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42. Codecs I Have Known, Part 2: AC-2 aka DolbyFax   8/1/13 by Dave Immer

The DolbyFax was one of the original codecs in the group of systems to appear on the scene back when ISDN was the hot new technology. Dolby stopped building them in the late 90’s but the community of users remains quite stable and dedicated. Systems are now sold and serviced by EDnet.

Employing the proprietary Dolby AC-2 algorithm, it is the preferred system of theatrical film post production studios, in use everyday for ADR. The DolbyFax may be the easiest system to deal with because users seldom, if ever, change settings and only 1 phone number needs to be entered into the dialing field. It was never packaged in a single chassis like the others and consists of a separate encoder, decoder and ISDN module connected by data cables.

AC-2, like MPEG, is a perceptive coding algorithm. It was designed to operate at 256kbs for full bandwidth stereo audio so it requires 2 BRI circuits (4 “lines”.) The DP503 encoder also includes the AC-3 algorithm that never really gained wide acceptance as AC-2 became a de facto standard. Also included is MPEG layer II, but the terminal adapter won’t play nice with Musicam or Telos so the DolbyFax remains an island among ISDN codecs.

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