Use of Quality Sound in Cinema

Dolby

In 1976, Dolby Stereo revolutionised the film industry by placing multichannel stereo sound in movie theatres creating a more involving experience for the audience.  In addition to left and right screen channels, theatres also use a centre channel to sharpen the perspective of on-screen sounds, plus a surround channel played over loudspeakers along the sides and rear of the auditorium to immerse the viewer in ambience and special effects.  This technology was then extended to bring multichannel soundtracks to viewers at home, and was called Dolby Surround.
Dolby Surround / Pro Logic
Dolby Surround delivers four-channel surround sound via regular stereo VHS video tapes, TV and FM broadcasts, laser discs, DVD-Video discs, DVD-ROM discs, video games and CDs.   Four channels (left, centre, right, and surround) are encoded into two-channel soundtracks, then recovered on playback by means of a Dolby Surround Pro Logic decoder.   Dolby Surround is a two-step encode/decode process involving both recording and playback.  When a Dolby Surround soundtrack is produced, four channels of audio information, left, centre, right, and surround, are encoded onto two audio tracks using equipment manufactured by Dolby Laboratories. These two tracks are then carried on stereo program sources such as video tapes and TV broadcasts into your home, where they can be processed by a Dolby Surround decoder to recreate the surround sound experience. 
There are two kinds of decoders: basic Dolby Surround and Dolby Surround Pro Logic.  Both recover the surround information from encoded program material and feed it to a pair of surround speakers placed up on the side walls adjacent to the listening area.   In a basic Dolby Surround system, left and right front speakers are fed with the entire program in normal stereo without any processing, while the surround speakers are fed with a surround signal derived by a relatively simple passive matrix decoder.  As with regular two-channel stereo, the left and right front speakers create a "phantom" centre channel but only for those relatively few viewers seated on centre.  By using the same directional enhancement system found in professional Dolby Stereo cinema processors, Pro Logic decoders derive a separate centre channel to keep dialogue and other central sounds firmly localised on the video screen.
Pro Logic also supplies higher separation among all four channels and more accurate sound positioning, which along with the centre channel enable a greatly expanded listening area.  Dolby Surround Pro Logic is the best way to accurately reproduce the Dolby Stereo theatre experience in your home and will ensure that you hear Dolby Surround soundtracks as the producer intended.  The basic Dolby Surround Decoder features passive matrix decoding to derive the surround channel and three output channels, left, right and surround.  Surround channel frequency response is limited to 7kHz plus modified Dolby B-type noise reduction.  Surround channel time is variable from 15ms to 30ms.   Dolby Surround Pro Logic Decoders feature in addition high-separation, active matrix decoding for enhanced directionality, four output channels: left, centre, right and surround, automatic input balance control,  test noise sequencer and level adjustments to balance all four channels.  The centre channel mode control gives Normal, Phantom and optional Wide modes.
Dolby Digital / AC-3
Dolby Digital delivers six totally separate (discrete) channels of sound.  Like Dolby Surround Pro Logic, it includes left, centre and right channels across the front of the room.  Dolby Surround Pro Logic provides a single limited bandwidth (100Hz to 7,000Hz) surround channel which is typically played back in the home through two channels of amplification and two speakers.  In comparison, Dolby Digital provides separate (discrete) left surround and right surround channels for more precise localisation of sounds and a more convincing, realistic ambience.  With Dolby Digital, all five main channels are full range (3Hz to 20,000Hz).  A subwoofer could be added to each channel if desired.
The sixth channel, the Low Frequency Effects Channel, will, at times, contain additional bass information to maximise the impact of scenes such as explosions, crashes, etc.  Because this channel has only a limited frequency response (3Hz to 120Hz), it is sometimes referred to as the ".1" channel.  When added to the five full range channels, the Dolby Digital system is sometimes referred to as having "5.1" channels.  All six channels in a Dolby Digital system are digital, which means that they can be transferred without loss from the producer's mixing console to your home playback system.  The Dolby Digital System packs all six channels into less space than a single channel on a compact disc.  This advanced approach to handling the digital audio data is what makes it possible for Dolby Digital soundtracks to be added to DVD discs, and to a whole host of other sources.  Dolby Digital has been used to bringing you Dolby Digital movies for several years, and now, due to the advent of Dolby Digital DVD discs, you are able to enjoy this unique performance in your own home.
Dolby Digital can deliver 5.1 channel sound from DVD Video Discs, DVD ROM discs, laser discs, digital TV broadcasts, satellite transmissions and cable systems.   It differs from Dolby Surround by providing two separate surround channels for greater realism and an additional ".1" or "LFE" channel for Low-Frequency (bass) special sound Effects.  The LFE channel is sometimes incorrectly identified as the "subwoofer" channel. While a subwoofer can be helpful in reproducing low bass in general, it is not necessary to have one if one or more of the speakers in the playback system have extended bass response.  Without a subwoofer, the LFE channel's bass effects can be directed to those speakers with extended bass.  Furthermore, not all multichannel soundtracks have the ".1" channel as it is used primarily for explosions and rumbles on movie soundtracks, and is not present on all programs.