Last-modified: 21 January 1998
Copyright: © 1994, 1995, 1996, 1998 by Martin J Leese
Distribution is unlimited
Dolby MP encoding, and Dolby Surround and Pro Logic decoding, are all described in the excellent article, Introduction to [Dolby] surround sound written by Bob Niland. The article suggests that:
Dolby Motion Picture matrix encoding (Dolby MP) is an encoding system designed for motion picture sound tracks. Four channels are encoded - left, centre, right and a mono surround channel. Dolby Stereo is the two-channel result of this encoding. Dolby Surround is a decoding process designed to decode Dolby Stereo in the living room. Pro Logic is an active decoding process, also designed for the living room. Lucasfilm Home THX is an enhancement of Pro Logic.
A Dolby MP encoded source is intended to be reproduced through a Dolby Surround or Pro Logic decoder. These assume rear speakers arranged only to create a reverberant sound field (typically, bipolar speakers facing the walls), and the use of a time delay to ensure that front sound effects are not localised to the rear.
Ambisonics is not limited to creating a reverberant rear sound field, and requires a different arrangement of speakers. Also, with Ambisonics all speakers cooperate to localise sounds, so the front-rear time delay is unnecessary (and would be detrimental).
Dolby Surround and Pro Logic decoders suffer from non-existent sound imaging to the rear and side of the listener. While not a serious impediment to the enjoyment of motion pictures, these limitations do make them unsuitable for music.
Of course, there is nothing to prevent you reproducing Dolby MP encoded material through an Ambisonic UHJ or Super Stereo decoder. Many people prefer this, and describe the seamless coherent sound field which results as "superb". But it will not be what the sound engineer who created the recording intended you to hear.