New entropy source proposal.

Chuck Swiger cswiger at
Mon Mar 7 23:07:37 GMT 2005

Ivan Voras wrote:
> Pawel Jakub Dawidek wrote:
>> Are you sure you used /dev/dspW? It give me really good results with my
>> sound card.
> Sound card noise REALLY depends on the type and age of the said
> hardware. In a good soundcard (traditionally, Creative Sound Blaster or
> whatever they're calling it now), the noise is fairly minimal to
> nonexistant (I'm saying this purely as as a user/listener, not RNG
> expert), while on mass-market hardware such as various AC97 cards,
> especially if older or embedded into notebooks, the noise is very apparent.

Should the RNG be looking at the sound card's output, or at the sound card's 
microphone input?  I think Pawel was talking about the latter, since a mike 
input is very sensitive to ambient EM and ought to generate some noise in the 
lower bits even if the mike input isn't connected to anything.

Of course, if the sound hardware mutes that input to all zeros, or if there is 
no mike input at all, then we're not going to obtain any useful entropy. 
Hopefully, the driver can guard against that case by checking for a repeated 
stream of all-zeros, which presumably would indicate no HW/muting.

[ BTW, thanks, Pawel, this looks like a neat idea, even if it may not be 
useable for all machines... ]

> It's also very annoying that DA hardware picks up various inteferences
> from surrounding buses - on many low end system moving a mouse, resizing
> windows or accessing disk drives will produce bursts of audiable noise.

Try using digital sound output via coax or TOSLINK (fibre & red laser); the 
inside of a computer is a horribly noisy place to do D/A conversion, and 
nowadays most home receivers can accept SPDIF input and give you a better sound.

The Creative Audigy and later have reasonably good digital audio and adequate 
DACs, but earlier Creative cards had miserable DACs and analog audio 
circuitry.  And yes, Creative cards sound pretty good compared to most 
on-motherboard audio, which is what laptops have.  Really good CD and DVD 
players have seperate power supplies for the motor+digital versus analog 
circuitry in order to avoid biasing the audio being generated by their DACs.


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