Journal of the Society of Motion Picture and Television Engineers (1950-1954)

Operating Procedure The operating procedure used to maintain accurate exposure control with the instruments described above may be classified in four categories: 1 . Calibration of equipment. 2. Peak-white measurements. 3. Black-level measurements. 4. Sensitometry. Calibration of Equipment. As indicated earlier, it is extremely important that all units in the recording plant be carefully calibrated, since amplitude drift or level discrepancies affect film exposure. The five operational steps for calibration are as follows: (1) Adjust the 60-cycle calibrating voltage to exactly 1.4 v peak-to-peak as read on the meter previously described. (2) With the oscilloscope selector switch hi the "calibrate" position, adjust the vertical amplitude of the TM5A master monitor oscilloscope until the 1.4-v calibrating signal exactly fits arbitrarily prescribed lines for the 1.4-v video signal. (3) Apply the 1.4-v calibrating signal directly to the input of the 7-in. oscilloscopes at each recording position and adjust amplitude to prescribed lines on the tube face. (4) Apply the 1.4-v calibrating signal to the input of the phototube amplifier and adjust its gain until the output as seen on the scope fills prescribed calibrating lines. (5) All electronic measurement instruments in the system have now been calibrated. Peak-White Measurements. There are several means by which peak-white measurements can be made satisfactorily. From an operational standpoint the most desirable is as follows: (1) The familiar step signal produced by the step generator is used and the three important steps, 1, 2 and 10, carefully adjusted to prescribed lines on the face of the master monitor oscilloscope. (2) This signal is then fed to each recording position through exactly the same distribution amplifiers and coaxial lines as will be used for the picture signal to be recorded later. The 7-in. oscilloscope is then connected across the output of the recording video amplifier, and the amplifier gain adjusted to the desired amplitude as shown on the calibrated oscilloscope. (3) There is, of course, a fixed relationship between signal amplitude and kinescope light output, hence also maximum film exposure. Black-Level Measurement. A somewhat different procedure is employed for black-level measurement since it has been found to be much more critical than peak-white measurement. (1) The blank raster, or pedestal, position of the step generator is used, carefully adjusted to 20 units on the master monitor oscilloscope. The 20unit adjustment, instead of the customary 10 for black level, was chosen because it produced a density of about 0.1 on the film, a value far enough removed from the base fog region to give more consistent operating data. (2) The 7-in. oscilloscope at each recording position is now connected to the output of the phototube amplifier. The black-level control is then adjusted to produce the desired amplitude as indicated on the calibrated oscilloscope. This, of course, will determine the minimum exposure, or black level, on the film. The two extremes of the anticipated video signal, peak white and black level, have now been set. Sensitometry. The final test of the above is to make an exposure, develop the film, and measure the resulting densities. (1) This is done using the step signal carefully adjusted as described. (2) The test exposure is made about two hours before recording time to permit development, analysis and readjustment. 232 March 1953 Journal of the SMPTE Vol. 60