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

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