> Consider taking the difference of the two stereo channels, if
> available. Doing so will reduce common mode signal, including mains
> hum.
It doesn't matter in the slightest how much signal there is in the noise.
What matters is how much entropy there is. Our experiments showed a
variance (cheap 16bit sound card, microphone input with nothing plugged
in, full record volume) of around 8-11, or over three bits per sample.
We're basically looking for quantum shot noise in the input transistors,
it doesn't make an appreciable difference if you short-circuit the input.
Differencing the two channels would only reduce the available entropy, which
holds true until the variance falls below 1 bit per sample - I don't know
any 16 bit cards with this property though.
We just use the bottom bit, but could use the bottom three (on this card),
and could for example have used the parity over the entire 16 bit sample.
Again, having signal in your noise doesn't hurt any as long as it doesn't
swamp the noise.
> IMO Rather than adding lots of random entry points, why not just
> add a RAND_pipe() or RAND_exec() which accepts random data from
> a seperate program?
Possible but requires more XP code and probably more elapsed time.
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