[sdiy] Analogue Drift (was Re: HF VCOs and tracking problems)

[Andrew Simper]

> I did quite a bit of this VCO drift characterisation in my lunch hour at
> work a few years ago now.  Sadly don't have the actual results files now as
> my lab PC at work died, or I would have sent them to you.

Great work Richie, thanks for sharing the results in textual form.

This tallies with what I have observed by ear and FFT, but not
measured in such detail. I have a few extra little notes on the
subject as well.

I have noted that at lower pitches VCOs have more fuzz too them than
at high pitches, which I am guessing comes from FM by hiss at both the
control signal input to the expo converter, and noise at the output
which is integrated by the cap. This hiss is small and doesn't lead to
drift as such, just FM fuzz. Another possible source of this fuzz is a
small amount of noise on the comparator voltage, which will could lead
to a decrease in pitch with increased oscillator frequency. This kind
of thing is pretty minor and I am guessing usually cancelled out when
the synth is tuned using the scale and offset trims, speaking of
which...

In poly synths, in addition to all this FM, LFO, and LED modulation
going on I think the main "fat" part of the sound is again not
actually from drift but the basic fact that the scale and offset trims
of the expo converters all different for each voice. This basic
detuning keeps things sounding interesting as the voices get allocated
pretty randomly to pitch when chords are played, so each chord sounds
slightly different and alive.

The filter also contributes to the tone, and has the same kind of
power supply and hiss FM happening on it.

All of these sometimes subtle factors contribute in their own way to
keeping things sounding interesting. I hate the drunken sound of
adding drift wobble from filtered white noise, which is the typical
thing digital synths do to make them try and sound more analog - yuk!

Here is an audio example I did for a product I coded a while back
called Synth Squad where I exaggerated the contribution of hiss and
mains hum at various control and main signal path voltages, all pretty
basic stuff but it really helps to make the sound come alive a bit
more in digital synths:

http://www.vellocet.com/dsp/SynthSquad/strobe_mellow_noise_arp.mp3

To my ears this doesn't sound convincingly analog, it sounds more like
a stylised version of analog, cleaner and neater, but still with its
distinctive and useful character.

All the best,

Andy