A question for Mark Hammer/Envelope Filters

[KMS]

Mark,

I read you’re “The technology of Auto-Wahs/Envelope-controlled Filters”

It makes since to me and everything but I am a little lost with this stuff still so I read “How it works” by R.G. Keen and that helps some more.  I’m still sort of lost. I understand the theory behind this stuff but one thing I’m not clear on when you say Envelope Follower, Envelope Detector are these the same?  Also, I think what I need for a learning tool is an example of a semi-complex FX circuit with block diagram showing where each component of the Envelope process is within the circuit.  I’m having trouble identifying the different “phases” of the Envelope/Filter/Detector/Follower/.  I need to understand how to see this in a real circuit.

Can you do it on a circuit for me? Like the Orange Squeeze or Fuzz Face or whatever?

Thanks

KMS

[Mark Hammer]

I dug my own damn grave here by using a variety of terms interchangeably. :?  I'll try and dis-obfuscate.

All devices, whether stompboxes or rackmount stuff, that adjust *something* on the basis of the current signal use/have what is referred to as a "sidechain".  It's called a "sidechain" because it is a signal path that runs in parallel with the signal path it will eventually influence/modify.

The most common type of sidechain will simply attempt to detect the overall level of the input signal, however this is not the only type/format/variation.  For example, de-essers use a sidechain to target objectionable upper mids that form sibilants and adjust them specifically.  Limiters use a sidechain to detect if a signal is not simply louder but if it is over a specific designated level.

ALL things in sidechains, however, have to engage in what is called "rectification".  That is, if it is going to be of any use at all, the AC audio signal has to be transformed into DC, so that it can now be a voltage that fluctuates from some baseline level, namely zero volts DC.  Only by having a reference point can the voltages that are detected play any useful role.

So, where does that leave us?

If someone talks about the "rectifier" or rectifying the envelope,  or envelope follower, they are talking about the same things.  I suspect the word "follower" is inserted because the rectifying typical of a power supply is acting on a steady-state AC input, not one that fluctuates over time.  An "envelope follower" rectifies but it rectifies something that is changing, so it is said to "follow".  

As well, since the most critical information contained is really what happens at the beginning of the plucked note, people will often talk about the same circuit as being a "detector" since it tells you when a note has started.  I might point out that since trigger/gate extractors for synths also use a envelope-rectifier circuit to indicate the start of a note when the envelope exceeds some preset level or threshold, that is another reason why "detector" will also be used with the word envelope.

There are many ways to rectify an audio signal, but the heart and soul of even the crudest one will be a diode.  Since it only conducts in one direction, simply sticking a diode in the signal path will chop off everything on one side of ground, leaving AC which swings from ground to some either negative or positive maximum (depending on which way the diode is facing).  

While the Fuzz Face has no such element to it, the Orange Squeezer does.  If you look at the OS schematic, you'll see that the output of the op-amp gain stage "goes" to two places: the audio output, and the sidechain/follewer/rectifier.  The diode the audio signal is forced to pass through in the sidechain assures that the resulting voltage will only swing in one direction from ground.  The medium-sized electrolytic capacitor to ground right after the diode is a way of smoothing out the gaps between all those little teeny audio peaks that are still left between waves after the diode has done its business.  Since the capacitor stores and hangs onto charge for a little while, it introduces some lag while essentially fills in the gaps between "micro-peaks" a bit better.

You may have noted something in my paper which talks about half-wave and full-wave rectification.  Where the half-wave type simply chops half the waveform on one side of ground, the full-wave type essentially "folds over" the audio signal so that what used to be on one side of ground is now its mirror image on the other side of ground.  That effectively shortens the gap between those little micro-peaks in the envelope voltage/signal, such that they become easier for the capacitor to smooth over and much less audible.

There I hope that's more coherent.  If you read the version of the paper at GEOFEX, take a look at the version at my site (http://hammer.ampage.org).  I originally sent RG a slightly shorter and less well-illustrated version, so there may be some additional useful content in the other version.

[KMS]

Thanks Mark,

That cleared up a lot.  I kind of thought that some of your different terms were describing the same thing.  I just need to learn all the jargon. Now it is much clearer and the example of the OS helps a lot.  It is interesting that you say the Fuzz Face is not applicable.  Do you mean in the terms your speaking?  It would seem to me that some kind of envelope would exist in the circuit.  How would the circuit “know what to do” without some type of “envelope system”?  I guess I might not really understand exactly what an envelope is but I think you made it pretty clear.  I’ll read the material at the other sites that you suggested and come back to this if I don’t grasp it later.  Thanks again.

KMS