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298 C. E. K. MEES [J. S. M. P. E.
m = *ma%. *av. = *av._^inm. = ^
J. av. *av. •*• 0
Since the amplifier current can never be less than zero, the value of m can never exceed unity.
If, now, the variations in the exposure of the film are proportional to the variations in the current I, the exposure of the film may be represented by equation 4 :
e = EQ + E sin wt (4)
where e is the value of exposure corresponding to a sound pressure, Po 5 EQ is the value of exposure when there is no sound before the microphone ; and E is the maximum value of the sinusoidal variation of exposure.
The condition for correct tone reproduction in the photographic phase of the problem is simply that the current in the photoelectric cell of the reproducer be expressible by an equation in the form of equation 2:
i' = V + KI sin a>t (5)
The constant term /0' in eq. 5 is independent of the constant term /o in eq. 2. However, the amplitude of the variable term in eq. 5 must be proportional to the variable term in eq. 2, as indicated by the proportionality constant K; that is, the transmission of the positive sound-track must be linearly related to the exposure of the negative. The transmission of the positive sound-track will vary, therefore, from an average value, TQ, to a maximum value, TQ + T, and a minimum value of TQ — T, and T/TQ is the modulation.
In the case of a variable-width record, the modulation is proportional to the width of the track covered by the silver deposit, the average transmission being that where half the width is occupied; the maximum transmission where all, or nearly all, the track is clear; and the minimum transmission where all, or nearly all, is covered. This average transmission in either type of sound record exists when no sound strikes the microphone. The transmission of the film, however, is not absolutely constant owing to surface imperfections, such as dirt, dust, scratches, etc., and also to the granular structure of the silver deposit. These changes in transmission cause a certain minimum of modulation which, when converted into sound, is known as ground-noise.