Tom Henry Tunable Noise Source

Haible Juergen Juergen.Haible at
Wed Nov 17 11:35:56 CET 1999

	>> (3) How is the pattern (distribution ?) of a Sample and Hold
changed when
	>>       you're sampling a white-noise-FMed VCO instead of sampling
	>>       directly ?
	>I can answer this one for certain.
	>White noise has a gaussian (bell shaped)
	>probability distribution relative to amplitude.
	>The further from zero volts the lower the
	>probability of getting a sample.
	>A triangle of sawtooth wave sampled from
	>a VCO modulated by white noise has and
	>equal probability distribution. All voltage levels
	>have equal probability of being sampled.

Hi Grant, thanks for your answer!

This is what I was *hoping* for: a means
to get equally distributed random samples.
Now if I'm sampling a fixed frequency triangle oscillator
at a fixed sampling rate, I get equal distribution of
discrete levels, but they form a regular pattern in time,
depending on the VCO freq and sampling rate.
This is obvious for LFOs (the famous OB-8
sampled LFO waveform), but it even happens with
very high VCO frequencies. (I guess the sampling
folds any high VCO freq. partly into the baseband.)
Now with white noise modulating the VCO freq one
can destroy the regular pattern quite easily.
But there's still questions remaining (for option (3)):
What amount of FM with noise will be enough to get
no audible repetition in the S&H pattern ? And will
the equal level distribution of a sampled triangle VCO 
be in any way disturbed by the noise-FM ?
What I *fear* is that if you get away from regular patterns
you might also loose the equal distribution. But it's
a very nonlinear operation, so I don't know the answer.


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