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Suround Sound
CHAPTER 3
SunnouruD Souuo
About surround sound
Sunound sound is the term used for several different systems which use multiple
channels of ptayback in the listening room to recreate a truly three-dimensional
sound experience. The practice of recording music-only in surround sound is a fairly
recent innovation. surround sound for video, howevet has been around for over ten
years. With modem encoding methods, multichannel audio may be delivered to
consurners on standard video tape, video discs, and even through broadcast, with the
proper encoders (devices whidr take multiple charurels of audio and combine them
into two charurels) and ilecoders (used at the receiving md to split the signal back into
rnultiple charurels).
Systems are in use which use three (eft, right, and surround), five (left, center, right,
left surround, right surround, often supplemented by a sixth subwoofer charureli
and more speakers. The original Dolby Sunound@ of the mid-eighties was a
matrixed threechannel-only system. It had left, right and a mono surround (rear)
track
^which
was usually playid through two su"rJund speakers. In 1982 D;by'pro
Logic@, a four-charmel matrix system, was introduced. Fro Logic added a ded.icated
center channel to which most dialog was assigned. The rear surround channel was
strll in mono. h 1996, Dolby Digital@ (Ac-3) and DTS, f'lly discrete six-charurel
audiodelivery plaffomrs with separate left-rear and right-rear channels first became
available to consumers on laserdisc and, more recently, on DVD (Digital versatile
Disc).
With so many listeners hearing music through their multimedia systems, it is
important for mglneers to produce music on surround systems, even if they,re not
working for film or video applications. Powered monitors like the Alesis M1 Active
Biamplified Reference Monitor have proven to be a simple, cost-effective way to add
five'channel surround to control rooms.
Center speakers in rnusic mixes
In video applications, with left, center and right channels all set to deliver equal
output, the ear tends to hear dialog as coming only from the center charurel. (It's a
psychoacoustic effect because au right and all left channel signals are fed to the
cmter.)
In Dolby Pro Logic, the center charurel information
is derived from sum and
difference information
encoded in the left and right charurels. The very attributes that
make this matrix-type
system viable for video application make it lesjdesirable
for
music-only reproduction.
In practice, there is a hard center dialog channel but there
is also a sort of phantom mono channel developed betwem the left and center
channel and the right and center channel. This between-channel
phantom mono
image exists because so much of the information
contained in each channel 0eft anc
center or right and center) is identical.
Hence, mono. In the few music-only
recordings encoded in Dolby Pro Logc
the derived cmter channel could lead to
some drastic changes in spatial perception.
Itr' ACTT'E REFERETCE MAIIUAL
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