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618 ACTIVITIES OF THE RESEARCH COUNCIL [J. s. M. p. E.
where A T is the change in transmission over a cycle in the test track.
In order to use this film to determine the gain in reproducing systems in terms of this film, an electrical film level is supplied with each print. This level is obtained by the same method used to determine the electrical film level of the ERPI ED-20 test film. For this reason, a cross calibration between any ED-20 film and our Standard Multi-Frequency Test Reels can be easily obtained by noting the difference in levels.
This electrical film level is expressed in terms of the level produced by this film with respect to 6 milliwatts at the output of some standard photocell pick-up system. In the case of the ED-20 reel this system was an average 3A cell and 10 megohms' internal impedance working into a 10-megohm load. The illumination is supplied by an average 8V2-volt, 4-ampere exciter lamp operated at 3.7 volts through a lens system having an optical transmission equal to the ERPI KS-6470 lens tube assembly corrected for zero slit width.
Such a set-up described above experimentally yields a level of 37.8 db less than the densitometric level obtained from the formula given above. Hence the electrical film level in db is equal to — 40 + 20 log A T 37.8 = -77.8 + 20 log A T.
All frequencies are rated in terms of the 1000-cycle level and the "deviation from 1000-cycle level" for each of the other frequencies, with the signs of all values reversed, are given as correction factors for testing. Corrections are used rather than deviations in output level so as to conform with field use where direct addition is used in making out field test reports.
For example, if a test film has an output level of 3 db lower at 8000 cycles than at 1000, it is necessary to add 3 db to the output level to compensate for this loss. The correction factor for 8000 under this condition would then be given as +3 db and the sign of the correction factor is therefore reversed in sign from the true characteristic of the film.
After the microdensitometer or electrical film level of this calibrating print has been established, individual prints are calibrated by comparing each print with this calibrating print on a sound reproducing system. We have been employing a conventional theater sound-head with a particularly steady film movement used in conjunction with an amplifier working into the continuous level recorder. This continuous level recorder gives a complete graphic record of the output of the film, and by comparing these graphs to the one secured