Re: What does PCM mean to you?

Jerry, this is an interesting topic indeed. It first caught my attention
some years back when someone made an argument in which they implied that
a particular signal was not just digitized, but was PCM, as if it were
some kind of special encoding; and I had to ask myself, is it not just
plain digital data (and what they heck they mean my PCM).

Anyway, I don't know the exact origins, and it's easy to come up with a
half dozens plausible explanations that *could* be right, but I'll just
make a couple of observations:

Modulate means "to change"--so it's pretty easy to make a lot of cases
for change in some manner. If you have digital data in some form (
decimal floating point values in a spreadsheet), and move it to another
form (fixed point values in binary in computer memory), you have a
modulation of some sort.

Also, the sampling process itself is a modulation. One model of sampling
is to take an analog signal (we usually bandlimit it first), and
modulate it by multiplying it by a unit pulse train. This gives us a
modulated pulse train. We then store the height of each pulse by
encoding it as a numerical value (the "code" part?). The result is our
PCM representation of the analog signal. You could say that the data is
our string of pulse codes, from the pulse modulation; the pulse codes
can be converted back to pulses and run through a low pass anti-imaging
filter for analog reconstruction later.

Again, this is something I made up--I don't know if this is the true
origin of the term, but it seems reasonable.


In <ObGdnZw7CNMbQ9_eRVn-ig@xxxxxxx> Jerry Avins wrote:
> Steve Pope wrote:
>> I'm not sure I understand the question. Any analog signal
>> passed into an A/D creates a PCM version of that analog
>> signal, and the A/D can be more than one bit wide.
>
> That's just a pulse code. There's no modulation.
>
> Jerry
.

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