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THE ADDITION OF SOUND TO FILMS 205
metal disc equivalent in all characteristics to the original wax recording, any number of positive copies, that is, discs in which the sound bands are raised up instead of being cut in grooves, can now be made and used to produce the final compound mouldings.
High pitched notes, especially when recorded at the centre of the disc, appear as exceptionally short period waves, originally cut with very acute corners. After the several copies which must be made to produce the metal 'moulding positive' these acute wave forms naturally loose some of their fine outline and, as we shall see later, these losses correspond to the slight loss in high frequency resolution which occurs when film sound tracks are repeatedly copied from successive duplicate negatives.
In the early days of sound films these discs were 16-inches (40-64-cms) in diameter and rotated at a speed of 33 1 /3 revolutions per minute. It was thereby possible to project a full 1,000-ft (304-8-metres) roll of pictures during the playing time of one disc and, naturally this made possible the synchronism of disc and film before the machines were started and, assuming some form of electrical interlock were employed, such synchronism would be maintained throughout the entire roll of film. It should be noted that, in those days, the picture film was still travelling at the silent film speed of 16 pictures per second and, therefore, a 1,000-ft (304-8-metres) roll of film would run for just over 16 minutes. As will be seen later, it was in order to improve the sound recording quality that the film speed had to be increased to 24 pictures per second when a photographic sound track was printed onto the picture film.
Modern Disc Recordings
It is interesting to note that disc recording is still extensively used in modern studio work although, of course, only as an intermediate stage in film production. Many instances arise when it is a great advantage to obtain immediate 'play-back' of sound on the studio 'floor'. One example of this is when an elaborate 'dance sequence' is required; it will then be necessary to have the speciality dancers performing so that, eventually, their movements may be accurately synchronised to music which, for many reasons, could not be recorded directly from a 'live' orchestra arranged on the 'floor' and repeatedly playing the number for every 'take' which may be necessary before a satisfactory result is produced. Under such conditions the orchestra is first recorded under