[sdiy] oscillator jitter / phase noise

[Richard Wentk]

At 08:09 03/06/2004 +0200, Czech Martin wrote:
> > > For the phase noise I could imagine current source noise,
> > > integrator noise and perhaps most important comparator
> > > noise. Also noise on the power supply comming from other
> > > modules could be considered.
> > >
> > > Anyway, has this ever been investigated?
>
>Where? Most work I see concentrates on narrow
>band rf oscillator phase noise,
>because this is a problem for nearly all on chip plls.
>So far I have seen nothing interesting for audio frequency
>oscillators of the kind we use in synth gear.
>
> > It's the bandwidth resolution which is your limit. You want
> > to zoom in like
> > hell on a single overtone to view the sideband spectra.
>
>Well, I thought that the interesting noise must be low frequency,
>in order to create those phasing patterns. i.e. 1Hz and below.
>Noise in higher frequency (1kHz-20kHz) would make the signal sound
>noisy (hissing), but avering out the instantenious frequency.

What about the band between 1Hz and 1kHz? If there were enough of a 
deviation I can imagine this could lead to a 'natural' chorus effect.

I've tried adding various kinds of random modulation to digital 
oscillators, but as yet nothing has done much to transform the sound. I 
suspect it's really more to do with waveshape and waveshape variability.

While it would be very tedious to do properly, I don't see any reason why 
you couldn't build a digital simulation by multisampling an analogue 
oscillator at various pitches and pulsewidths.

Actually just recording a static tone would be interesting, to see if the 
waveshape changes at all, even when it's supposed to be fixed.

>Using the square output of the oscillator (given that the comparator
>is good enough, else use the original saw or tri)
>it should be possible to make such statistics
>for every cycle. This time domain analysis has also the advantage
>that no computer memory constraints apply, only hard disk space.
>
>With 48kHz sampling rate the time resolution apears to be rather poor:
>20.8 us. But is this really the case? Could the real resolution
>not be a little bit better due to interpolation techniques?
>
>Since most people do not have access to old gear, audio sampling
>would be also interesting. It is easier to obtain a DAT tape
>of a pure VCO recording then to get hold on the machine.

I suspect digital recording will lose any analogue jitter effects.

It's possible, at a stretch, to imagine that any noise added by analogue 
jitter would act as a kind of natural dither/noise shaping. Adding high 
quality noise shaping to a digital system makes a blindingly obvious 
difference in smoothing out the sound, making it warmer and fuller - 
exactly the qualities usually ascribed to analogue.

Richard