Birds, bats, and beats

[Mark Hammer]

About 35 years ago, I'm walking across campus when suddenly something zips by in front of me.  By the way it moved, I knew it was alive, so it wasn't a leaf or piece of paper, but it didn't seem to move like a butterfly or bird.  It was....different.  A split second later, one of the magpies found all over the region also zipped by me, at chest height.  I followed where it went and it disappeared into a small bush, where there seemed to be a scuffle.  I walked over to see, and learned that the thing that had flown past me moments earlier was indeed NOT a bird or butterfly.  It was a bat that the magpie had spooked out of its resting/sleeping place, chased down, caught, killed, and was in the process of eating.  I mentioned it to one of the ornithologists in the biology department, and he was elated.  Apparently, there had been many sightings of magpies eating dead animals, but nobody ever knew whether the birds had simply stumbled on a fresh corpse or had created the corpse themselves.  My anecdote was written up and reported as confirmation that, yes, magpies can and will hunt sometimes.

So, sometimes, when you have the feeling that something is different from the normal, your feeling may be right.  And the difference may be something you never ever considered before.

This morning, I was soldering away, listening to a Tangerine Dream concert (great soldering music, BTW; easy to concentrate to), and there was a pulsing sound in one of the synths.  It was sort of like tremolo, but not really.  I tried to concentrate hard and dissect why it sounded ever so slightly different from tremolo, and I realized that it was not the result of an LFO modulating the volume directly.  Rather, it was an envelope generator -presumably attack/release type - being repetitively triggered.  There was a consistent pattern and level to the pulsing sound, but what was very different was that each pulse had a perceptible decay.  More importantly, there was a small brief space between the decay and the onset of the next pulse.  In other words, it wasn't the sound of one continuous event being amplitude-modulated.  It was the sound of many "events" following each other.  And it was the spacing that did it.

There are plenty of devices and ways of achieving "nonstandard" modulation waveforms (i.e., something other than simple square and triangle).  Our own Tom Wiltshire (AKA electricdruid) makes a number of such devices using PICs, and they're excellent and elegant solutions.  Where these differ from what I heard and described, however, is that an LFO, no matter how exquisite, begins the next cycle as soon as the previous one is complete.  So, let us say that a digital pattern generator of some type, using a wavetable, was able to perfectly mimic the envelope/transient applied to the synth sound I was listening to, the attack of note N+1 would begin one clock cycle after the decay of note N had ended.  The spacing between successive notes could certainly be altered by the speed of such an LFO.  But the attack and decay times would always be yoked to the rate.  That is, one could not separate the space between notes/events, from the "feel" of each note, created by its envelope.

In analog modular synthesis, this is comparatively easy to do.  You feel the VCO (or other signal source) to a VCA, and you control the VCA with an envelope/transient generator that is triggered by an LFO pulse.  Easy peasy, right?  And cumbersome.  If you're Radiohead, I suppose you can do it, but we'll leave that universe alone.  In our universe, it would likely need to be something small and digital, that could permit dialing up an "event-onset rate" separate from the other individual event parameters.

I've been waiting for a relatively simple Arduino project to get into, and I think this is it.  I realize there is a separate sub-forum for digital stuff and may move there.  But for now, I view it as a challenge in modulation, so I started the topic here.

Comments?

[GibsonGM]

I'm interested, Mark. I'd like to find some things I can work that little bugger into.  For a long time, all I've really used it for is cycling the fan near my wood stove at night, with a relay! 

[diffeq]

I'm interested as well, as I haven't had the chance to tackle MCUs yet.

Some synths have a re-triggerable ADSR, where the envelope just cycles endlessly. I had a chance to play with such some time ago and as you mentioned, the biggest problem with it was that changing envelope shape also changed the modulation speed. For instance, shortening attack would speed it up greatly, making usable tremolo-like effect range of ADSR parameters very narrow.
Your approach solves it completely, as triggering signal is completely independent from the envelope generation. And since it's done in software, adding features like pulse triplets, tap tempo and external control will be easier than in any analog solutions. 
While tweaking ADSR modulation on a synth, I wished there was a more modern way of envelope generation. Perhaps a graphical display with touch input that allows "drawing" of the envelope and applying functions like randomising, smoothing, exponentiating, re-trigger points and so on. In short, adding more programmability and precision. After, that is what digital excels in.

[ElectricDruid]

I think you should definitely have a go at this, Mark. You're full of ideas for interesting modulation sources, and producing them digitally will let you experiment with stuff that (as you said) would be cumbersome to do in analog and take up half a rack of modular synth.

BTW, glad to help in any way if required. I don't know Arduino specifically, but digital mod sources are definitely something I've learned about.

[antonis]

About 35 years ago,

Say no more, Mark... 

[anotherjim]

Synths can have enough mod range to cause apparent pauses in either envelope or LFO sweeps. Easier to get the situation where a filter or whatever is fully open or closed before the extremes of the sweeps are reached. Funnily enough, in guitar fx, designers tend to scale control so there is less chance of any dead action.