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A fax demodulator is a device used to intercept fax messages by listening in on a telephone line[1] or radio signal.[2]

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.


  1. ^ Polishuk, Paul (May 11, 2007). Fiber Optics Weekly Update; Vol 27 No. 19. Information Gatekeepers. p. 3. ISSN 1051-189X.
  2. ^ Rains, Patricia (2006). Mexico Boating Guide. Mexico Boating Guide. p. 7. ISBN 0-9638470-5-8.