Kinematograph year book (1944)

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216 The Kinematograph Year Book. seats, 65 watts. The average power of amplifiers in British kinemas is well below these figures. Apart from this, however, one of the first advances likely is an increase in the power of amplifiers owing to the further increased volume range expected in future recording. Stereophony. — In a previous paragraph, reference has been made to U.S. experiments to obtain a directivity of sound in the auditorium. Two or three systems have been demonstrated in America. RCA technicians are working on the modification of their original " Fantasound," which used the triple sound tracks and a frequency modulated control track. This system necessitated the use of a sound film separate from the picture film, and the employment of no fewer than 36 speakers back stage, and 44 in the auditorium. In the simplified system, the multiple tracks are combined in a single track, with the control track which, of course, is an essential on all systems of stereophony. Not only does it provide automatic volume control, but it also cuts out or cuts in speakers when the dialogue or music demands it. The Western Electric system was also based on the multiple track theory, but as in RCA, the complications of a separate sound head, with four optical systems and other equipment requirements in installation, have compelled the production of a simplified version. This comprises a twin sound track and a separate control track — -a very narrow line located in the space between the picture and the existing sound track. A third method, " Stereo-control," has been demonstrated by Warners, who also fathered another system, " Vitasound," which carried the control track in addition to the sound track. In " stereo-control," two sound tracks and a control track — a clear line of varying width over the perforations — are employed. One of the sound tracks is recorded for a central microphone and reproduced for central speakers, while the other combines the sound from the two outer microphones, and can be reproduced upon either or both of the side speakers. The choice as to which of the two side speakers the second track shall be reproduced upon and the relative intensities of the control and side speakers is made by the control track, which can vary the intensity of any of the three sets of speakers and can fade in and out from one to another to produce the effect of movement of the sound source. The adoption by the Trade of any of these systems or, indeed, other methods of stereophony, will entail considerable expenditure upon the exhibitor. The essential installation will include : new sound heads, triplicate amplifiers, additional groups of speakers, besides a lot of other extra equipment, and, moreover, will involve the provision of considerably more space in which to house it. Under these circumstances, the Trade is entitled to demand that the whole question of stereophony shall be the subject of conferences between the sponsors of all practicable systems at which standards shall be settled, and, if possible, a decision made for the adoption of one single or composite equipment. No important new features have been introduced into the sound reproducer systems in this country where the following are in the most general use. British ThomsonHouston Co., Ltd. — Three models of B.T.H. sound reproducers are available ; the first, having an undisturbed output of 70 watts, is suitable for theatres seating over 2,500 ; the second is for theatres with a capacity between 1,100 ana 2,500, and the third and smallest is for theatres up to 1,100. Each model utilises the B.T.H. rotary magnetic soundhead and dual output arrangements on the amplifier feeding high and low-frequency speakers. The two larger equipments also incorporate the exclusive " volume expansion " feature, and are the only ones which will reproduce faithfully the original volume range of sound in spite of the necessary compression of this range recorded on the film itself. Conversion plant has been eliminated for 50-cycle A.C. supply, and the static conversion units, together with the whole of the amplifier and all fuses for the sound equipment, are mounted on the amplifier rack, thus affording centralised control.