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Since the Standard Electrical Characteristics do not extend higher than 8000 cycles/sec, this frequency has been taken as the cutoff frequency for convenience in expressing slit height as a ratio to the cutoff wavelength, Xc. Because the electrical response cannot suddenly cease at 8000 cycles/sec, the equalizer curves of Fig. 2 have been carried beyond this frequency at a rate of loss which appears reasonable and practical. High-frequency discrimination by electro-acoustic and acoustic elements has not been included in the calculations. These factors, when known, can readily be taken into account. Their inclusion will
-20
50 100 500 IK 5K
Frequency in cycles per second
Fig. 1 Standard Electrical Characteristic.
IOK
theoretically shift the optimum slit height slightly in the direction of higher slits. Likewise no attempt has been made to include subjective factors which might weight the noise spectrum.
Theoretical Optimum Slit Height
Consider a sound reproducer consisting of an optical system which projects on the film plane a sharply defined, uniformly illuminated scanning beam. The scanning beam impinges upon a film which has constant average transmittance and a percentage modulation which does not vary with wavelength. The light transmitted by the film falls upon a phototube which is coupled to an amplifier. The amplifier is considered to be so designed that constant percentage light modulation produces an output level, the frequency dependence of which is specified by Fig. 1. If the height of the scanning beam, h, is changed, the average light level upon the phototube and the average phototube current will change proportionately. The shot-noise power generated in the phototube is proportional to the phototube current. Thermal-noise power, arising in the coupling circuit, is constant per cycle of bandwidth for a c — ' temperature.5-6 For frequencies
fixed
-30
50
100 500 IK
Frequency in cycles per second
Curve
1 0.133
2 0.318
3 0.477
4 0.634
5 0.823
6 0 . 938 1.111
h = slit height, Xc = wavelength at 8000 cycles/sec.
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Fig. 2. Equalization required to match Standard Electrical Characteristic.
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November 1952 Journal of the SMPTE Vol. 59