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Name: Dan
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1995


Question:
What in a modem changes a computer signal to a phone signal?



Replies:
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.

G Bradburn


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