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1949
TEST-FILM CALIBRATION
443
must be included in the over-all system gain if the latter is to have any meaning. Combining (1) and (2),
output-power level — film level
amplifier gain
+ relative pickup sensitivity. (3)
Since power level is an observation and amplifier gain is already defined, (3) establishes the relation between film level and relative pickup sensitivity.
In order to define film level in terms of powder, a particular pickup system is borrowed, and film level will be defined as the power level developed in a load on that pickup system. The reference pickup chosen years ago consisted of the elements used in a sound system which was very widely distributed at the time. The phototube impedance was taken as 10 megohms; consequently the reference load for the reference pickup was set at 10 megohms. It was found that
OSCILLATOR °~
(IMPEDANCE NOT IMPORTANT)
— T
— 'vww-J
-0 IDAMPLIFIER
SYSTEM
RL>
1
IM,
1
Mt
I
1
Mi and M2, similar decibel meters.
cnn
Gain = Mt decibels Ml decibels + log ^ + 49 decibels. Fig. 6 — Measurement of gam of phototube amplifier.
an idealized film recording, having 50 per cent average transmission and 100 per cent modulation (this being the maximum sinusoidal film level theoretically possible), would produce a power in the 10-megohm load on the reference pickup of just 1.0 microwatt or a power level of —37.8 decibels relative to 6 milliwatts. The level which any film would develop in the load on the reference pickup is the film level, which is therefore defined as
film level = 20 log 2TM 37.8 decibels (4)
where T is the average transmission and M is the modulation expressed as a decimal fraction. If the densitometric constants of the recording are known, the film level may be computed from (4).
With film level completely defined, the related definition for relative pickup sensitivity is established. The reference pickup is assigned a relative pickup sensitivity of 0.0 decibel. The idealized recording with 50 per cent average transmission and 100 per cent modulation, will cause the phototube current to swing from /o, the phototube current with no film, to zero, or a total peak-to-peak swing of /<>