Audio formats for the General Public
"In its literal sense, "Surround" means "enveloping" the listener in sound.
Dolby Surround is an analogue procedure for encoding and reproduction allowing for the addition of a supplementary channel in a stereo recording. This track is used to power a rear channel consisting of two so-called "Surround" speakers, both delivering the same signal. The Surround signal is thus "hidden", encoded in the left and right channels and is completely transparent during the reproduction of the stereo recording on a normal stereo amplifier connected to a pair of speakers.
On the other hand, if you possess a Dolby Surround amplifier and four speakers (two main right and left speakers and two rear speakers – right and left), you’re able to decode the Dolby signal and will have a different, supplementary sound ambience between the front and the back of the room.
The Dolby Surround signal encoded in the stereo recording is not a signal of hi-fi quality. In fact, it is mono, and in addition, its bandwidth is quite limited within the sound spectrum. While the front right and left channels reproduce the entirety of the sound spectrum ranging from lows of 20Hz to highs of 20,000 Hz, the Surround channel only reproduces the sound spectrum from 2,000 to 7,000 Hz. This difference is not very harmful, from the point of view that 95% of the action of a film takes place in front, rather than behind you. The rear speakers are used as support, but are never very important, in order not to distract the viewer.
The Surround channel is thus used to reproduce the action in a film going on behind the spectator: a plane flying over, jungle atmosphere with birds squawking, music… in brief anything that is able to help you plunge even deeper into the sound atmosphere and ambience, as wished by the director of the film.
We thus find the mention "Dolby Surround" or its logo on videocassettes, video games, Laserdiscs, DVD’s, as well as on the opening/closing credits of programmes or films broadcast in NICAM on television or in stereo via cable or satellite. This mention indicates that the programme possesses a Surround channel, which can be used if one has the right equipment. "