In article . com,
Radium wrote:
I'm curious to why there are no purely-analog devices which can
record, store, and playback electric audio signals [AC currents at
least 20 Hz but no more than 20,000 Hz] without having moving parts.
Most of those voice recorders that use chips [i.e. solid-state] are
digital. Analog voice recorders, OTOH, use cassettes [an example of
"moving parts"].
The fact that it's an AC (inherently-varying) signal being recorded,
means that *something* has to move... if only some amount of
electrical charge. If the electrons don't move, the output can't vary
and all you have is a DC voltage.
And, in fact, this concept of moving electrical charges is the basis
for one type of analog signal storage and playback device which has no
moving (mechanical) parts... the CCD, or Charge Coupled Device. It
consists of a large number of charge storage devices (typically MOSFET
transistors with dielectrically-isolated gates) hooked up as a sort of
shift register or "bucket brigade". Each gate stores a charge which
is proportional to the input signal present at a given moment in time.
Several thousand times per second, a clock pulse causes each storage
cell to generate an output voltage proportional to the charge in its
storage gate, and then to "capture" onto its gate the signal being
presented by the previous gate in the chain.
In effect, the signal is propagated down the chain at a rate
proportional to the clock rate.
Why aren't these devices used more than they are? They're not very
efficient, and they're noisy. Every time the charge is copied from
one cell to the next, a bit of imprecision (noise) creeps in... so the
fidelity isn't great. And, because the device has to be able to hold
a very wide range of charges (since the charge is directly
proportional to the signal level) the storage gates have to be fairly
large.
The net result is that an audio CCD is capable of storing a
decent-quality signal for only a few tens or hundreds of milliseconds,
from input to output.
Another sort of a purely analog signal-storage device, with no moving
parts other than the electrons which convey the signal, is a simple
length of transmission line (with perhaps some amplifiers mid-way).
Put a signal in at one end, get the same signal back out the other end
some number of microseconds or milliseconds later.
Once again, they're not terribly efficient and are prone to be noisy.
For storage of large amounts of information, in a small space, with
high fidelity, using digital storage techniques is much more
efficient - largely because each storage cell must only store 2
different information states (0 and 1) rather than a large number of
possible levels.
--
Dave Platt AE6EO
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