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1949 TEST-FILM CALIBRATION 441
The scanning width evaluated in Fig. 4 (b) would produce an 8000cycle scanning loss of 0.34 decibel. Personal errors — the only kind known which could produce negative scanning-loss errors in calibrating film with the inverse-speed system — are minimized by carrying out the calibration with care, repetition, and reference to films previously calibrated. (See Appendix.)
Since all other calibrators compared reveal lower high-frequency level, it is concluded tentatively that the inverse-speed calibrator is the most nearly accurate. In the interest of industry standardization of test-film calibration, it is urged that this conclusion be challenged by others who possess means for calibrating multifrequency film.
PART II — FILM-LEVEL CALIBRATION
One important difference between film-reproducing systems is manifested by differences in output power developed when a particular film is reproduced. The fundamental property involved here has been called over-all system gain or, for brevity, system gain. However, any given system will develop various output-power levels (up to the limit of its power capacity) depending upon the particular sound track being reproduced. The term film level has long been used to describe this pertinent property of the sound track.
Specific definitions were established for system gain and film level in the earliest days of film calibration and these have been in continuous use ever since by one part of the industry. The same definition for film level was adopted elsewhere in the industry a decade ago, but it appears that the principles have not been well understood since we have found some of these film-level figures as much as 10 decibels apart from those measured by our methods for the same films.
With the thought that others in the industry will also find valuable use for the film-level calibration of multifrequency test film and for the measurement or specification of system gain, definitions for these quantities and methods for measuring them will be proposed for consideration by the Standards Committee.
The basic relationship between these quantities is shown in the following: A test film is reproduced and a particular output-power level (less than the power capacity of the system) is developed. Then output-power level = over-all system gain + film level. (1)
Since gain of an amplifier system has long been clearly defined, it is convenient to divide over-all system gain into two parts :
over-all system gain = amplifier gain + relative pick-up sensitivity. (2)