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Surround Sound - a Brief History

The first commercially successful multichannel sound formats were developed in the 1950s for motion pictures. At the time, stereophonic sound was a dazzling new concept to the general public and the film industry, concerned by the increasing competition from television, promoted it heavily. From the outset, film stereo featured several channels across the front plus at least one channel played over a speaker towards the rear of the theater. This was known then as the 'effects' channel.

As movie sound developed over the next several years, Dolby Laboratories developed their revolutionary matrixing technique for encoding the multi-channel audio needed for large movie theaters. Four channel of information - left, center, right and surround - were contained in just two optical stripes on the film, aided by a sophisticated new steering technique, and quickly became the de facto standard for motion picture sound reproduction.

In 1958, stereo sound made it into the home. Home stereo used just two channels (compared to four to seven in movie theaters), since that was all the LP phonograph record could accomodate. Two-channel stereo radio broadcasting began on FM in the early 60s. Excluding a poorly-conceived, badly-executed and deservedly short-lived trial with so-called 'Quadraphonic' sound in the early 70s, stereo audio in the home remained two channels. It was the video revolution in the 1980s that changed that. The rebirth of the stagnant movie industry was largely made possible by the advent of the video cassette and the ability for consumers to playback movies in their homes, and soon, the TV set became a 'video monitor'. 'HiFi' technlogy provided good-quality stereo sound for video cassettes, and television began broadcasting in stereo, as well.

In late 1982, Dolby Surround was introduced for the home playback of video cassettes of theatrical films produced with Dolby encoded soundtracks, keeping intact the original four channels of sound. In the beginning, Dolby Surround decoded merely made it possible to decode the surround channel at home, but soon the more sophisticated Dolby Pro-Logic also decoded the center channel and utilized the advanced steering circuitry, as well.

As usual, it was for the motion picture industry that Dolby began the application of digital audio to 35mm film soundtracks, with a separate Dolby Digital optical track placed between the sprocket holes. This technique debuted in theaters in 1992, and allowed the presentation of sound in the 5.1 configuration advocated by industry study groups - left, center, right, left surround, right surround, and a discrete sixth channel for low frequency effects (this LFE channel is referred to as a ".1" channel because it requires just one-tenth the bandwidth as the other five). The encoding method was originally known as AC-3, but now is simply referred to as Dolby Digital.

Now, the consumer equivalent of Dolby Digital film sound forms the final link from multichannel program producer to home listener. Unlike analog Dolby Surround with its single band-limited surround channel (usually played over two speakers), Dolby Digital features two completely independent surround channels, each offering the same full-range fidelity as the three front channels, greatly expanding the depth, localization and overall realism of home theater sound. Consumer electronics manufacturers have developed receivers, amplifiers and other components which deliver amazingly high quality Dolby Digital sound reproduction, and are becoming the heart and soul of today's home entertainment systems.

Surround Sound Formats - The Basic Differences

Dolby Pro Logic is the simplest, cheapest format. It has 4 channels that are compressed into two analog channels, so you can record it on your stereo VCR and it can also broadcasted even over analog radio or cable TV. Another advantage is that Dolby Pro Logic can be easily downconverted from a Dolby Digital signal.

These are 2 discrete and 2 matrixed channels: 2 discrete, full-bandwidth channels (left front, right front), 1 matrixed full-bandwidth center channel, 1 matrixed, limited-width channel that goes to 2 surround speakers.

The disadvantages are both surround speakers produce the same sound from the same channel (mono sound) and are limited in bandwidth. Additionally, there is no discrete subwoofer channel specified in the standard, so subwoofer channel can extracted from other channels rather than specified explicitly.

Dolby Digital is a 6-channel format, referred to as 5.1-channel. All 6 channels are discrete, 5 channels are full-bandwidth, while .1 refers to subwoofer, which supplies Low Frequency Effects (LFE).

To compress data with low loss in the sound quality, audio coding, known as "perceptual coding" is used. Dolby Digital uses a third generation audio coding algorithm (AC-3).

Advantages over Dolby Pro Logic include the fact that all channels are digital, discrete and 5 of them are full-bandwidth. Additionally, the surround speakers use separate left and right channels (stereo).

DTS is also a 6-channel format (5.1-channel). The main difference from Dolby Digital is that DTS uses less compression thus providing marginally better sound. DTS is less widespread than Dolby Digital.

THX Surround EX takes the Dolby Digital signal and creates another additional matrixed channel, which is then sent to one or two surround speakers, thus giving a 6.1 or 7.1 sound. Sources at the moment are specially encoded THX Surround EX DVDs, regular Dolby Digital sources with the use of a THX Surround EX decoder.