Patent GB-880053-A (GB)
Title: Improvements in or relating to television signal converting apparatus
Inventor: HELSDON PETER BENNETT
IPC: H04N7/01
Filed: 1959-12-15
Abstract: 880,053. Electric signal transmission systems. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. July 1, 1960 [Dec. 15, 1959], No. 42574/59. Class 40 (4). [Also in Groups XL (b) and XL (c)] In a scanning standards connector in which pictures reproduced from television signals having a first set of scanning standards are " viewed " by a television camera operating according to a second set of scanning standards to produce signals having the latter standards, the field scanning of the camera is controlled by synchronizing signals derived from an oscillator operating at a common multiple of the field frequency of the two standards and controlled in phase and frequency by the field synchronizing signals of the first scanning standard. With this arrangement a fixed " phase relationship " between the field scanning frequencies of the two standards is maintained and this allows correct operation of a compensating gains control circuit to compensate for " flicker " due to an unwanted signal amplitude variation which occurs at a frequency equal to the difference of the two field frequencies in the television signals produced by the camera. Fig. 1 shows one embodiment for connecting television signals of 405 lines and 50 fields per second to signals of 525 lines and 60 fields per second by displaying pictures according to the former standard on a picture tube 10 (which has an after-glow period of about one field period of these standards and is operated with " spot-wobble ") which is " viewed " by a television camera 9 the scanning of which is effected according to the latter standard. In accordance with the invention the field scanning of camera 9 is controlled by synchronizing signals (60 c.p.s.) derived via a generator 8 and - 5 divider 7 from a freerunning oscillator 4 operating at 300 c.p.s. (i.e. 50 x 60) and the frequency and phase of which are controlled by the output of a phase comparator 5 to which the 300 c.p.s. signal is supplied via + 6 divider 6 and the field synchronizing signals (50 c.p.s.) derived from a field pulse separator 3. The input to the latter is derived from a separator 2 which separates out the line and frame synchronizing signals in the composite television having the first scanning standard and supplied at terminal 1. To compensate for the " flicker " (referred to above) which, considering the two field scanning rates involved, occurs at a frequency of 10 c.p.s. the composite signal at terminal 1 is supplied to the intensity modulating electrode of tube 10 via an amplifier G the gain of the valve G 1  of which is controlled in accordance with curve (b), Fig. 2, by supplying to the common connection of the control grids of G 2 , G 3  via amplifier 19 a voltage waveform having a characteristic as shown by curve (a), Fig. 2. This waveform is generated by a chain of five trigger tubes F 1  . . . F 5  the first of which (F 1 ) is " fired " by the simultaneous occurrence on the trigger electrode of the field synchronizing signals of the two standards (supplied via amplifiers 11 and 15 respectively and a D.C. bias source 18) and the remainder (F 2  . . . F 5 ), by the occurrence and for the duration of the next four field synchronizing signals respectively of the first standards (from amplifier 11). By adjustment of resistors F 6  . . . F 10  an integrated voltage is built up across capacitor F 16  which has the waveform shown by curve (a), Fig. 2, the repetition period of which is 10 c.p.s. and each cycle of which commences with the " firing " of the tube F 1  and extends over five field periods of the first standards. To improve the linearity response of the tube in camera 9 the black level of the signals supplied to reproducing tube 10 are clamped to an adjustable reference level (e.g. 20% of peak white) controlled by known means 20 such that the clamping signals (applied to the anode of G 1  are also subjected to the same compensating gain control. The converted signals having the second scanning standards are then derived from camera 21 for utilization in any desired manner, e.g. for recording on magnetic tape. Instead of inserting the amplifier G in the signal supply to the reproducing tube 10 it may be positioned in the signal output of camera tube 9 (Fig. 4, not shown). In this case each cycle of the gain control voltage must extend over six field periods of the second scanning standards (Fig. 5, not shown) and, consequently an additional trigger tube is necessary in the chain (Fig. 4, not shown). The arrangement described may be adapted for conversion between standards other than those described above.