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March, 1930] BLOCKING OUT SPLICES IN SOUND FlLM 353
The patch which proved most successful was so made that it covered the entire width of the sound track completely and extended as far as possible toward the center of the film strip without entering the picture area. Some of the factors which entered into consideration of the best design for the patch are discussed below.
Design of Patch. — As mentioned above, the patch or a "paint-out" performs its function by masking off an area of sound track of varying width so as to reduce the total transmission of the area illuminated by the slit in the reproducer at a rate which is insufficiently rapid to cause the recorder to generate an audible sound, and then, when the splice is past, uncovering the track in a like manner.
The reproducers now in use are capable of generating sounds of a frequency not less than 20 to 50 cycles per second. Therefore, if the splice is to be designed so that it will cause no noise of itself, it should vary the transmission as it would be varied by a signal whose frequency is not more than 20 cycles. Such a signal would be represented by a patch whose contour would be described by a sine curve of an amplitude corresponding to full modulation. Its length for 20 cycles would be
18 inches
20
= 0.9 inch.
Now it might be argued that this length causes a noticeable discontinuity in the sound. This is not so serious as might seem. A patch having straight instead of curved sides has been considered because it is much more easily made, especially if it is to be cut by hand. If the patch is shorter (about one-half this length, as has been recommended), the harmonics introduced by using a straight edge for the cut-off as an approximation for an edge of curved contour, are of a higher frequency and therefore more prominent. Also, the fundamental is well within the range of the reproducer.
The following tests were made with a view to arriving at a design which would be a compromise between one which would be audible and one which would obscure too much of the sound record.
A number of patches having dimensions indicated in Fig. 4 as shown in Table I were made and applied to (1) an oscillator record of low modulation (frequency 540 cycles) ; (2) a strip of clear film of density about 0. 1 ; and (3) a strip of film flashed and developed to produce a uniform density of about 0.7. In each of these films two splices were made with 5 feet of film between them and then 17 feet