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

tion. When the amplitude of the deflection is adjusted to a critical value, the apparent number of lines in the picture is thus doubled. The size of the scanning spot and limiting resolution of the film emulsion and optical system then prevent line structure being recorded. In America the spot-wobbling tech nique is not normally applied because the resolving power of the 16mm film, mostly employed, is not adequate to produce a well-defined line structure with 479 lines in the picture height. The more extensive use of 35mm film in America would almost certainly render spot-position modulation necessary. (5) THE VISION-FREQUENCY EQUIPMENT (5.1) General Layout The operational experience obtained on the two experimental recording equipments originally installed at Alexandra Palace led to the conclusion that a recording suite, intended for the continuous recording of programmes, should consist of three channels. This would allow one channel to be in use while the second channel was being loaded ready for change-over, the third channel acting as a spare. In the event of breakdown, it should be possible to have the spare channel in operation within two or three seconds. The three recording monitors should be adjusted and ready for use at all times, and, should a change be made in the operating conditions during a recording, it should occur simultaneously on all three monitors. At the Lime Grove television studios, the signals to be recorded are passed into a vision channel where the synchronizing signals are first removed. Control of gain, lift, frequency correction and contrast-gradient correction are then applied. The corrected vision signals and separate line and frame-synchronizing signals are passed to distribution amplifiers which feed the recording picture monitors or display units. The electrical controls for the vision channel are centralized at a control desk, which has built-in picture and waveform-monitoring facilities. It is possible to observe the incoming picture on a white cathoderay tube and to compare this with a duplicate of the picture appearing on the recording cathode-ray tubes, provided by the additional display unit No. 4. A spare vision channel is provided. Before the start of a recording, test waveforms are applied to the vision channel in use and the local amplitude and cathode-ray-tube bias controls on each display unit are adjusted to give the same transfer characteristic. Measurements are made using a multiplier photocell mounted in a probe held in contact with the glass surface of the cathode-ray tube, so that the brightness of a small area of the screen can be taken. A typical test waveform produces a picture of 15 squares, each of a different brightness. Five squares are disposed horizontally and three vertically. The amplitude of the voltage corresponding to each square varies between zero and 100% of peakwhite modulation, in linear steps. By measuring the brightness of each square in turn, the picture-monitor transfer characteristic can rapidly be checked and made similar on the four display units. To facilitate change-overs the cameramotor controls and the sound and vision faders are situated on the control desk. (5.2) Electrical Correction The electrical corrections introduced in the vision channel are as follows : Frequency Correction: To compensate for the loss of fine detail introduced by the camera optics and film, a high-frequency lift is applied in the vision channel. Since the losses can be expressed as an aperture effect, the amplitude of the lift is made approximately equal to the square of the frequency. A typical value of lift would be 9 db at 3 me. W. D. Kemp: Television Recording — Abstract 379