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76 Transactions of S.M.P.E., Vol. XIII, No. 37, 1929
tlie width of which is varied according* to electrical modulations created by the sound waves to be recorded. Soimd recording with a slit of this type known as a light valve has been described by MacKenzie.^°° A description of another type of light valve called the ''Kerr Cell" has also been given.^°^ Hardy ^°^ has dealt with the optics of sound recording systems. Wax recording methods have been described by Frederick/^^ The intricate equipment necessary to insure synchronization of sound and picture during recording has been described by Stoller.^°* Koller^°° has written on the use of the photo-electric cell and Kellogg^°^ on the vacuum tube.
Descriptions of sound recording processes in use abroad include variable density record types, one the invention of A. J. Roberts^"^ and another known as the Kiichenmeister system.^°^ Others are the Gaumont-Peterson-Poulson process^°^ whereby the sound and picture are recorded separately, and a somewhat similar system called the ' ' Meisterton " process.^^^ In this latter process the recording is done with a Reisz microphone on a separate negative in a sound camera which is synchronized with the picture camera. The amplified microphone current is recorded with a constant width slit and Riihmer's singing arc lamp which operates on a constant direct current and is activated by having the amplified microphone current impressed on the constant supply. The sound record is of the varying density type.
Tri-Ergon^^^ Sound Films also yield a variable density record obtained with a ''glow-lamp," but a film of 42 mm. width is used.
Causes of ground noises, extraneous sounds recorded by the film, have been analyzed by Sandvik^^^ who showed that the processing conditions, even though very carefully controlled, introduce considerable ground noise. A giant amplifier installed on an airplane made clearly audible to persons on the ground the voice of a man speaking in ordinary tones when the plane was at an elevation of two thousand feet.^^^ An amplifying system installed in the Central Casting Bureau at Hollywood, California, in conjunction with the telephone switchboard makles possible the segregation of any voice for testing purposes for sound pictures.^^*
In Hollywood, according to J. A. Ball's report, there has also been a considerable extension of the length of the transmission line between the point of recording and the point of pick-up. Pick-up has frequently been extended to the back lot of the studio and
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