circuit.
Regards, Mike

Yep, sorry, i see your point - didn't mean to lecture you or anything.  I've seen a similar effect in a metering application I worked on but it was more related to the lack of samples of the signal and the signal being close to a sub-multiple of the sampling rate.  The samples "rotate" slowly at the difference between the actual frequency and the nearest sub-multiple of the sampling rate.  e.g. if sampling at 48KHz and you meter a 4KHz ish signal the envelope beats up and down at a low rate.  There isn't much which can be done in this case.  Your scenario of two closely related signals summing and cancelling is similar and I don't think can really be avoided.  I'm not sure it would cause major problems though in reality - it may even sound good.

Varialblender,
Your idea of selecting the loudest note only would be a big improvement, don't know how you'd do it though, DSP might be the best way.

Dave, I just mentioned the acoustic beat phenomena to indicate it was another source of trouble separate from the beating which occurs during rectification.  Audio books talk about linear mixers, while RF books talk about a very different kind of non-linear mixer, I seldom see acoustic beats discussed anywhere.

Yup, this is mostly academic, people have been using env flwrs for years now, just seems it would be nice to have one where playing additional notes would only increase the overall envelope level without adding low-freq ripple.

And it would be really neat to see a box where you plug in your axe, and each individual note gets auto-wah'd or individually fuzzed etc without interaction with the other notes.... sort of turn your ordinary pickup into a hex pickup with a dedicated set of fx per string. Any digital multi-fx boxes out there that can do this....?
Regards, Mike

Hi Mike,

Well my Roland GR-300 used to do that (not digital).  Hex pickup (Ibanez ZIMG), each string processed separately by the GR-300 with fuzz and a filter section and a VCO dedicated to each string.  That was a really cool box.  I'm sorry I sold it now... :icon_cry:
Cheers,

Peter

Oops! As usual I misread your post :-\.  You were asking about a device that would allow a regular pickup to act like a hex pickup.  Sorry... :icon_redface:

(GR-300 is still cool though :icon_cool:)

Peter

made a hex-pickup 25 years ago from 6 reed-relay coils (can`t find a pic).

One could multiplex/modulate them (on-board), and transmit that (HF-)signal via the normal cable (which could also carry DC for phantom-supply) "on top" of the normal signal from the normal pickups...
(and then filter out normal and multiplexed signals, using the latter for pitch-to-voltage,
trigger, and envelope purposes,
while using the normal PU`s for "sound".

Peter,
Seen a number of GR-300 etc but have never heard one in person (AFAIR). I just think that such a facility would make a great addition to those do-everything boxes out there.  Might not be so simple though  ;)

OK here's the solution... each string is paired up with a 2nd (much higher pitched) string, both are picked simultaneously, the normal set of 6 strings provides the audio, while the new set of 6 high strings feeds the env flwr for greatly reduced ripple   :D
That's the ticket.....   ;D
Mike

That's got them thinking Mike - Roger McGuinn will be OK!!
Stephen

Geeez Mike,

I have enough trouble negotiating 6 strings!  :icon_biggrin:

Peter

Franchises coming soon to a plaza near you!   ;)
Mike

Mike, assuming you are 5 hours behind us how do you manage to stay up until 5am or have you just got up!!!
Stephen

Steve,
Your observational skills are finely honed    :D
That's what (sometimes) happens when I get a circuit idea in the wee hours.
Luckily, this doesn't happen too often...
Nothing to do with env followers (might be LFO-related though)  ;D
Cheers, Mike

Missed this thread earlier somehow. Oh yeah, on vacation. GREAT THREAD, guys.

Anyone around here familiar with the RNC (Real Nice Compressor) ? I've been told it uses an old motorola microprocessor (like a 68705R4 or something) to generate the envelope follower signal controlling the VCA... and I would suspect using a method not too far removed from Harry's.

If you use a FWR followed by a resetable peak detector like Harry used in his EDN Design Idea, and sample the voltage into a uP with a ADC-input channel every 1-5 ms or so, then the software can keep up with all of Harry's peak-selection processing (and then some - maybe even use an exponential decay prediction algorithm to tame down those low Hz beat frequencies Harry's worried about), and then output the control signal on a DAC pin into a simple medium-short time constant RC smoother that can also have it's decay rate sped up if you mute the strings by using another uP pin to turn on a transistor switch to add a lower value resistor to ground across the "smoother" cap. Practically no ripple, low-distortion, responsive envelope-follower circuit. Would do it all, yes? ;D

Maybe not one IC, but should get it down to maybe 2 (uP and quad op amp) and a dozen passive components. Isn't there a PIC version with a 8-or-so bit ADC and DAC built-in that comes in a compact, low pinout DIP? Seems like I remember one. No? :icon_question:

Also - Harry, do you have a schematic laying around of the one you made using the CMOS analog switches that you would be willing to share? I'd like to see it, too. You seem to be a very inventive fellow. Glad to see you around here! :icon_biggrin:

Hello Mr. Coffee

see... speak of the devil and you get a quick reply. Now I have to figure out how to
include the schematic you requested. If I understand correctly, I have to link from
some other website.  I'm working on it...
H^) harry

I think the only way to do it is to stick a cat in a box with a spring-loaded, grounded volume pedal...and guitar amp output connected to its tail.  When the dynamic level gets too high, the cat jumps off the volume pedal.  When it calms down a bit, the cat relaxes and beds back down on the volume pedal.  You have forward and reverse depending on which way you spring-load the volume pedal :icon_twisted: :icon_twisted:

The cat won't respond to beat frequencies, because if the dynamics are loud, it's getting zapped and it's time to move off the pedal.

Hi Harry,

I dunno if it's a link or just a personal mail thing. You said

QuoteThere are other ways to do this circuit.   I did a version using a 4053 CMOS switch to run the
sample holds AND the resets.  Its a little more compact.

and it got my curiousity up just trying to figure exactly how it was you hooked that up and made it compact. It sounds pretty cool.

Do you see any problems with doing the logic and peak holds with a small uP with an 8 bit ADC-DAC?

My cat's not that talented :icon_lol:


Thanks,

Mr Coffee & list,
Harry forwarded me the schemo for the CD4053 version of his envelope follower and asked if I could post it (he tried but his machine crashed in the process).

Now just how do you post the gif....???
Anyone?
Cheers, Mike

Hola Mr. Coffee

Once Vsat or I figure out how to post, I'll send the schematic.  I use the 4053 to pass the
signal to the hold caps, and the other part of the switch to dump the charge. It requires some
low leakagew diodes to work really well, but does not have the voltage drop of my original circuit. That might be a blessing or curse.

I'm using a 4093 and a 4017 for the gating signals, you might do better. Still looks like 4 ICs so its
not stompbox ready.

Bop over to wiseguysynth.com and get the schematic for my "morphlag" circuit to finish the job. Do it
quickly because the owner (Larry Hendry) passed away in a tragic motorcycle accident and it might disappear any time.  Larry was a great member of the DIY scene and he'll be missed, and often.

H^)harry

OK, that's a 4017 to count cadence, two 4053's to let you have three throws per pole (sample, open, discharge), a 4093 to do misc gateing. Isn't there a single peak detector circuit there to generate the "sample me now" pulse?

As requested previously, I have the updated CD4053 schematic here....
Now if someone will tell me how to post the schematic to the list, I will do so...
Mike

you gotta upload it to a website of yours, (or photobucket or the like),
and the post the link to that here...
(either link to the URL of that pic,
or insert the URL xxx your url (http://xxx%20your%20url) of that link to the pic as an "IMG"(http://xxx%20your%20pic-url)

To post a picture to the forum, first you have to upload it to any web page where it can be browsed. You can't (that I know of) upload it directly to here.

Then to post it, you enter a message and put in the  terms "img" and "/img" around the picture url, but where I have quotes, use the left square bracket "[" and right square bracket "]" to enclose img and /img. As in (img)www.website.com/folder/imagename.gif(/img) but using square brackets [ ] instead of parentheses.

Thanks R.G. and Puretube...
Let's see if this works:

http://putfile.com/pic.php?pic=11/32315330961.gif&s=x12

(image hosting courtesy of www.putfile.com)

Regards, Mike

OK... Harry's updated schematic is online now

I'm a bit late, but welcome to the forum Harry! Nice to see you here. 8)

The 4017 / 4093 would easily disappear into an Atmel ATtiny12 or an 8 pin PIC. At first I was thinking great, now it's only three chips. A second later the need for a voltage regulator brought it back to four. Oh well, at least it's a total of just 11 pins for the digital section.

Mr Coffee,
I have an RNC1773 and absolutely love the thing. It does indeed use a 68HC705P6A clocked at 4MHz.

I've never looked around the board with a scope as it runs, but it uses a MAX318 along with buffering from a TL074 in what looks like a sample and hold at first glance. I don't see any diodes after the sidechain input so it looks like it does all the peak detection in software. The control voltage for the VCAs is supplied by an 8 bit D/A. I would love to see a schematic, but it looks like it may be a somewhat involved software/hardware hybrid setup.

It has two algorithms to choose from while running, regular and super nice. Super Nice mode is a set of three different algorithms running in series with the output fed to the VCA. You get the performance of three compressors in series, with noise from a single opamp and a THAT Corp. VCA. Very cool design! 8)

Ooooooooo, this is such a great thread. My FAVORITE subject!!!!!

Many thanks to Harry and Vsat for posting the cool schematic. The CD4093 is there just to give a nice sharp, squared-up quick reset signal for the S & H's, then?

Nice way to get the op amp output node up one diode drop to overcome the Vf on the S & H diodes, Harry. ;D

Just a thought... couldn't we simplify the circuit just a little bit more by bumping the output of the input buffer up another diode drop and replacing the 3 op amp buffer\rectifiers with 3 good-quality pn diodes, as long as the following stage had a fairly high input impedance? The S & H circuits only have to hold up the charge on the .01 uf caps for 15 ms. Diodes don't leak that much, do they?

Hey maybe we can get this thing really small yet.  :icon_mrgreen:

And Peter,
I'd love to hear more about the RNC - I haven't ever had one to study\trace out, but it seems like a fantastic design! They *do* sound really nice according to everybody I know who has played with them.

Hmmm, 3 algorithms in a row... didn't Mark MCQuilken designed this thing? Wonder if he'd join this discussion if we asked nice - anybody here know him? 8)

another pitch detector:
http://www.pat2pdf.org/patents/pat4351216.pdf (http://www.pat2pdf.org/patents/pat4351216.pdf)

Hi TELEFUNKON,

Nice patent circuit to check out, but this thread was on Envelope detectors, not pitch detectors.  ???

Takes a lot of circuitry to "bust humps" and derive a pitch signal from an electric guitar string.

don`t worry, Bart.
early contributors to this thread will welcome that p-file.
it`s a crossover topic.
crosses over into the DSPDIY too.
this thread has touched the pitch detection (aka fundamental extraction) realm
a couple of times.

Quote from: puretube on October 12, 2005, 09:06:23 AM
after I just had a phenomenal idea for the perfect env-fol, (whose patentworthiness has yet to be checked...),
maybe it would be interesting to see what Mr. Bissell thinks of :this (older)  quadrature approach (http://www.diystompboxes.com/smfforum/index.php?topic=25313.0)...

nice idea! ( 30 years late!   ) :icon_razz: (http://www.pat2pdf.org/patents/pat3789143.pdf)

now where have I heard THAT "Blackmer" before?