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VACUUM
Fig. 8. An enlarged view of the cathode-ray-tube screen.
Phase Correction: This is required because the frequency correction tends to introduce some additional phase distortion, and also to make more apparent phase errors occurring earlier hi the chain of equipment preceding the television-recording system.
Contrast Gradient Correction: Owing to the density /log-exposure characteristic of the film it is necessary to overemphasize the tone separation in the highlights and darker tones of the picture to obtain correct tone reproduction on the film. By the use of a nonlinear amplifier, the contrast gradient is increased by a factor of about 2:1 (see Sec. 6).
Lift Correction: For a television picture of full tone range, the difference between the voltage corresponding to the darkest
tone in the picture (picture black) and the voltage corresponding to the tip of the synchronizing pulses ( black level) is known as the lift. For television signals derived j from most cameras, this is varied by a manual control manipulated by the camera operator. The contrast-gradient correction employed renders the vision channel j particularly susceptible to changes in lift, and a control is therefore provided to .1 enable minor adjustments to be made.
(5.3) The Display Units
To minimize the loss of quality in a television-recording system, it is essential < that the recording monitor should be of the highest attainable standard. Fine focus, maintained over the whole screen even at peak modulation, is particularly important. A substantially perfect interlace is also essential. Freedom from drift in cathode-ray-tube bias and drive over long periods of time is a necessity, since no adjustments to individual display units can be made once a recording has commenced. The geometrical distortion introduced by the recording monitor must also be extremely small.
The vision frequency equipment installed at the Lime Grove studios was built to a B.B.C. specification covering the above points.15 The response between 10 kc and 3 me was specified as constant within ±0.5 db.
(5.4) Cathode-Ray Tubes for Recording Purposes
Film emulsions, even those of the panchromatic type, tend to possess maximum sensitivity at the blue end of the spectrum. This alone would suggest the choice of a blue phosphor such as silveractivated zinc sulfide for the recording cathode-ray tube, and this choice has the following additional advantages:
(a) The phosphor can be made with a very fine grain structure.
(b) The efficiency is high.
(c) The afterglow is short.
(d) The use of a single colour helps optical resolution as the lenses used need not be so accurately colour-corrected.
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April 1953 Journal of the SMPTE Vol.60