International projectionist (Jan-Dec 1937)

THE PUSH-PULL SOUND RECORDING AND REPRODUCING SYSTEM By FRANK T. JAMEY, Jr. FOR the past few years there has been considerable publicity given to a newly-developed system of recording and reproducing sound on film. This new method has been represented as. being a definite step in advance, and for this reason, there has been some speculation as to why it has not already reached the theatre. Before we analyze the reasons for this let us consider just what is involved in the recording and reproducing of this new type of recording. When sound motion pictures were introduced it at once became evident that considerable progress would have to be made to eliminate the extraneous and objectionable noises that came from the loudspeakers in the theatre reproduction of sound from film. At that time the most important cause for such noises was the large amount of transparent sound track on the positive print. Any dirt or oil which might find its way on the sound track, thus intercepting some of the light that should have reached the photo-cell, would certainly occasion such noises. In addition, some extraneous light reached the photo-cell. In 1929 the introduction of ground noise reduction equipment, which essentially reduced that amount of useless transparent sound track, permitted a material reduction of the extraneous and objectionable noises. Even after this important development there was plenty of room for further progress. Recognizing the objective of reproducing sound in the theatre that was directly comparable to the sound originally created in the studio, engineers realized that 1930 sound quality fell far short of this goal. The reproduced sound was not sufficiently realistic because too many of the important harmonics were missing, and also, it still was impossible to reproduce the sound in the theatre at the amplitude or volume level comparable with the original. A Two-Headed Problem The research engineers found that this problem could be attacked in two ways. First, it was necessary to refine to a considerable degree each individual unit of a complete recording and a complete reproducing system. Second, it was essential to develop a new system of recording which would permit an even further reduction in ground noise. Late in 1932 the first stage of this engineering development was sufficiently completed to permit the introduction commercially of recording and reproducing systems for extended frequency and volume ranges. The recording and reproduction of a frequency range of 40 to 10,000 cycles per second, with reasonable uniformity and a volume range of approximately 50 decibels, resulted in sound in the theatre that was startlingly realistic. For the first time voices assumed a naturalness that permitted the auditor to recognize them without closely watching the lip movement, and musical instruments could be heard individually with a quality easily comparable to the original. While the results of this development were widely recognized and exploited, it was evident to the engineers that the second stage was still as important as ever. Two years later this work was completed and the new system of recording known popularly as "pushpull" was introduced. The volume range was thus extended to about 65 decibels. Push-pull recording involves the splitting of the standard sound track on the film into two sound tracks which occupy the same space on the film (76 mils). The tracks are split so that all the positive halves of the signal waves (alternating current waves) created by the microphone and amplified are recorded on one track, and all the negative halves on the other. Each track is 35 mils wide, and they are separated by 6 mils in case either overshoots. This is accomplished in the recorder by using a mask which exposes one track for the positive halves of each wave, and the other for the negative halves. To reproduce this type of recording few fundamental changes are required to the standard reproducing system. The light beam from the exciter lamp is accurately focused on the sound track so that it scans a width of approximately 84 mils, overlapping the track on both sides. When a push-pull track is thus scanned, two varying beams of light eminate from the other side of the film. These must be separately converted into electrical waves by being directed to separate photo-cells. A full signal wave consists of 360 degrees. Since only half of each wave is on one sound track (180 degrees), it is evident that the tracks are 180 degrees out of phase with each other. As electrical waves, then, they must be properly mixed before being amplified and converted into sound waves. This may be accomplished by connecting the cathodes of the photo-cells to opposite ends of the primary winding of a photocell transformer, and the common connections to the center tap. The waves are then mixed in proper relationship and the two tracks are added together f90 J_ TO AMPLIFIER TO AMPLIFIER +90 Comparison of circuits: Above, push-pull; below, standard [19]