[sdiy] Digital Noise, FYI

[Scott Gravenhorst]

"David G. Dixon" <dixon at interchange.ubc.ca> wrote:
>> If you replace that CPLD *thing* with an analog one FET based S/H
>> and one NPN based noise source you end up with a cheaper and
>> simpler and better sounding result.
>
>Possibly, but it depends on finding a transistor with a significant amount
>of thermal noise (they don't all have it).  Also, you might find that you
>are picking up local radio stations on the unconnected collector pin (yes,
>even if you cut it off), which you can hear quite clearly when you amplify
>the noise signal a million times or so.  While this might be cool in its own
>right, it probably isn't what you want from a noise source.

And these are some of the principal reasons for using a digital noise source.  Digital
noise source output is consistent across units and requires no special selection of parts.

And I disagree with both "better sounding" and "cheaper" from Karl D.  Yes, a short LFSR
makes noise that sucks audibly.  However long length LFSRs (for me that's 64 bits) run at
decent sample rates (for me, that's 100 kHz to 1.0 MHz) are, IMO at least as good audio
quality.  For designers using CPU or FPGA, the "expense" comes in terms of instruction
cycles or slice count and for such a component this can be rather small.

No, I am not disparaging analog NPN transistor noise.  It has it's place as does digital
noise.


>With the digital noise source, you have a couple of different options for
>how to harvest the noise.  You can feed some or all of the random word to a
>DAC, which gives "square noise" which you can then filter to make less
>square if desired, or you can feed the random output bit to an integrator
>(with an input capacitor to prevent the integrator from hitting the rails).
>You can even do both simultaneously.

Or the raw words taken from the shift register can be used directly in DSP computations.

It just so happens that I am currently working on a synth design that depends heavily on
digital noise...

-- ScottG
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-- Scott Gravenhorst
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