ABSTRACT
Multi-channel surround sound originated in the cinema industry, with the development in Hollywood of the ‘Fantasound’ system in the late 1930s for the film release of Walt Disney’s Fantasia in 1941. A separate, synchronised sound film carried three channels of audio with a tone track that enabled the automated panning of sound around the auditorium to between three and eight loudspeakers depending on which system version was used. A dedicated array of equipment was needed to be installed
before each theatre could show the film, and development of the equipment showed considerable technical changes between each of the many versions. The MkX set-up, for example, used in a Los Angeles cinema, had left centre and right screen speakers with two speakers at each rear corner of the cinema, in a similar fashion to the current 5.1 channel digital cinema and home theatre systems. The signals to the rear speakers in this particular version were switched by notches on the edge of the film which operated relays. After this great step forward in surround sound, no further
commercial multi-channel development was made until the introduction of magnetic stripes on the film stock, which enabled some films in the 1950s and 1960s to carry up to fourand six-channel audio tracks on 35mm and 70mm stock respectively. This process was expensive to manufacture because of the magnetic coating process and also the extra recording stage. It was prone to wear (of both the magnetic stripe and the expensive playback heads) and other problems such as alignment and noise, so it was not until Dolby applied their A-type noise reduction to the existing dual mono optical stripes on 35mm film in 1974 (Callan) and then encoded threeand, later, four-channel sound onto them in 1975 (Lisztomania) that surround sound emerged as a viable commercial success. In the late 1960s, Ambisonics were developed in Britain by
Michael Gerzon, Professor Peter Fellgett and others. Using a
special phase coherent Soundfield microphone, a flexible playback system was produced where the number and placing of loudspeakers could be chosen to suit the requirements of the listener, and then this layout could be correspondingly selected in the playback decoder. Many music vinyl albums and CDs have been released with this format, and it is still in use today. In the early 1970s, the recording industry was starting to
release quadraphonic encoded vinyl albums and tape cartridges. These quadraphonic surround media could carry the four signals (Left, Right, Left rear, Right rear) in one of several competing encoding formats. The CBS SQ system carried the extra information on a supersonic carrier signal, whereas the Sansui QS system employed a matrix encoding technique which would later be adapted as the basis for encoding analogue cinema surround tracks. Home enthusiasts in the 1970s would connect two speakers
across the left and right positive output leads of a twochannel stereo system to give a difference signal, an invention attributed to David Hafler, and this was used to produce the rear or ambience signal, the first domestic instance of matrix decoding. By employing the matrix encoding process (using the rela-
tive phase and amplitude differences), Dolby discovered that four audio tracks (Left, Centre, Right, Surround) could be reduced to two, Left total and Right total (Lt and Rt) signals, which when decoded would then reconstitute the four original signals fairly accurately. The surround signal was band limited (100Hz-7 kHz), and represented the original use of the surround (or ‘effects’) channel for occasional ambient effects. The cinema industry traditionally strives to maintain the dominant part of the soundfield at the front of the auditorium in order not to distract the audience away from the screen action. The first surround channels were only occasionally used for subtle effects and the early cinema surround loudspeakers were quite small compared to the screen loudspeakers, hence they could not reproduce realistic levels of low frequencies, and so the LF roll-off was introduced at 100Hz.
