On 09/12/2013 10:38 PM, Thor Lancelot Simon wrote:
The audio subsystem actually posed *two* obvious opportunities:
amplifier noise from channels with high final stage gain but connected
by a mixer to muted inputs, and clock skew between system timers and
audio sample clocks. The former requires a lot of interaction with
specific audio hardware at a low level, and with a million different
wirings of input to mixer to ADC, it looks hard (though surely not
impossible) to quickly code up anything generally useful. The latter
would be easier, and it has the advantage you can do it
opportunistically any time the audio subsystem is doing anything
*else*, without even touching the actual sample data. Unfortunately,
both of them burn power like the pumps at Fukushima, which makes them
poorly suited for the small systems with few other sources of entropy
which were one of my major targets for this. So they are still sitting
on some back back back burner. Someday, perhaps... Thor
There are a class of hyper-cheap USB audio dongles with very
uncomplicated mixer models. A small flotilla of those might get you
some fault-tolerance.
My main thought on such things relates to servers, where power
consumption isn't really much of an issue. Similarly these hyper-cheap
($10.00)
DVB-T dongles based on the RTL2832U can be made to run in "SDR" mode,
and give you a basebanded sample stream of a wide variety of tuned
RF frequencies--put a terminator on the input, chose your frequency,
crank up the gain, and pull samples until you're bored....
This topic has suddenly become interesting to me in my work life, so I'm
currently looking at the sensors API for Android. I thought I had left
Android work
behind, but it's coming back to haunt me. I was playing with the
sensor outputs on a Nexus tablet today, and it has an impressive array
of sensors.
I suspect each of them could contribute a few bits/second of entropy
without too much trouble. More investigation is necessary.
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