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 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 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 enhancement) 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^[8] giving the modern 5.1 channel allocation retained today by Dolby Digital.

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
Right Total

where j = 90° phase-shift

This 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.^[7] In addition to the logic circuitry the surround channel is also fed via a delay, adjustable up to 100 ms 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 localize the sound to the intended direction.

DOLBY STEREO DOLBY SVA

Dolby Stereo is Dolby Laboratories' trademark for its various analog stereo cinema sound formats.

Two basic systems used this name: the Dolby SVA (stereo variable-area) system used with optical soundtracks on 35mm film, and Dolby Stereo 70mm, which referred to Dolby noise reduction on 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 7-track magnetic stereo had been used in Cinerama films since 1952, and Fox had introduced 4-track stereo magnetic sound as part of the CinemaScope system in 1953, the technology had proved to be expensive and unreliable.^{[citation needed]} 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^[1]) 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 35MM

Edge of a 35mm film print showing the soundtracks. The outermost strip (left of picture) contains the SDDS track as an image of a digital signal; the next contains the perforations used to drive the film through the projector, with the Dolby Digital track, the grey areas with the Dolby Double-D logo, between them. The two tracks of the analog soundtrack on the next strip are bilateral variable-area, where amplitude is represented as a waveform. These are generally encoded using Dolby Stereo matrixing to simulate four tracks. Finally, to the far right, the timecode used to synchronize with a DTS soundtrack CD-ROM is visible.

ANALOG SIGNAL

• An analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal.
• The instantaneous voltage of the signal varies continuously with the pressure of the sound waves. 
• It differs from a digital signal.
• The term analog signal usually refers to electrical signals; however,mechanical, pneumatic, hydraulic, and other systems may also convey analog signals.
• An analog signal uses some property of the medium to convey the signal's information. 
• For example, an aneroid barometer uses rotary position as the signal to convey pressure information. In an electrical signal, the voltage,current, or frequency of the signal may be varied to represent the information.
• Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure. 
• The physical variable is converted to an analog signal by a transducer. 
• In sound recording, fluctuations in air pressure (that is to say,sound) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condensor microphone.
• The voltage or the current is said to be an "analog" of the sound.

HOW IT ALL BEGAN

THE HISTORY OF SOUND

The history is of importance because it gets into the creation of sound-on-film

PROBLEM WITH 

east to lose sync 

no amplification 

Limited-recording time

THE INVENTION OF SYNC SOUND

1906- DR. LEE DEFOREST

OPTICAL SOUND PRINTING 

THE JAZZ SINGER 1927

THE STUDIO ERA

RECORDING THE 

ANIMATION BECOMES POPULAR

STUDIO ERA ENDS

WHAT WAS THE NEXT THREAT HOLLYWOOD FACED ? WHY?

HOLLYWOOD AND SOUND