16-mm sound motion pictures : a manual for the professional and the amateur (1953)

Record Details:

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KOUTINE AMPLIFIER CHECKING 273 of lateral-cut records are available: a 12-inch 78-rpm Vinylite disk for the range of 70 to 10,000 cps, and a 16-inch 33 1/3-rpm Vinylite disk for the range of 60 to 10,000 cps. Both are recorded as logarithmic sweeps from the low to the high frequency; the repetition rate (20 per second) makes viewing on a cathode-ray oscilloscope practicable. The 78-rpm disk is recorded with a 500-cps crossover* frequency; below 500 cps the recording is constant amplitude, above 500 cps the recording is constant velocity. The 33 1/3-rpm disk is recorded with the NAB (National Association of Broadcasters) preequalized characteristic. For frequency identification, marker "pips" are located at convenient frequencies such as 1, 3, 5, 7, and 10 kilocycles ; these are of 200-microsecond duration. When these records are reproduced on a good turntable, such as the Western Electric 1304 Type with a reproducer of the Pickering, GE Reluctance, RCA transcription, or Western Electric types, and adjusted to provide a flat frequency characteristic, the electrical signals produced are convenient for testing amplifiers, galvanometers, or other similar equipment. In testing amplifiers a frequency scale may be superimposed on the face of a cathode-rayoscillograph to show the frequency characteristic directly on the face of the tube, thereby avoiding the time consuming and laborious point-by-point plotting of responsefrequency characteristics. As the phenomenon is repetitive, a still camera may be used to photograph the image shown on the face of tube if a record is to be placed in permanent file as is usually desired. The traces of this sweep on the face of the cathode-ray tube will also show to some extent dynamic axis shift, harmonic distortion, transient distortion, and a number of other factors that remain obscured when the conventional point-by-point response-frequency characteristic is taken with a sine-wave oscillator. * Constant velocity recording in a disk record is recording in which the amplitude of the groove is inversely proportional to frequency. For a constant level of signal, the groove amplitude is small at high frequencies and comparatively large at low frequencies. Since the spacings between adjacent grooves is limited, it is necessary to attenuate the low-frequency amplitudes to prevent "cutting over" (the cutting of one groove into another). The point in the frequency range where this change occurs is called the crossover frequency or turnover point; below this point the recording is customarily made at constant amplitude. Constant amplitude recording in a disk record is recording in which the amplitude of the groove is constant for all frequencies. For a constant level of signal, the groove amplitude would be independent of frequency. Reproducing equipment is customarily equalized to correct for the recording characteristics used.