Journal of the Society of Motion Picture and Television Engineers (1950-1954)

I I I HIGHLIGHT SIGNAL-ELECTRODE CURRENT= 0.35/ia 1 MEGACYCLE = 80 LINES (APPROX.) TEST PATTERN: TRANSPARENT SQUARE-WAVE RESOLUTION WEDGE 10 1 1 tr 4.5 MEGACYCLE -•—BROADCAST BANDWIDTH-*-] < cc X. ^S> m a. o^ Ny \ ; ^OMPENS ATED PONSE-A \\ \ \ i \ \ \ u UNCOMPENSATED\ \ \j D \ V Sy EC UIVALEN" RESPON SE^S;^ -\ C\ "^ 100 200 300 400 TELEVISION LINE NUMBER Fig. 4. Improved horizontal resolution (compensated curve) obtained by applying aperture correction to the output of the 6326 vidicon. The curve labeled "Equivalent Response" represents the geometric mean of the horizontal and vertical resolutions after aperture correction. mentioned previously which enable the vidicon to produce superior performance. Aperture correction is the process of compensating for the drop-off in amplitude response shown in Fig. 3. Correction for this drop-off is accomplished by boosting the high-frequency response of the video amplifier in such a manner that the amplitude response curve is linear up to some predetermined cutoff frequency and compensating for the phase shift introduced in the frequencyresponse boosting process.2 The vidicon is the first camera tube to develop a high enough signal-to-noise ratio to allow effective use of aperture correction. The application of aperture correction does not increase the limiting resolution of the vidicon, but it does make it possible to boost the horizontal resolution to 100% over the entire transmitted bandwidth as shown in Fig. 4. The role which the high signal-tonoise ratio of the vidicon signal plays in aperture correction is as follows: The noise energy in the signal from the vidicon camera is contributed principally by the first amplifier stage. This type of noise is proportional to frequency, with the result that nearly all the noise energy in the video signal is concentrated in the higher frequencies. Boosting the signal information at the higher frequencies does not increase the overall signal amplitude, but it does boost the noise, with the result that the signal-to-noise ratio decreases almost directly as the amount of boost increases. A maximum boost of approximately 3 to 1 at the 350-lines point will reduce the signal-tonoise ratio of the picture to approximately 100 to 1, which is still a very satisfactory value. It should be noted that this type of aperture correction does not boost the vertical resolution of the vidicon tube. The resulting equivalent amplitude response of the vidicon tube output signal is therefore a combination of the vertical and the horizontal amplitude responses. The resulting equivalent response is shown in Fig. 4. This curve is the square root of the product of the uncompensated and the compensated curves, and represents the overall performance of the tube with aperture correction of 3 to 1 at the cutoff frequency of 4.5 me. The resulting signal-to-noise ratio is still higher than that of any camera tube or film-pickup system in current use. Signal-Electrode Voltage We have seen how the high signal-tonoise ratio of the vidicon can be used to improve its resolving capabilities. The second operating technique which clears up the remaining possible objections to the vidicon as a film-camera tube is the use of a signal-electrode voltage considerably lower than the value recommended for maximum sensitivity. The use of reduced values of signalelectrode voltage for the vidicon improves 146 February 1954 Journal of the SMPTE Vol. 62