Last-modified: 21 January 1998
Copyright: © 1994, 1995, 1996, 1998 by Martin J Leese
Distribution is unlimited
Ambisonics cannot contribute to the Dolby Surround AC-3 encoding or decoding processes; it can make contributions before the 5.1 discrete channels are encoded and after they are decoded.
Dolby Surround AC-3 is described in technical publications available on the Dolby Laboratories Inc Web page. These suggest that:
AC-3 is a digital encoding technique that exploits "audio masking" to achieve high bit-rate reductions. AC-3 can be used to encode between 1 and 5.1 audio channels. Dolby Stereo Digital film sound format uses AC-3 to encode 5.1 audio channels onto film stock. The 5 channels, left, centre, right, right surround and left surround, are all full bandwidth. The .1 channel is a band limited (20 Hz to 120 Hz) bass effects channel. Dolby Surround AC-3, also called Dolby Surround Digital, is the consumer equivalent of Dolby Stereo Digital film sound and is also based on AC-3 coding of 5.1 channels.
Dolby Surround AC-3 does not use matrixing and the 5.1 audio channels have complete separation. Sadly, this is not sufficient for realistic surround sound reproduction, the problem being the "pair-wise" mixing style. Dolby Surround AC-3 is just a delivery mechanism and is not tied to pair-wise mixing, however, to date all Dolby Surround AC-3 movie sound tracks have been mixed using the pair-wise mixing style.
Dolby Surround AC-3 was designed to enhance the enjoyment of motion pictures. The limitations of pair-wise mixing are not a serious impediment to this, however, they do make pair-wise mixed Dolby Surround AC-3 unsuitable for music.
One solution is for sound engineers to use a mixing style other than pair-wise mixing to mix the Dolby Surround AC-3 format. Happily, an alternative exists - Ambisonics.
Another poorer solution is for the 5.1 pair-wise mixed channels to be converted into W, X, Y and additional signals, and to then use Ambisonic technology to reproduce the sound field. This is described in the Gerzon 1992b reference.
Pair-wise mixed Dolby Surround AC-3 is only impressive; Ambisonics is accurate and can be impressive or subtle as required.