[sdiy] jitter analysis

[Richard Wentk]

At 14:49 09/07/2004 +0200, Czech Martin wrote:

>We do not have to argue about the real limitation of the human ear.
>At some frequency all perception will be lost. My ears can go as far
>as 16-17 kHz, others may reach 22kHz. But not much more.

That's for steady tones though. The perception of transients is much more 
complicated.

You're also assuming that sampling at 44.1kHz will give you perfect 
reproduction. Of course it won't, so just from the point of view of common 
sense it's a good idea to sample at high a rate as possible.

>My assumption is that the noise component which is interesting here
>is very low in frequency, < 10Hz.
>After all, we do not percieve those waves as noisy, but drifting.

I suspect that's likely to be wrong. This isn't just random drift we're 
talking about. Random drift is extremely easy to simulate in digital 
synths, and while it makes the sound more interesting it certainly doesn't 
make it fatter. What you get instead is more of a chorus effect, which is 
rather different.

As an interesting digital synthesis experiment you can create (in something 
like Csound) a generic oscillator from a bank of harmonically tuned 
bandpass filters, each channel fed by uncorrelated noise, with very fine 
control of Q up to oscillation.

If you turn up the Q to oscillation the sound is indistinguishable from an 
arbitrary oscillator with the same spectrum. If you back off the Q a little 
the sound fattens up in an interesting way. That's analogous, if not 
exactly identical, to what I suspect is happening here.

What we actually perceive is fatness as a timbral quality. It's easily 
audible in a good single oscillator design.

I'm interested because I want to know how to model that sound digitally. 
The fact that the quality of fatness survives digital recording and 
reproduction proves that its absence isn't inherent to 'digitalness', so 
much as poor modelling and DSP design. If you can replay the bitstream that 
creates the effect, you can obviously create it from scratch digitally too.

>btw.: I expect that very old transistors have a lot of 1/f noise,
>and also shot noise. New transistors have improved a lot, simply because
>of general semiconductor advancements over the last 40 years or so.
>This would mean that replicas with new hardware are not the same!

This wouldn't surprise me in the least.

Richard