BACK & Re: PCM=PWM?

[be053520]

-----Original Message-----
From: Rob <cyborg0 at GlobalEyes.net>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: zondag 27 december 1998 5:18
Subject: PCM=PWM?


First:

After 2 years I'm finally back,... Hi everybody, I'm very happy to be
back!!!!!!!!!!

Rob wrote:

>Is it true that pulse code modulation is the SAME as pulse width
>modulation?

No,

Normally, pulse code modulation is often used to generate a specific
data(communication) pattern (no synth-use :-)), for example in
telecommunications.
However some years ago the PCM-technology was used in synths.
I don't really know how it exactly works but I always thought that the PCM
was used to control an IC which outputs a sampled sound??

I thought that:

In a PCM signal the timebase is not divided in HZ, but rather in timeslots.
Each timeslot at his turn represents a number of bits.
So, when you create a PCM signal with  32 timeslots (channels) for example,
your synth can/will be 32-voice polyphonic.

a PCM signal can be:

1_  _     ___ _    _  _      ____
                                                           the period marked
with *
0  --  ----       -  ---  --  -----         ------  represents one timeslot
of 8 bits
  *************
      !!!!!!!
to create this same signal with PWM
you should have a square-wave wich starts with a '1', duty-cycle
(=pulse-width) is aproximatly 50% for the first 2 bits...OK, that couldn't
be hard to generate :-)
BUT NOW, the next 2 bits (marked with ! )  have as duty-cycle value 1/3th
and a larger period (slower frequency) as the previous 2 bits.

So while a PCM controller would say: the signal read is 10100111,
a PWM controller says 1;50%;1 33,3%;1 99%.

Bye

Tom Verheyden,