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Curve C gives the relation between film noise level and slit height (signal level held constant) with measured values indicated by the circles. Note that the location of these curves on the ordinate scale has no significance. The absolutelevels depend upon such factors as illuminance level, phototube sensitivity, phototube load resistance, film density, and film granularity. Accordingly, the experimental curves have been adjusted on the ordinate scale for best fit with the theoretical curves. A further check of the theoretical results may be obtained by making use of the relation between the phototube d-c output voltage and the ratio of shot noise to thermal noise. The measured gas-amplification factor (anode volts = 52) is 2.3, so that unity noise ratio should be obtained when the d-c voltage drop across the phototube effective load is 22.5 mv. The measured value is 23.8 mv.
Discussion of Results
The optimum slit height for minimum electrical noise level is, from Fig. 7, some value of h/\c between 0.71 (minimum phototube noise) and 0.81 (minimum amplifier noise). Using the former value gives an electrical noise level not more than 0.4 db higher than the true optimum. An h/\c value of 0.71 gives actual slit heights of 0.64 mils and 1.60 mils for sound-reproducer speeds of 7.2 in. /sec and 18 in. /sec, respectively. The corresponding slit loss at a frequency of 8000 cycles/sec is 8.9 db. A slit loss of this magnitude is undesirable, partly because of the required equalizing, but chiefly because relatively small differences between actual slit heights and design value will cause appreciable changes in overall frequency response. If phototube noise is much greater than amplifier noise, there is considerable tolerance in the choice of slit height. Then h/\c may have values from 0.44 (at which value the slit loss is but 3 db) to 0.9 without increasing the noise level by more than 1 db.
1-4
0)
6-8
-12
-14
6. Amplifier noise I I I
.6 h/X,
8 1,0
Fig. 7. Relative noise power vs. h/\c:
= Theoretical curves; O, D, X = Experimental points; Curves adjusted to 0 db at h/\c = 0.133.
Because phototube noise voltage is proportional to the square root of illuminance, the most adverse conditions, with respect to the ratio of phototube noise to amplifier noise, exist when noisereduction sound tracks are used. The illuminance on the phototube during a silent portion of a noise-reduction track may be only about 5% of the illuminance with no film in the reproducer. Assuming the phototube to have a gas-amplification factor of 6 and no film in the light path, the d-c level across the effective phototube load resistor must be about 300 mv (this is equivalent to 106 rms mv for 100% light modulation, as by a sinusoidal light chopper) for phototubenoise power to be double the amplifiernoise power when a fully biased noisereduction sound track is reproduced.
Grimwood and Horak: Reproducer Slit Height
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