Alesis M1 S URROUND S

Speaker Installation

26 M1 ACTIVE REFERENCE MANUAL

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.

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