Modem - Modulator and Demodulator

A modem (modulator and demodulator) is a device that modulates a carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or “bps”.

Various standards of a modem

v.32

Normally the phone system sends a small amount of the outgoing signal, called sidetone, back to the earphone, in order to give the user some feedback that their voice is indeed being sent. However this same signal can confuse the modem, is the signal it is “hearing” from the remote modem, or its own signal being sent back to itself?

Echo cancellation was a way around this problem. By using the sidetone’s well-known timing, a slight delay, it was possible for the modem to tell if the received signal was from itself or the remote modem. As soon as this happened the modems were able to send at “full speed” in both directions at the same time, leading to the development of the 9600 bps v.32 standard.

v.34

It is also known as 28,800 bps v.34 standard. Eventually, the manufacturers were forced to use more “flexible” parts, generally a DSP and micro controller, as opposed to purpose-designed “modem chips”.

v.90

Also known as 56k standard, systems use in-band signaling for command data, inserting one bit of command data per byte of signal.