Journal of the Society of Motion Picture Engineers (1930-1949)

Moviolas or any other screening equipment to magnetic sound reproduction, since the regular photographic sound reproducing equipment may be used to reproduce either the push-pull or onehalf thereof as a standard single track. Only the re -recording equipment required for the final transfer to the release photographic equipment needs to be modified to reproduce from the magnetic track on the composite film. Dual Recorder The recorder chosen for this work was the Westrex RA-1231-C Variable-Area Recorder as modified to lay down a 200-mil push-pull direct-positive variable-area track as previously described in the Journal? The optical schematic of the direct-positive variablearea modulator is shown in Fig. 2. It includes a check visual monitor and an improved photocell monitor. In the latter the ingenious scheme is employed of using the light transmitted through the ribbons for actuating the photocell monitor, the light reflected from the ribbons being used to expose the photosensitive emulsion. The recorder was further modified by adding a magnetic recording kit similar to that described in the March 1950 Journal? Since the film travel is in the reverse direction, the monitor head has been moved as shown in Fig. 3 to a position above the recording head, instead of to the customary position below and to the right of the latter. The location of the recording head at the drum position makes it possible to make the magnetic and photographic lines of translation exactly collinear. In fact, the location of the recording head at any other position in the recording machine would defeat the purpose of this dual-recording technique. One of the problems encountered in this recorder was the tendency to partial magnetization of the recording head by the stray field from the light-valve permanent magnet. This was somewhat alleviated by placing a sheet of mumetal between the modulator and film compartments of the recorder. Under this recording condition, an overall signal-to-noise ratio of about 55 db is readily obtained on the magnetic track. Since this is considered satisfactory in view of the ultimate transfer to a standard photographic release track, no further isolation of the disturbing source of magnetization seems to be immediately warranted. Film The film used to date in this process is the Eastman Fine Grain Sound Recording Safety Film, Type 5372 (35 mm), variable-area type film with the magnetic stripe added to the base side of the photosensitive film. The pioneering work in the coating of this raw stock was carried out by Reeves Soundcraft Corp., and in spite of the rather hazardous process of working with a light-sensitive film the production of the early batches of the dual-purpose film has been singularly free of serious defects. Further experience should tend to make this operation a purely routine affair. The processing of the film is handled in the film laboratory without any precautions other than those dictated by normal operating practice for the proper development of variable-area tracks. No damage to the magnetic stripe by the photographic developing process has been observed. The Record-Reproduce Transmission System The transmission system of the photographic-magnetic transfer channel in use at Columbia Pictures Corp. is shown in block-schematic form in Fig. 4. The original magnetic recording is reproduced by a modified RA-1251 Rerecorder, the signal being fed to a dividing network through a level-control attenuator and line amplifier. The dividing network provides two input signals, one of which is recorded by 390 November 1952 Journal of the SMPTE Vol. 59