Dolby Stereo

Dolby Stereo
Dolby Stereo Logo.png

Dolby Stereo, is the trade mark that Dolby Laboratories used for the various analogue stereo cinema sound formats that they produced.

Two basic systems used this name. The first was the 'Dolby SVA' (stereo variable area) system used with optical soundtracks on 35mm film. The second was Dolby Stereo 70mm, which referred to Dolby noise reduction on the 6-channel magnetic soundtracks on 70mm prints.


Dolby SVA

Of the two, Dolby SVA was by far the more significant, as it brought high-quality stereo sound within the reach of virtually every cinema. Though stereo magnetic sound had been used on film since the 1950s, the technology had proved expensive and unreliable. The majority of movie theatres did not have facilities for playing back magnetic soundtracks, and the vast majority of films continued to be produced with mono optical soundtracks. Dolby SVA provided a way out of this impasse by putting a high-quality stereo soundtrack on optical sound prints.

The optical soundtrack on a Dolby Stereo encoded 35 mm film not only carried left and right tracks for stereophonic sound, but also—through a matrix decoding system similar to those 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 in the track space previously allocated for one mono optical channel. Dolby also incorporated its A-Type noise reduction into the Dolby Stereo process. The brand of Dolby Stereo became a world leader, and synonymous with high quality sound in thousands of movie theaters across the world.


Dolby Lab's original involvement in movie sound was when film studios used Dolby A type noise reduction on studio magnetic film recordings. The first film that used Dolby noise reduction throughout the production process was "A Clockwork Orange," 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.

At that time (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" was still in use. This curve was the standard frequency response for cinema playback of optical tracks as specified by the Academy of Motion Picture Arts and Sciences in 1938, it involved a drastic roll-off in the high-frequency response of the theatre system with the intention of reducing the audibility of noise and distortion. Dolby proposed ditching the Academy curve and instead using Dolby A type noise reduction on the track. Starting with the 1974 film Callan, ten films were released with a Dolby encoded mono soundtrack. Dolby made available a unit for installation in cinemas that included a Dolby A type noise reduction module and a 3rd octave equaliser to equalise the electro-acoustic frequency response of the speakers/auditorium to a new international standard for movie theatres[1]

Although this system worked well it made little impact on theatre owners who were disinclined to spend the necessary money for what seemed to them a relatively small improvement. Something more was needed to get them to dig their hands in their pockets, and that something extra was stereo.

Eastman Kodak and RCA had been working on a stereo variable area system since 1973, Dolby Labs now joined them and added their noise reduction process to the Kodak/RCA SVA system. Dolby also suggested employing a matrix similar to those currently being used for domestic "Quadraphonic" systems; by this means it was possible to provide a four channel stereo system using the same speaker layout as had been used for the CinemaScope 4-track magnetic stereo system of the 1950s but at far lower cost. Dolby Stereo, as this system was now branded, was first used on the 1975 Ken Russell film Lisztomania, in a 3-channel Left, Centre, Right configuration. In the following year A Star is Born was the first film to use the full 4-channel Dolby Stereo system. Initially the Sansui QS matrix was used, but from the Spring of 1979 onwards a new custom-designed matrix was employed. This new matrix was first used for Hair and Hurricane.[2]

The success of 1977's Star Wars, which used the four channel system to great effect, did much to encourage movie theaters to convert to the 4-channel LCRS speaker configuration. The 1979 re-release of 101 Dalmatians was the first Disney Classics film to use Dolby Stereo. A key feature of this system was its backward-compatibility: the same print could play anywhere, from an old drive-in theater with mono sound to a cinema that had upgraded to a Dolby Stereo processor. Thus, there was no need (nor expense) in carrying a double inventory of prints for distribution. The success of Dolby Stereo resulted in the final demise of magnetic stereo on 35mm release prints. From then on, only 70mm prints used magnetic sound.

Dolby SR noise reduction began to replace Dolby A type NR in 35 mm motion picture exhibition from the early 1990s onwards. A Dolby SR analogue soundtrack is still included on all theatrical release prints encoded with Dolby Digital, as the default track if something goes wrong with decoding the digital track. Also the Dolby SR track is used in theaters not equipped for Dolby Digital playback.

The Dolby Stereo Matrix

The Dolby Stereo Matrix is straightforward: the four original channels: Left (L), Center (C), Right (R), and Surround (S), are combined into two, known as Left-total (LT) and Right-total (RT) by this formula:-

Dolby Stereo Mix Left Right Center Surround
Left Total 1 0 \frac {1}{\sqrt 2} +j \frac {1}{\sqrt 2}
Right Total 0 1 \frac {1}{\sqrt 2} -j \frac {1}{\sqrt 2}

where j = +90° phase-shift

Thus center channel information is carried by both LT and RT in phase, and surround channel information by both LT and RT but out of phase. This gives good compatibility with both mono playback, which reproduces L, C and R from the mono speaker with C at a level 3dB higher than L or R, but surround information cancels out. It also gives good compatibility with two-channel stereo playback where C is reproduced from both left and right speakers to form a phantom center and surround is reproduced from both speakers but in a diffuse manner.

A simple 4-channel decoder could simply send the sum signal (L+R) to the center speaker, and the difference signal (L-R) to the surrounds. But such a decoder would provide poor separation between adjacent speaker channels, thus anything intended for the center speaker would also reproduce from left and right speakers only 3dB below the level in the center speaker. Similarly anything intended for the left speaker would be reproduced from both the center and surround speakers, again only 3dB below the level in the left speaker. There is, however complete separation between left and right, and between center and surround channels.

To overcome this problem the cinema decoder uses so-called "logic" circuitry to improve the separation. The logic circuitry decides which speaker channel has the highest signal level and gives it priority, attenuating the signals fed to the adjacent channels. Because there already is complete separation between opposite channels there is no need to attenuate those, in effect the decoder switches between L and R priority and C and S priority. This places some limitations on mixing for Dolby Stereo and to ensure that sound mixers mixed soundtracks appropriately they would monitor the sound mix via a Dolby Stereo encoder and decoder in tandem.[3] In addition to the logic circuitry the surround channel is also fed via a delay, adjustable up to 100msec to suit auditoria of differing sizes, to ensure that any leakage of program material intended for left or right speakers into the surround channel is always heard first from the intended speaker. This exploits the "Precedence effect" to localise the sound to the intended direction.

Dolby Stereo 70 mm Six Track

Dolby Stereo 70 mm Six Track refers to 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 70mm format was introduced in 1955 and included multi-channel magnetic sound from the start, it does not have an optical soundtrack (although in recent years some 70mm prints have used an optical 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 enhancement) utilising 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[4] giving the modern 5.1 channel allocation retained today by Dolby Digital.

Ultra Stereo

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, 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.[5]

Dolby Surround

Dolby Surround' was the earliest consumer version of Dolby's multichannel analog film sound format Dolby Stereo.

Due to the compatibility of the Dolby Stereo matrix with mono and stereo playback, when films originally made in Dolby Stereo were released on stereo domestic video formats, such as VHS-HiFi or laserdisc, or broadcast on stereo TV the original two-channel Dolby Stereo soundtrack could be used. Some domestic listeners were keen to hear these soundtracks in a manner more akin to how they would have sounded in the theater and for that market some manufacturers produced simplified surround decoders. To keep the cost down these decoders dispensed with a center speaker output and the logic circuitry found on the professional decoder, but did include the surround delay. To distinguish these decoders from the professional units found in cinemas they were given the name "Dolby Surround" decoders. The term "Dolby Surround" was also licensed by Dolby for use on TV programs or straight-to-video movies recorded through the Dolby Stereo matrix.

Dolby Pro Logic

By the late 1980s integrated-circuit manufacturers were working on designing integrated-circuit matrix decoders. A typical early example is the SSM-2125 from PMI.[6] 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 given the name "Dolby Pro-logic"


  1. ^ "The production of Wide-Range, Low-Distortion Optical Soundtracks Utilising the Dolby Noise Reduction System" by Ioan Allen – Journal of the SMPTE Vol84 Sept1975.
  2. ^ "Mixing Dolby Stereo Film Sound" by Larry Blake, Recording Engineer/Producer Vol 12, No.1 - Feb 1981
  3. ^ Mixing Dolby Stereo Film Sound by Larry Blake, Recording Engineer/Producer Vol12, No 1 Feb 1981
  4. ^ "The CP200 - A Comprehensive Cinema Theater Audio Processor" by David Robinson. Journal of the SMPTE Sept 1981
  5. ^ Film Journal International August, 1999, page 34.
  6. ^ PMI Audio Handbook Vol 1, 1990

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