After I had posted my idea, I realized that there would be a possibility of ring-oscillator/ring-oscillator interactions if the delays of the individual inverters were of identical technology.  (invertor delay).  I thought of an idea to vary the size of the transistors (and/or capacitive loading) in the invertors such that the shortest-loop oscillator inverters were smaller, having perhaps 1-2% less delay, while the longest-loop oscillator inverters had a 4-5% greater delay, and the two intermediate-loop oscillators had 0-1% greater and 2-3% greater delays.  I think this would tend to prevent inadvertent synchronization between these four ring-oscillators.  Naturally, this would have to be tested, or at least characterized by the manufacturer.
Another, belt-and-suspenders, approach would be to add a long-period LFSR to the above circuitry (48-64 bits, say) and XOR the ring-oscillator outputs with themselves, as well as with that LFSR.  If the resulting signal had some sort of pattern, it would be of extraordinarily-long pattern.
            Jim Bell

From: Lodewijk andré de la porte <l@odewijk.nl>
To: James A. Donald <jamesd@echeque.com>
Cc: "cypherpunks@cpunks.org" <cypherpunks@cpunks.org>
Sent: Monday, October 21, 2013 3:43 PM
Subject: Re: Curious RNG stalemate [was: use of cpunks]

2013/10/18 James A. Donald <jamesd@echeque.com>
You can, however, be sure a microphone input is a reliable source of entropy, since fake entropy would interfere with its microphone function.

This is a syntatic non sequitur. Why would fake entropy interfere with a microphone's function? How is the microphone guaranteed to have "its microphone function"? Is a microphone input just the microphone's jack or an actual soundwave-modulated-magnetic-power-factor?

In either case it's also a semantic non sequitor. If someone plays a darn loud sine wave in the serverroom you can be sure the microphone will replicate it. It'd be doable to make any microphone always output it's maximum value, through a plenty of means. The sad thing is that it's sound, so it might even be doable at distance!

(scenario: people breaking into a running-but-physically-controlled server through manipulation of it's random numbers)

I think an internal radioactive source such as a smoke alarm makes great sense. Be wary to isolate it very well to prevent outside interference. If it just goes to MAXINT if someone holds his cube of madam curie next to the server's case it'd be a shame.

@Jim Bell: wouldn't such a ring oscillator aggregate be subject to patterns? If you have something that can create more out of fewer pieces of randomness, isn't there plenty bad-randomness-sources to go on?