Blue Box + compression == good tracking?

[PenPen]

I am in a discussion on another board about the benefits of putting a compressor in front of a blue box. The way I understand it, the compressor will level off higher order harmonics to eliminate the 'warbling' effect on low notes. Is this correct?

If so, would it be a better design to build in a small compression circuit in the blue box on the line going to the cmos flip flop? Would it help the tracking?

[Mark Hammer]

The compression should make the level more consistent, however note that tracking would depend on the quality of compression.  For instance, an Orange Squeezer, with its fast attack and decay, might not provide as much of an improvement in tracking as a Dynacomp or CS-2 with their comparatively longer recovery times.  Note as well that compression and absolute level are two different things.  One could easily set the output of the compressor to a level that was consistently just below tracking threshold and resulted in poor tracking.

In the case of the Blue Box, there is considerable gain applied to the signal in order to: a) generate a rectified envelope signal to operate the transistor gates, and b) make the 4013 flip and flop.  Given the amount of gain applied and the clipping that results, I don't know how much of an added improvement a compressor would make to that circuit.  It would depend on the guitar itself.  For instance, if your primary axe was a semi-acoustic with a wooden floating bridge, I think a compressor might be critical.  If it is a solid body with a meaty output and lots of bottom and sustain, the compressor might be superfluous.

What may be more critical to your tracking is adjusting the gating action and the gain in the stage that feeds the flip-flop.  I written about this before, so I'll simply diect you to the search function rather than write it all out again.

[PenPen]

Understood. I'm glad you responded, I searched and read your threads on this issue. What I took away from it (and this could be incorrect) was that a compressor keeps the harmonic levels closer to the fundamental note, keeping them from taking over the flip flop circuit for a moment and resulting in the warbling effect. With low notes, the harmonics peak in between the intervals and cause the divided harmonic octave to be very pronounced. Is this incorrect?

[Mark Hammer]

Partly.  The harmonics will rarely have the amplitude that the fundamental does.  You can see this on just about any scope picture that shows the distribution of frequency content for a waveform of this type or that.

The flip-flop itself is triggered on rising edges that exceed a given amplitude..  Suppose we have a distorted waveform that, on a scope, looks like a profile of a molar tooth.  That is, there are obvious rising and falling edges to the cycle, and it is vaguely square-ish, but at the "top" and "bottom" there are some "wiggles".  The wiggles are harmonic components.  None would trigger the flip-flop of their own accord, but if the overall amplitude of the fundamental is insufficient, I could see the addition of the harmonic having some impact on the likelihood of triggering.

Let's use the analogy of money in your pocket.  There is paper money, and there are coins.  Here in Canada, we have $2 coins and $1 coins.  It can happen that when you empty your pocket, that you have far more money (in total) in coins than you have in bills.  If I ask you "We're going to the movies, you wanna come?", and you're response is to check your pocket for bills, you might underestimate how rich you are at the moment if those bills are a pair of $5 bills.  Ten minutes after I say "Well, I'll tell you about it tomorrow", you might dig in your pockets and realize there is an additional $12.43 in there, such that the pocket change actually makes movie attendance feasible.  So your judgment of your wealth, based on bills was wrong.  If you reach in your pocket and there is a $100 and a $50 bill in there, then whatever the coins add up to is moot, isn't it?  You will say, on the basis of the bills "I can afford it".

Similarly, if the fundamental level is modest, the harmonics added to that might be sufficient to trigger the flip-flop (Hoorah, my input signal is rich enough to divide down!!) or perhaps not.  In a sense, the "judgment" of the flip-flop is unreliable.  If the fundamental is robust enough, then the variations of the harmonics added to that become irrelevant to the general outcome.  As long as what the flip-flop sees is supra-threshold, it will fire.

As the note decays, however, while the harmonics have long since disappeared from the scene, the fundamental starts to reach the point where it might be subthreshold.  A compressor can maintain it supra-threshold for a longer period, hence result in consistent triggering of the flip-flop for longer.

Or would it?

In any side-chain driven device (noise gate, compressor, limiter, envelope filter, etc.), you can observe "envelope ripple" if the rectification is imperfect.  Devices that use half-wave rectification (the OS and Dr. Q are prime examples; all the rectification is a product of one diode and one cap) can produce subtle but measurable changes in whatever is being controlled during the decay phase.  So, with an imperfect compressor, as the note decays, there is a kind of "wobble" superimposed on the level of the note as it dies out.  Depending on the rate and amplitude of that wobble/wiggle/ripple it can be heard/perceived/described as distortion by some as a kind of trill or even a Scottish burr.

Now, perturbations in amplitude level are not the exclusive crime of cheap compressors.  They can occur with older strings, and in other circumstances too.  The point is that such subtle compressor-produced ripples, if added to whatever ripple is inherent in the plucked string, can position the signal level to be supra-threshold for this instant and subthreshold the next.  In other words, although compressors introduce a consistency to the signal level at the macro-level, they can introduce inconsistency to the signal at the micro-level, particularly during fadeout.  Bottom line is that compression is not a perfect panacea to poor tracking.

What MXR did was to use a built-in gate such that once the input signal declined below a certain level, the circuit was not even going to consider feeding that signal to the output.  In a sense it avoids mistracking (or reduces the likelihood of hearing it) by bailing out before it gets to the point where mistracking might occur.  This is why I suggest that people monkey around with the envelope detection/following portion of the circuit such that it optimizes the gating action to suit the qualities of their picking style and instrument.  In that sense it is no different than toying with the envelope follower section on a Mutron or Nurse Quacky.

Make sense?

[PenPen]

That was a fantastic explanation, Mark. I bow to your wisdom.

Yes, that makes perfect sense now. I'm curious how some of the more expensive boutique octave pedals achieve their better tracking then, I assumed it was due to added a compression circuit on the line going to the flip flop. But now I see this could be wrong.

For example, DOD produced the Buzz Box that was a distortion + octave. Everyone claims its simply a Blue Box + Grunge distortion, however a friend that has one and uses it solely for the octave portion swears its 10x better than a stock Blue Box in tracking. I assumed DOD had done the compression trick, but now I'm wondering if they used a completely different method?

Anyway, you answered the primary question I had very well, the rest of this is just chatter. Thanks again.

[Mark Hammer]

Well keep in mind that fuzz IS compression.  If I goose the level up 300x such that everything will be the level permitted by a diode's forward voltage or an op-amp's voltage swing, then I will have effectively "clamped" the signal level at some point.  This is what the Blue Box does.

Look at the Tonepad diagram for reference purposes.  In stage 1, gain is set by R8 and R5 for a gain of 471.  If that wasn't bad enough, in stage 2 that is further amplified by the gain set by R6 and R23 (x101).  By my calculations, that is a cumulative gain of 47,571.  Hot enough for you?  You will note that even though there is nary a diode to be seen (outside of the rectifier section), the Blue Box makes a damn fine fuzz.  If that doesn't result in "compression" out of a 9v battery-powered device I don't know what will.

Having said that, the PAiA Rocktave still has one of the best records for tracking as far as I'm concerned.  So why am I badmouthing compressors one minute, saying they are easily replaced the next, and touting their virtues the next?  I suspect it is because: a) there are compressors and there are compressors (the 570 does a better job than simpler devices), b) fuzz yields consistent level but it also adds harmonic content (by definition), c) high-gain clipping can add noise and you also don't want the flip-flop responding to amplified hum, and d) the 570 would appear to simply alter dynamic change, somewhat independent of absolute input level (though I may be wrong about that).  In any event , although a high-gain clipping circuit will accomplish many of the same conceptual tasks as a compressor, the audio path is likely to be better behaved with the 570 than with the high-gain clipper.  As well, what the Ropcktave adds over both a divider with a high-gain clipper on board or a divider with a compressor up front, is that the expandor half is linked to the compressor portion such that the octave division will always fade out before the compressor decides to let up.  It is the yoking of expandor to compressor that does the trick in this instance, not just the presence of a compressor.

[Paul Perry (Frostwave)]

If you want to help the blue Box wihtout actually modifyig it, I'd put a low pass filter (ideally, a graphic set as a bass boost / treble cut pattern) then the compressor, then the blue box.
Because, what you want to emphasise is the fundamental.
In an ideal world, the low pass filter would be INSIDE the compressor loop. But, we do what we can...

[Mark Hammer]

The simplest way to do that internally to the Blue Box is to stick a 47pf cap in parallel with R6, the 1M feedback resistor in the second stage.  That's not the greatest LPF in the world (6db/octave from about 3.4khz onward, or with 100pf a rolloff above around 1.6khz), but it's a start.  Note that filtering the signal at that point will not only impact on the harmonic content seen by the flip-flip but also the tone of the fuzz signal that forms the alternate channel.  You may find that's too big a sacrifice.

[trevize]

maybe a little off topic, or non technical:
I had a good tracking  with BB only with a guitar with humbucker at the neck and tone rolled off.
It was very good from the B on the 5th string.

[Processaurus]

A good article on octave ups, that inlcudes a schematic for a 3 pole low pass filter that helps a good amount with octave downs:
http://www.geocities.com/sunsetstrip/studio/2987/octave.html

It noticably improves the blue box's tracking, and does a better job of wacking the some of the guitars overtones and high frequencies than rolling your tone knob down (which does work), or the simple one pole filter made of the cap to ground on the input to the first opamp stage in the BB.  If you want the same tone from the fuzz that you had without the filter, it wouldn't be much work to duplicate the input section with the two opamps

Mark, even though clipping is the crudest form of compression, it should be noted that below the clipping threshold, the waveform is unaltered from the input, except it is louder.  So during the loudest part of the note, its possible that the harmonic overtones will be louder than the the fundamental will be later on in the notes decay.  With a compressor (or in this case we would want a hard limiter, with a fast attack and release) the waveform would hopefully have the fundamental be a more constant level (to be extracted by the comparator section of the BB) throughout more of the envelope of the note.  After limiting, tailoring the clipping threshold like you were suggesting would work great, to try and get just the very tops of the limited wave form (hopefully at the fundamental frequency) to trip the comparator.

For the mod free approach, if you don't mind having a bunch of pedals to do one sound, Pauls suggestion of the graphic eq followed by compression is going to give you about the best tracking the BB design has to offer.  The eq is good because if you are playng in one area of the neck, you can dial those fundamental frequencies in to make 'em stick, even the low E.  As a side note the Dyna/ross type compressors act like a limiter, ie. they try to keep the output at a constant level no matter what.

[choklitlove]

i like the "warbling".

[PenPen]

All of this discussion is gold. Prior to asking this question, I never had a need for an octave pedal myself. Well, it turns out that there is now a need for it, the band I just started has been declared to remain a two peice, with the two of us alternating the vocals, guitar and drumming. Now, the problem is thicking up the sound with only one guitar. So, today at practice we brainstormed on ways, and one solution I thought of was the classic split the guitar output then run it into an octave down and into a bass amp. So it looks like I'm going to be seriously expermenting with designing an octave pedal that can accurately track. I'm thinking of starting with the Blue box, but adding in a small and simple compression/limiter and low pass filter on the line going to the flip flop.

Thank you to everyone who replied with suggestions, I'm going to take to work on a design that incorporates some of these theories.

[trevize]

lower the tuning of your guitar and split the signal with an octave up effect!
or use just a baritone guitar