A fax demodulator is a device used to intercept fax messages by listening in on a

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...

or radio signal. A typical ( Group III) fax transmission requires a two-way conversation between two

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

s (that is, each participant must both transmit and receive). Each modem may be part of a “fax machine” incorporating an image scanner and a printer. A demodulator can only be used to observe a conversation between two fax modems, usually to record the image transmitted. It cannot be used to receive a fax transmission in the usual sense, because a sending modem cannot transmit an image without first negotiating a connection with a receiving modem.

References

Communication circuits Fax {{Telecomm-stub