14. Can Ambisonics reproduce Dolby Surround AC-3?
Description
This article is from the Ambisonic Surround Sound FAQ, by Martin Leese with numerous contributions by others.
14. Can Ambisonics reproduce Dolby Surround AC-3?
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.
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