Ask A Scientist© Computer Science Archive

Index Key:   COM034
Author:      dan
Subject:     Modem operation.
What in a modem changes a computer signal to a phone signal?

Response #:  1 of 1
Author:      gregory r bradburn
As you know, a telephone carries analog signals and a computer uses digital
signals.  In order to send the computer signal over the telephone lines it
must be converted to an analog signal.  This is done by your computer modem. 
The modem creates a high frequency carrier wave that is running continuous-
ly.  It uses the digital signal from the computer to "modulate" the carrier
wave.  The digital signal is a series of 0's and 1's.  When the modem gets a
0 from the computer it sends out a very weak (or maybe no) signal to the
telephone line (at the carrier signal frequency).  When the modem gets a 1
from the computer it sends out a strong carrier signal to the telephone
line.  The modem at the other end listens for the carrier frequency.  When
it is strong, it sends a 1 to its computer and when the carrier signal is
weak it sends its computer a 0.  Of course, for this to work, the carrier
frequency must be higher than the fastest rate you will send signals over
the telephone line.  In addition, there is a need to keep the two modems
"synchronized" so that both the sending and receiving modems know when a
change from a 0 to a 1 (or vice versa) might occur.  While I am pretty sure
this is the process that was used for modems up to around 4800 baud I am not
sure that it is used for the faster rates achieved today where data compres-
sion algorithms are used, etc.