Dolby Stereo: История и Технологии Кинозвука

Sound format

Dolby Stereo is a sound format made by Dolby Laboratories. It is a unified brand for two completely different basic systems: the Dolby SVA (stereo variable area) 1976 system used with optical sound tracks on 35mm film, and Dolby Stereo 70mm noise reduction on 6 channel magnetic soundtracks on 70mm prints. Dolby SVA significantly improves the development of sound effects in films and theorization of sound design by Walter Murch. Except in large cities, most movie theaters did not have facilities for playing back magnetic soundtracks, and a majority of films continued to be produced with mono optical soundtracks. Dolby SVA provided a method for putting high quality stereo soundtracks on optical sound prints. The optical soundtrack on a Dolby Stereo encoded 35 mm film carries not only left and right tracks for stereophonic sound, but also—through a matrix decoding system (Dolby Motion Picture matrix or Dolby MP) similar to that developed for "quadraphonic" or "quad" sound in the 1970s—a third center channel, and a fourth surround channel for speakers on the sides and rear of the theater for ambient sound and special effects. This yielded a total of four sound channels, as in the 4 track magnetic system, in the track space formerly allocated for one mono optical channel. Dolby also incorporated its A Type noise reduction into the Dolby Stereo system.

Dolby Labs became involved in movie sound when film studios used Dolby A type noise reduction on studio magnetic film recordings. The first film to use Dolby noise reduction throughout the production process is A Clockwork Orange (1971), though much of the benefit was lost when it was released with a standard "Academy" optical soundtrack. This led to a proposal from Dolby that A type noise reduction be applied to the optical soundtrack on release prints. Through the early 1970s, there was renewed interest in improving the quality of optical soundtracks, which had changed little since the 1930s. In particular, the infamous "Academy curve" (the standard frequency response for cinema playback of optical tracks as specified by the Academy of Motion Picture Arts and Sciences in 1938) was still in use. It involved a drastic roll off in the high frequency response of the theater system with the intention of reducing the audibility of noise and distortion. Dolby proposed replacing the Academy curve with Dolby A type noise reduction on the track. Starting with the 1974 film Callan, ten films were released with a Dolby encoded mono soundtrack. Theaters were equipped with a Dolby A type noise reduction module and a third octave equalizer to equalize the electro acoustic frequency response of the speakers/auditorium. It created a new international standard for cinema sound. Though the system worked well, theater owners were reluctant to invest in the technology until stereo was added to the mix. The idea of putting a two channel optical stereo soundtrack in the normal soundtrack area of a film print was not new; the stereo pioneer Alan Blumlein had made experimental stereophonic films using such a system as early as 1933, and J. G. Frayne of Westrex had proposed a similar system to the SMPTE in 1955. By 1970, however, it was apparent that magnetic recording methods were not going to displace optical soundtracks on most release prints, and in the early 1970s Eastman Kodak revived the idea, as described by R. E. Uhlig in a paper presented to the SMPTE in 1972. Initially using a two channel 16mm film recorder built for them by RCA, Kodak recorded two channel stereo soundtracks much as Blumlein and Frayne had done before, but added Dolby noise reduction to improve the limited dynamic range available from these half width tracks. He considered the possibility of splitting the soundtrack area three ways to provide a third center channel, but dismissed it because of the negative impact it would have on dynamic range and the problems involved in converting film projectors. Instead he suggested feeding a center channel speaker with a simple mix of the left and right channels; however, this is not entirely satisfactory as it degrades the stereo separation. After he brought his idea and worked with Dolby, Dolby SVA, a 35mm stereo variable area optical encoding and decoding system, came up and later was adopted as an industry standard ISO 2969. is straightforward. The four original channels of Left (L), Center (C), Right (R), and Surround (S), are combined into two, known as Left total and Right total (LtRt) The SSM 2125 is a complete Dolby Stereo matrix decoder (except for the surround delay) on a single chip, it allowed domestic decoders which used the same logic system found in professional decoders to be marketed to the consumer. These decoders were thus named Dolby Pro Logic.

Dolby Stereo 70 mm Six Track is the use of Dolby noise reduction on the six magnetic soundtracks of a 70 mm print. This was first used on some prints of the MGM film Logan's Run released in 1976. The Todd AO format was introduced in 1955 and included multi channel magnetic sound from the start, it does not have an optical soundtrack, although in some 70mm prints have used a DTS digital track in place of the analogue magnetic one. The original layout was for 5 front channels and one surround. But by the 1970s the use of the intermediate (left center and right center) tracks had been largely abandoned, these channels either being left blank, or filled with a simple mix of the adjacent channels. Dolby did not approve of this later practice, which results in loss of separation, but instead used these channels for LFE (low frequency effects) utilizing the bass units of the otherwise redundant intermediate front speakers. Later the unused HF capacity of these channels was used to provide for stereo surround in place of the mono surround of the Todd AO layout giving the modern 5.1 channel allocation retained by Dolby Digital.

By 1984, Dolby Stereo had a competitor. Ultra Stereo Labs had introduced a comparable stereo optical sound system, Ultra Stereo. Its cinema processor introduced improvements in matrix decoding and convolution matching with greater channel separation. An included balancing circuit compensated for film weave and some imbalances between the left and right tracks that previously caused voice leakage into the surround channel. The Ultra Stereo sound system won a 1984 Technical Achievement Award from the Academy of Motion Picture Arts and Sciences.