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The channels are 'read' (decoded) by the theatre sound processor and reproduced by left, centre and right screen speakers and an array of surround speakers at the rear and sides of the auditorium. In addition to the usual left and right audio channels, a centre channel sharpens sound position and dialogue clarity, and a rear 'surround' channel immerses you in ambience and special effects.

To fit these four channels into the film's two audio tracks, a special computer program, the Dolby Stereo encoder is used. During playback in the theatre, a Dolby Stereo cinema processor decodes (turns the digital information into an electrical signal) and recovers the four channels exactly recreating the sound images crafted by the filmmakers.

Playing these back in regular stereo lets you hear the entire soundtrack but the exciting sound 'envelop' effect is completely lost. Dolby Surround systems unlock the full potential of movie soundtracks.

The original generation of decoders, using a simple design, play the surround channel through additional rear speakers. The vital front-to-back dimension is then restored, giving a complete cinema experience at home. Advanced generation surround decoders with Pro Logic technology use sophisticated signal processing and an additional speaker for the centre channel.

While the results are similar for both generations of Dolby Surround decoders, the extra power of Pro Logic sharpens sound positioning for a wider listening area. The Pro Logic encoding and decoding process ensures that noise levels (extraneous sounds not intended by the movie makers) are lowered when the audio signal is small or not present.

Coupled with this, strong audio signals are used to cover (mask) noise at other signal levels, even when audio signals are present only in some parts of the sound spectrum, Dolby noise reduction reduces noise in the other parts of the spectrum, rendering noise imperceptible.

Dolby Surround technology may be found in amplifiers, receivers, separate decoders, and televisions from around forty manufacturers. Besides the many films available on video, increasing numbers of sports, dramas, talk shows, and even commercials are being produced for television in Dolby Surround.

Additional features of Pro Logic are simplicity of system set-up and operation. A centre channel speaker with Pro Logic decoding places sound with pinpoint accuracy for everyone in the viewing area.

A built-in test signal allows all speaker levels to be quickly and accurately adjusted for good sound balance. Several interpretations of the technology offer automatic circuits to continuously balance the source material for best decoding.

Pro Logic can also accept several speaker configurations, making it possible to get the highest performance in any system: left and right speakers create stereophonic effects, surround channel speakers add spatial depth and increase viewer involvement, the centre speaker gives precise sound positioning over a large viewing area.

Adding a hi-fi video recorder and extra amplifier/loudspeaker channels to your stereo system will make it possible to reproduce the original Dolby multidimensional soundfield. Ideally, most of the time you should not be acutely aware of the surround channel ... but would very much notice its absence.

In addition, Dolby Surround cannot be used to make up for inherent deficiencies in the soundtrack elements or the mixing process itself. Indeed, home video and television systems equipped with Dolby Surround clearly reveal such deficiencies, just as good stereo music systems reveal the inadequacies of inferior music recordings.

 

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