Today new multichannel digital technology is poised to foster even closer industry co-operation, leading to a more consistent approach to home sound reproduction. It has the potential to ease the burden on both consumer and producer, while providing unparalleled fidelity not just to the tonality of live sound, but also its spatiality.
The first commercially successful multichannel sound formats were developed in the early 1950s for the cinema. At the time, stereophonic sound, a concept new to the public, was heavily promoted along with new wide-screen formats by a film industry feeling threatened by the rapid growth of television. Unlike the two-channel format later adopted for home stereo, film stereo sound started out with, and continues to use, a minimum of four channels.
With such stereo film formats as four-track CinemaScope (35 mm) and six-track Todd-AO (70 mm), multiple sound channels were recorded on stripes of magnetic material applied to each release print. To play these prints, projectors were fitted with playback heads like those on a tape recorder, and cinemas equipped with additional amplifiers and speaker systems.
From the outset, film stereo formats featured at least one channel played over speakers towards the rear of the cinema. At first this was known as the effects channel, and as its name implies, it was reserved for the occasional dramatic effect—ethereal voices in religious epics, for example. In fact, some formats even switched this channel off when it wasn't needed (by means of trigger tones), because the track on the film was particularly narrow, and thus very hissy.
Although film stereo lost favor in the 1960s and early 1970s due to high costs of the magnetic formats and a slump in the film business, sound mixers continued to experiment with the effects channel. Formats such as six-track 70 mm magnetic provided consistent signal-to-noise ratios on all channels, so mixers could use the effects channel to envelop the audience in continuous low-level ambient sounds. The effects channel came to convey greater sonic realism overall, not just the occasional dramatic effect.
This expanded, more naturalistic practice came to be known as surround sound, and the effects channel as the surround channel. The extra speakers at the rear—and now along the sides of the theare as well to create a more diffuse soundfield—came to be known within the industry as "the surrounds."
To enable compatible playback in mono theaters, it was necessary to fit the new stereo soundtrack into the same space on the print occupied by the traditional mono track. Experiments showed that two channels, treated with Dolby A-type noise reduction, could give excellent fidelity. But trying to squeeze in more than two channels raised noise to an unacceptable level, even with noise reduction.
Two channels, however, are not enough for movie stereo. For one thing, movie screens are so wide that a separate center channel and speaker are required to localize dialogue for viewers seated off-center. For another, "stereo" and "surround" had become synonymous to most of the film industry. To ensure mono compatibility, high audio performance, and acceptance by the industry, some way had to be found to encode just two physical tracks on movie prints with four channels of information.
The method developed has its roots in the matrixing techniques first tried for quad home stereo. However, the playback channels are configured in the cinema stereo norm—left, center, right, and surround—and the Dolby Stereo process is itself far more sophisticated than the earlier home systems.
Stereo optical proved so practical that today there are more than 21,000 Dolby Stereo theaters worldwide, and almost every major title is released in the format, for a total of more than 5,000 films. What's more, the film industry experienced a kind of renaissance in the 1980s, helped by a new commitment to high-quality presentations exemplified by the adoption of Dolby Stereo. That rebirth was also helped by the "video revolution."
The Dolby Stereo Digital track provides five full-range channels for left, center and right screen speakers, plus separate left surround and right surround speaker arrays in a configuration known as stereo surround. A sixth, bass-only track for subwoofers gives rise to the term "5.1" channels. Less than two years after the new format's introduction, more than 50 feature films had been released in the new format, and nearly 600 cinemas in 27 countries had been equipped for playback of the digital track.
Bell Laboratories' famous early experiments with stereo sound in the 1930s used three channels. Cinema stereo in the 1950s was using no less than four channels, and as many as seven. Yet when stereo finally made it into the home in 1958, only two channels were used. This was not because of listener preference, or some predisposition on the part of audio professionals. Rather, two channels (left and right) were all that the then-prevalent LP phonograph record could accommodate.
Two-channel stereo music reproduction was such a dramatic advance over mono, and so comparatively easy to implement, that it soon became the norm, even as filmmakers continued to regard four channels as the minimum necessary to create a lifelike soundfield. A few years later two-channel stereo FM broadcasting was introduced, further entrenching two-channel sound in the consumer's mind as "stereo." Indeed, as time went on, only dedicated film buffs were aware that film stereo, rare as it had become, continued to use more than two channels.
As home stereo grew in popularity, equipment manufacturers began to look for ways to expand their market. This was one motivation behind the ill-fated four-channel, or quadraphonic ("quad"), home stereo format promoted in the early 1970s. It required two additional speakers in the rear corners of the listening room, to reproduce two extra channels from specially encoded program sources.
Because the existing home stereo media had only two channels and could not easily accommodate more, several schemes were developed to encode the basic two channels with additional sound information. Most were based on matrixing techniques which enabled "hiding" extra channels within the two regular channels by recording them with different relative phase.
The failure of quad had several causes. Different record companies and stereo equipment manufacturers backed different, incompatible encoding/decoding systems, creating consumer confusion. Producers and recording engineers couldn't decide on how to best utilize the extra channels. To top it all off, most consumers perceived little advantage from the format. As a result, they rejected the whole idea by not buying the new equipment.
Quad was never associated with the multichannel stereo formats used in movie theaters, and the term "surround sound" still wasn't heard outside the film industry. After all, the only home visual medium at the time was broadcast TV, which provided distinctly low-fidelity mono sound. For the next decade home stereo, movie stereo, and TV sound would remain separate entities supported by essentially isolated industries.
In 1958 when the stereo LP was introduced, high-fidelity sound was itself a rarity. By the 1980s, high-performance stereo music systems were the norm. Audio as well as video had expanded to include new program sources (audio cassettes, CDs). Car stereo was coming of age, and personal headphone portables were providing a new way to listen to music. A generation had grown up listening to rock music that was often as dependent upon audio technology as musician ship the same generation that was now being regularly exposed to multichannel surrou6nd sound in the cinema. Against this background, Dolby Surround was introduced late in 1982 for playing videos of Dolby Stereo theatrical films.
The original four-channel encoding of Dolby Stereo film soundtracks remains intact when they are transferred to stereo video cassettes and Laser discs, or broadcast on stereo TV. Dolby Surround made it possible to decode the surround channel at home. Soon Pro Logic made it possible to decode the center channel as well, and to take advantage of the advanced steering circuitry developed originally for theatrical playback.
Unlike quad, Dolby Surround has gained considerable marketplace acceptance. For one thing, the channel configuration and how to best utilize it were firmly established within one industry in advance of its introduction to another. For another, it was developed with a clear objective, specifically to enhance the viewing experience. And third, software and hardware standards for both the film and consumer electronics industries are defined by one organization (Dolby Laboratories) independent of, and having credibility with, both. As a result, more than 12 million consumers have perceived Dolby Surround sound as well worth the investment.
Today Dolby Surround has gone further, to include television broadcasting. Not only Dolby Stereo films, but also more than 20 regular TV series in the US are transmitted in Dolby Surround, as are an increasing number of specials and sports events. Athough Dolby Surround was developed as a sound-with-picture format, record companies such as RCA Victor/BMG Classics and Concord Jazz are releasing music-only CDs and audio cassettes encoded with Dolby Surround. Surround sound has crossed the traditional format boundaries, to the advantage of not only the consumer, but also of the film, prerecorded software, and consumer electronics industries.
The first video cassette recorders were introduced in the early 1970s. They were promoted initially as time shifting devices to record TV broadcasts for viewing at more convenient times.
Before long, the video cassette took on another, hugely popular function: —the playback at home of theatrical films. This created a voracious appetite for program material which the film industry was more than happy to satisfy. A whole new business grew up around the production, sales, and rental of prerecorded video cassettes.
While the increased penetration of broadcast television had contributed to the film industry's woes in the 1950s and 1960s, the video cassette did quite the opposite. Movie attendance actually increased with the growth of the prerecorded video cassette. Films that proved popular in theatrical exhibition proved just as popular for home viewing, and in some instances, films which did not do well at the boxoffice fared better in video release.
The video cassette was not the only element of the video revolution. More homes than ever were connected to cable systems, providing viewers with still more program sources (and the film industry with still more program producing opportunities). The Laser Disc, the highest-quality home video program source, was introduced. Television set manufacturers began to offer products with what might be called high-fidelity picture quality, and consumers bought them. Thus television became video, and the home TV set became a video monitor to display a wide variety of visual program sources, much as stereo systems play music from a variety of sources.
As the video revolution got underway, consumers were not only accustomed to high-quality stereo sound from their music systems at home, but were also becoming used to high-quality stereo sound in the cinema as Dolby Stereo spread throughout the film industry.
While at first the video cassette offered only relatively low-fidelity mono sound, machines and prerecorded tapes with stereo sound were soon offered, first by means of linear soundtracks and then the HiFi technology. The LaserDisc was conceived from the outset to provide high-quality stereo sound.
In addition, a stereo TV broadcasting system was soon adopted for over-the-air and cable use. Thus the familiar two-channel stereo format long established for home music reproduction was adopted for all video media.
Dolby Stereo Digital cinema sound is made possible by a new digital coding technique, Dolby AC-3, that enables high-quality, 5.1-channel surround sound in less space than is needed for just one channel on a CD. AC-3 will also provide digital surround sound with the future US HDTV system, and is being used now in two-channel form for direct broadcast satellite systems. It is the basis for Dolby Surround AC-3, the upcoming equivalent for the home of Dolby Stereo Digital in the cinema.
Like the film format, Dolby Surround AC-3 provides separate channels for left, right, and center front speakers; two surround speakers; and a subwoofer at the listener's option. Two surround channels provide more depth, accurate localization, and overall realism, while superior channel separation makes Dolby Surround AC-3 as suited to music-only recordings and broadcasts as to video formats. It may very well open up new worlds of multichannel sound reproduction.
Dolby Surround AC-3 program material can be decoded according to preference, budget, and listening space. It can be heard in mono, two-channel stereo, four-channel Dolby Surround, or full-bore 5.1-channel Dolby Surround AC-3. Low bass can be routed only to the system's wide-range or subwoofer speakers, and the listener can even tailor dynamic range to the environment.
Dolby Surround AC-3 has already been demonstrated from prototypes of a new LaserDisc. Another major step in its progress has been the introduction of ICs capable of AC-3 decoding. While consumer products may become available in 1995, costs are likely to be relatively high. However, the basic five-channel speaker configuration is the same as today's analog Dolby Surround, and to ensure compatibility with Dolby Surround program material, products with digital decoding will also feature Pro Logic decoding as standard.
So far, digital technology's most direct benefit for the home listener has been the Compact Disc. Yet for all its ergonomic virtues and resistance to wear, in terms of fundamental sound quality the improvement afforded by the CD over the best analog formats is incremental. Multichannel digital surround sound, on other hand, might bring about a true revolution in the way sound is reproduced and enjoyed in the home.
E-Mail address: jfedele@fedele.com