Sustain is determined by...

[turdadactyl]

Another Pedals 101 question:

From a circuitry/voltage/current perspective, what do we want to look for when trying to increase/decrease sustain in a pedal that's not a compressor (e.g. the sustain control of the Big Muff)?

Electrosmash says:

The 100K sustain potentiometer controls the level of signal going into the clipping blocks. If the level is high, the signal will be more clipped and the distortion effect will hold even if the guitar input signal is not strong. The resistor R23 prevents the signal from coming from the Input Booster from being cut-off when the sustain pot is at lowest setting.

So stating my question another way, why is greater clipping leading to greater sustain?

[R.G.]

why is greater clipping leading to greater sustain?

"Sustain" is a tricky thing. To humans, the simple idea is that a note rings for a long time before fading out. How long it sounds is "sustain".

There are three ways to get sustain.
1. Make the original note source ring longer. That's a great idea, but it's not practical for guitars, because the strings ring for as long as they do. How long that is depends on how much of the string energy is sapped out of the string by the damping of the nut, bridge, fretting pressure, dirt on the string, surrounding air, etc.
2. Continuously adjust the gain during a note so that the first, loudest part is "turned down" and turn up the gain as the note loudness fades down, so there's a longer period of the note seeming about the same loudness. This is what compressors do. They compress the loud parts down and the quiet parts up.
3. Amplify the $@*@_= out of the signal so the soft, quiet parts get amplified up to louder, then clip off the amplified signal at a certain level. The clipping limits the apparently loudness of the loudest parts, and clips off EVERYTHING to the same level until the signal decays so much that it falls below the clipping level.

This last is the answer to your question. "More clipping" doesn't make more sustain, but greater amplification makes there be some signal for longer, and the clipping makes the amplified signal be about the same loudness for longer. The human mind interprets the "about the same for longer" as more sustain. Notice that it's the relative amount of clipping compared to the amplified signal level that makes this, not just the clipping level.

[turdadactyl]

RG, you're always a huge help.  Much appreciated.

This seems like it must be a simplistic follow up, but how are we managing to amplify the crap out of the signal, to the point of clipping, without introducing significant distortion?

[Transmogrifox]

...how are we managing to amplify the crap out of the signal, to the point of clipping, without introducing significant distortion?

You aren't.  In the BMP you ARE introducing significant distortion.

Sustain in the guitar effect sense is dynamic range compression -- whether you do it "cleanly" with a dynamic range compressor, or do it "dirty" by clipping, in the end the dynamic range is compressed.

It's either "clean" compression, distortion or both.  You can certainly design BJT gain stages to dynamically adjust their gain (this kind of thing is seen in AM receiver circuits where the transistor bias current is dynamically adjusted by rectified signal level), but the BMP doesn't do this.  You can overdrive a traditional AGC type of circuit so you get both distortion AND clean dynamic range compression effect on the lower level signals, but it's tantamount to feeding a compressor into an overdrive circuit.

Other kinds of sustain that RG did not explicitly state are of the feedback type, but generally fit under the category of

1. Make the original note source ring longer.

This is like either the eBow, Fernandez Sustainer System, or just placing your guitar headstock on a cranked amp.  These are the more "clean" ways to get sustain.

[Mark Hammer]

Guitar signals are dynamic.  There is a sharp transient when the string is initially plucked or strummed, and things quickly settle down after the first few 1/10s of a second.  How quickly harmonic content from the string disappears and leaves the string vibrating only at the fundamental is going to be a function of the design of the guitar.  A big hollow body with a floating bridge is going to "turn off" the harmonics post-pluck faster than a Telecaster will.

I mention this because that initial peak is what normally prompts clipping, since it is often considerably greater amplitude than what the string will display only brief moments later.  Much of what we try to do as contemporary guitarists is to prolong both the level and the harmonic content by ramping up gain, post-pluck, so that the signal continues to provoke some clipping from the rest of the signal chain.

But here's the thing: the harmonic content that is elicited from the rest of the signal chain is a function of the frequency content it receives.  So if the upper harmonics have pretty well disappeared after 300msec, then what gets "generated" as the note lingers is multiples of lower-order harmonics.    This is going to be a less harsh-sounding tone.  Still brighter than what the string itself is providing, but not quite as harsh as if the level and harmonic content of the string was maintained for more than the initial transient.

What we often refer to as "sustain" is really an attempt to mimic a signal generator, or oscillator.  And how do oscillators work?  By continuing to feed energy into a loop.  How do guitars sustain?  By having their amplified sound vibrate the body of the instrument such that the vibrating string is "helped along".  That is also how sustainers work, albeit with a near-field magnetic continuation of the string's output through the pickup.

Gotta run catch a bus, so I'll leave it there.  If others want to jump in be my guest.

[R.G.]

Yep.

One ugly thing introduced with any sustain system other than the feedback systems is noise. All amplifying add noise. Huge amplifying to bring up tiny signals as the note tails off also amplifies any noise too. Happens in both compressors and amplify/clip systems.

Feedback systems have their own issues. The frequency response  of the feedback loop (pickup ->preamp->tone/volume->power amp-> speaker -> cabinet -> room -> strings -> pickup) can color the fed back signal, or feed back a harmonic or even sum/difference frequency that is noticeably different from the original signal. So can a pickup sustainer.

A feedback sustainer makes the string/pickup into the frequency determining element in an oscillator. Oscillators make more or less distortion and other artifacts unless their gain is adjusted to 1.0000000000... by some mechanism. In an amp/speaker feedback system, the gain controller is ....you.... - get the gain too low, the signal dies out, get it too high and something somewhere gets overdriven. Pickup sustainers have the same issue, but the gain control is taken away from you and put elsewhere.


And yes - what Mark said while I was maundering on about phase locked loops. 
The Feedbacker pedal(s) do things a different way. They phase lock an oscillator to a signal, then "freeze" the oscillator to that frequency. The original note is gone, but an oscillator tuned to it lives on.

Another way to do this is to use a DSP, and have it figure out what the period of the signal is, make a copy, then fade in the backup copy and play it overandoverandoverandover somehow.

[pappasmurfsharem]

Another way to do this is to use a DSP, and have it figure out what the period of the signal is, make a copy, then fade in the backup copy and play it overandoverandoverandover somehow.

http://www.ehx.com/products/freeze

:-)

Never played with one, but seems like it would be fun for a few moments