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To: Lost in New York who wrote (16606)3/25/1997 7:02:00 PM
From: Skipper   of 18024
 
David,

The ISP thingies really are modems. There are two different types of
A/D conversions in the connection you describe. The first is the
modulation/demodulation of a serial bit stream. What this means is
that the ones and zeroes of the serial bit stream are converted into
audio frequencies (modulation) and vice versa (demodulation). The
second is when the resulting analog signal is converted into a PCM
(pulse code modulated) digital stream.

The steps that take place on a 56K modem connection, starting at your
computer are:

1) A serial bit stream is output on your COM port to your modem.

2) The modem converts this data to audio frequencies.

3) The audio frequencies are carried on an anolog line to the telco.

4) The telco converts the analog signal to a digital PCM signal.

5) The PCM signal is sent on a PRI to a MAX 4000.

6) The Rockwell chip in the MAX 4000 converts the PCM signal into
audio frequencies (internally), and then into a serial bit stream
which is processed by the MAX. The chip is doing a D/A/D conversion
that used to be two separate steps.

7) On the return trip the MAX sends serial data to the Rockwell chip.

8) The Rockwell chip converts the serial data to PCM encoded audio
frequencies.

9) The PCM data travels to the telco, where it is converted to an
analog signal and sent to your modem (USR no doubt).

10) The modem converts the audio frequencies into a serial bit stream
and sends it to your COM port.

The modem does one D/A conversion while a codec (coder/decoder) does
the other (PCM). The new 56K chips now combine these two functions.

Skipper
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