[sdiy] Analysis of frequency variation in analogue synths

[Colin f]

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom 
> Wiltshire
> Sent: 03 May 2007 10:41

> For myself, before I can think about the next step of whether a  
> certain type of error or drift is associated reliably with 'the  
> analogue sound' or the 'moog sound' or 'arp sound' or whatever, I  
> need to see some measurable effect, which I haven't managed to do  
> reliably.

Here's a simple test you can do.
Take an analogue synth, and record several seconds of bare oscillator sound
into an audio app of your choice, through a half decent audio interface.
Now take a single cycle of that waveform, and create a new file by pasting
the single cycle repeatedly until you end up with the same length of sample
as the original recording.
Set up a blind listening test, where you alternate between the original
sample, and the loop of a single cycle.
Make sure you match the levels, and make sure you monitor the original wave
through the same converters as you recorded it - not live from the desk.

If you want to try this test with my sample file, there's a 256k mp3 of it
here:
http://www.colinfraser.com/misc/mini-saw-alternate.mp3
1 minute or each wave, with a short gap between. Play it in a loop, turn
down the volume, hide the audio app window so you can't see which point
you're at in the file, then go away for a random period of time.
When you return, you'll have no preconception of which wave you're listening
to, so it should be an accurate test of whether you can detect the
analogueness of the real VCO.

In isolation, can you tell the two waves apart ?
If you can, how long do you need to listen to the waves to decide which is
analogue and which is 'digital' ?
I can tell this sawtooth on my relatively unstable RA minimoog VCO from the
single cycle loop, but it takes me several seconds of careful listening to
decide. And I have little doubt that just a hint of random modulation of
pitch and amplitude of the digital wave would completely fool me, without
having to go into greatly detailed analysis of the original VCO behaviour to
derive the 'right' modulation.

The important question for me is, "does a single oscillator minimoog patch
sound 'analogue' ?".
To which the answer is a resounding yes.
But if I can't hear any extra 'analogueness' in the raw oscillator compared
to a single-cycle sample of it for the typical duration of a note played on
the synth, surely that means other factors have a much more important
influence on analogueness than oscillator jitter ?

Clearly things will be different with a two oscillator sound.
A worthwhile test there would be to see if you can pick out a pair of real
analogue oscillators in unison, against a pair of samples in unison where
the sampled oscillators have small amounts of random modulation.

Cheers,
Colin f