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Analog waveform circulating in a large coil (through feedback).
A coil of wire resembling a large electromagnet is mounted within a cage of thick, grounded metal plates to exclude virtually all RF interference. The power supply is carefully filtered as well. Digital data is fed into the control circuit and is immediately represented as a unique analog waveform which,
when sampled by same circuitry, is easily converted back into its original form. The waveform is fed into one end of the coil, and whatever exits the other end is sampled for the read-circuit as it passes it, then is amplified just enough to maintain uniform signal strength throughout the coil, and fed back in. The size of the coil, and the frequency at which the loop is completed along with the density of the D/A + A/D encoding determines the amount of data one may store. Advantage in comparison to ordinary RAM: dirt-cheap per bit of storage, possibly.
Delay line memories
A brief article on the subject [wiml, Jun 07 2001, last modified Oct 04 2004]
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||Delay-line memories were baked, baked, baked in the early days of computing. The classic system uses tanks of mercury with transducers at each end; you pump bits in (as sound waves) at one end, and a while later they come out the other end. (Why sound waves? Because they travel slower, so you can fit more bits in the same length.) You clean up the bits, possibly do some processing on them, and feed them back into the input end of the tank.
||One calculator manufacturer (calculating machines being desktop-sized at the time) managed to make their machines much more cheaply after discovering that if you use torsional vibrations, instead of pressure waves, you can store a lot of information in a coil of cheap iron baling wire, instead of an expensive tank of mercury.
||("A lot" being a relative term, of course.)
||Analog delay lines themselves, if
not recirculating delay line
memories per se, are still around.
Most VCRs have one, for example
(or at least they used to).
||The coil-of-wire design is
terrible -- you want to *avoid*
self-inductance between one part
of the delay line and another; a
coil is specifically configured to
*maximize* inductance. You might
do better with a very long coiled
fiber, though as [wiml] points
out, real world systems tend to
use ultrasound rather than
||I have, in the past, postulated
halfbaked delay line memories
||- "ping" packets bounced off hosts
or routers all over the Internet
||- modulated light or radio signals
reflected off distant space probes
or celestial objects