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26 M1 ACTIVE REFERENCE MANUAL
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HAPTER
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URROUND
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OUND
About surround sound
Surround sound is the term used for several different systems which use multiple
channels of playback 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, however, has been around for over ten
years. With modern encoding methods, multichannel audio may be delivered to
consumers on standard video tape, video discs, and even through broadcast, with the
proper encoders (devices which take multiple channels of audio and combine them
into two channels) and decoders (used at the receiving end to split the signal back into
multiple channels).
Systems are in use which use three (left, right, and surround), five (left, center, right,
left surround, right surround, often supplemented by a sixth subwoofer channel),
and more speakers. The original Dolby Surround of the mid-eighties was a
matrixed three-channel-only system. It had left, right and a mono surround (rear)
track, which was usually played through two surround speakers. In 1987, Dolby Pro
Logic, a four-channel matrix system, was introduced. Pro Logic added a dedicated
center channel to which most dialog was assigned. The rear surround channel was
still in mono. In 1996, Dolby Digital (AC-3) and DTS, fully discrete six-channel
audio delivery platforms 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 engineers 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 music 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 channel. (It’s a
psychoacoustic effect because all right and all left channel signals are fed to the
center.)
In Dolby Pro Logic, the center channel information is derived from sum and
difference information encoded in the left and right channels. The very attributes that
make this matrix-type system viable for video application make it less desirable for
music-only reproduction. In practice, there is a hard center dialog channel but there
is also a sort of phantom mono channel developed between 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 (left and