We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
2 Ratio of
46 8 10 12
shot noise power per cycle thermal noise power per cycle
Fig. 5. Optimum h/\c vs. noise-power ratio.
total noise must be at an h/\c ratio between these two minima. The optimum slit height will thus be a function of the ratio of shot noise to thermal noise. The individual noise components can be added vectorially for various assumed ratios of shot noise to thermal noise, and the total noise plotted against h/\c, with noise ratios as a parameter. The locus of the minimum of this family of curves may then be plotted as optimum h/\c versus the ratio of shot noise to thermal noise, as in Fig. 5.
It has been shown5-8 that shot noise and thermal noise are equal when the product of the average phototube current by the effective* phototube load resistance is 50 mv. This relation may be used to plot shot noise-to-thermal noise ratio versus the average voltage across the phototube load. Combining this
* Whereas the d-c voltage is developed across the phototube anode resistor, the noise components appear across the effective a-c load impedance in the anode circuit.
information with the data of Fig. 5 results in the curve of Fig. 6 in which optimum h/\c ratio is related to millivolts drop across the phototube load resistance.
The preceding relations apply when the phototube is a simple vacuum type. The results are modified in two respects when gas phototubes are used. The high-frequency discrimination of gas phototubes modifies the spectral distribution of the phototube shot noise relative to the spectral distribution of the amplifier thermal noise since the frequency discrimination arises in the phototube prior to the source of the thermal noise. Calculations have been made taking this factor into account, with the result that the effect of gas-phototube frequency discrimination is one of negligible magnitude in the determination of optimum slit height. The second effect of a gas phototube is to multiply the ratio of shot noise to thermal noise by the gas amplification factor. Thus, the millivolt scale of Fig. 6 must be divided by the gas amplification factor when this curve is applied to gas phototubes. This same effect would apply were a photomultiplier tube to be used for sound reproduction. The obvious practical effect of using a photomultiplier tube would be the elimination of thermal noise, so that only the curve of Fig. 3 would be pertinent.
Measured Optimum Slit Height
Measured data were taken using a 16mm sound-film reproducer designed some years ago by J. G. Streiffert, of these Laboratories. This machine is well adapted to measurements with various slit heights by virtue of its doubleslit optical system with a series of interchangeable secondary slits. An 8.5-v, 4.0-amp lamp and conventional soundreproducer optical system are used to form a slit image, approximately 1.2 mils in height, at the film plane. The slit image at the film plane is enlarged 6.5 times by a microscope objective and
382
November 1952 Journal of the SMPTE Vol. 59