[Jerry Avins]

Dave Platt wrote:
Come on, Dave, a CCD is a digital device, subject to aliasing. The
charges represent the signal at a particular instant of its average over
a particular interval. (My CCD digital camera can take time exposures.)
A CCD's content may not be quantized in amount, but it is quantized in
time. In a camera, where the charges pertain to individual pixels, the
result is also quantized in space.

"Digital" and "subject to aliasing" are two different things.

As I believe the term "digital" is usually meant, it implies a
two-state (on/off) storage representation. It's not just that the
signal amplitude is quantized, but that the quantization uses a
power-of-two representation and storage system of some sort.

I can buy that, but it's not how I would have used the term. I call a
two-state representation "binary". A storage system that is clocked is
subject to most of the restrictions and permits most of the useful
techniques of digital signal processing. Early transversal filters used
op-amps, with the coefficients being set by the resistor values.

In that sense, an audio CCD uses a digital clocking structure to move
the charge along, but uses a non-digital system for representing the
signal level (a linear number of electrons). Yes, it's quantized in
time, and the electron charges themselves are quantized... but I don't
think that either of these qualifies it as "digital".

I agree to use your term for the sake of this discussion.

"Analog" is a very fuzzy and imprecise term, and I think that a CCD
can reasonably be called an analog system.

Yet I'd lay a bet that you call the pictures made by means of a CCD
image sensor "digital".

Even audio cassette tape is quantized in both time and amplitude, at
the level of the individual magnetic domains in the oxide or metal
particles.

Oh, sure. In that case, the crystal radio that I built in the 40s was
digital too. The electrons came down the antenna one at a time even if
closely spaced. We need to draw a line somewhere, and I don't like the
idea of calling a flashlight a digital photonic device.

There are in principle purely analog storage devices. A loop of analog
delay line with a repeater in it qualifies. A memory based on that
principle was used to store digital signals in an early computer, even
though the device itself is analog. It used an acoustic delay in a
column of mercury.

Jerry
--
Engineering is the art of making what you want from things you can get.
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