Patent GB-673933-A (GB)
Title: Improvements relating to pulse generating apparatus
Inventor: WHITE ERIC LAWRENCE CASLING
IPC: H04N5/06
Filed: 1948-07-01
Abstract: 673,933. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 17, 1949 [July 1, 1948], No. 17694/48. Class 40 (iii). [Also in Group XL (c)] In a pulse generating circuit a stepped waveform is generated under the control of master pulses so that the steps occur at points determined by the master pulses and periodic recurring pulses are generated, the timing of the leading and trailing edges of which are set by predetermined increments in the steppedwaveform. Fig. 1 shows such a circuit used in a television transmitter system in which pulses of frame frequency have their leading edges accurately located in the line periods. Pulses of double line frequency and of duration equal to 0.25 of a line period, Fig. 2 (a), are generated by the master pulse generator 1 and are applied to the line synchronizing pulse generator 2 and to the line scanning generator 3 associated with the television transmission tube 5. Pulses from the master oscillator 1 are also applied to the frequency divider circuit 6 where the frequency of the input pulses is divided by 405 times to give output pulses, Fig. 2 (b), of frame frequency. The divider may be of the type described in Specifications 471,731 and 659,799, [Group XL (c)]. The pulses are applied to a trigger-pair circuit 7 and cause it to change over to a second stable condition, thus generating a positive pulse, Fig. 2 (c), once per frame, which is applied to diodes 8, 9 which conduct only during the presence of these frame pulses to let master oscillator pulses through to the differentiating circuit 15, 16. The trailing edges of the differentiated pulses which are passed by diode 17 are applied to the step counter valve circuit 19 which produces a stepped wave, Fig. 2 (d) at its anode during each positive pulse of Fig. 2 (c) as described in Specification 673,935. The stepped wave has increments at twice-line frequency occurring at approximately 0.25 and 0.75 of line periods and is fed back to reset the trigger-pair 7 at the last increment of the stepped-wave, thus terminating the frame pulse of Fig. 2 (c). The frame pulse of Fig. 2 (c) is used as a frame suppression pulse and is fed to the video amplifier 23 to suppress the amplifier output during the intervals between alternate frame scanning traversals in tube 5. The paraphase versions of the stepped wave of Fig. 2(d) are D.C. restored by circuits 25, 26, 27 and 28, 29, 30, respectively, and are fed to the mixing circuit comprising diodes 31, 32 to produce an output waveform, Fig. 2(e), across the common anode load 33 which has a most positive portion, e.g. as shown at 36 in Fig. 2 (e) during a given increment of the stepped wave which is predetermined by the setting of the bias resistor 34. Units 37-40 are mixing circuits similar to that comprising diodes 31, 32, each of which has its biasing resistor set at some predetermined value, e.g. mixer 37 has an output waveform as shown in Fig. 2(f). The output from the mixer 37 and that comprising diodes 31, 32 are applied to trigger and reset the trigger pair circuit 43 to give an output pulse Fig. 2(g) having a duration dependent on the most positive increment in the waveforms of Figs. 2(e) and (f), respectively. The pulse of Fig. 2(g) is used as a frame blanking pulse and is applied to cut-off the beam of tube 5 during the frame fly-back periods. The outputs of mixers 38, 39 are applied to trigger and reset the triggerpair circuit 42 to give an output pulse, Fig. 2(h), which is delayed in unit 45 to produce an output pulse shown at 46 in Fig. 2(h) which is used as a frame keying pulse. The frame keying pulse is applied to a gating circuit 47 to let through double line frequency pulses from the master oscillator 1 via the shaping circuit 48, in periods when the frame keying pulse is not present the gate passes line synchronizing pulses from the generator 2. The combined line and frame synchronizing signals from the gate 47 are mixed with the video signals from amplifier 23 and modulate the carrier in modulator 49. The output of mixer 40, Fig. 2(j) is fed to amplifier 54 to render it conducting during the maximum positive-going increment 53 of Fig. 2(j) to produce an output pulse, Fig. 2(k), at frame frequency which is used to terminate the output of the frame scan generator 55 during the intervals when the tube 5 is cut-off by the frame black-out pulses of Fig. 2(g). By adjustment of the bias resistors 34 &c. the duration and timing of the frame frequency pulses may be varied by intervals which are any integral number of halfline periods. A modification shown in Fig. 3 produces a frame suppression pulse which starts and stops at the beginning of a line period in all frames of an interlaced scan, thus necessitating unequal spacings between alternate frame suppression pulses. The output of trigger-pair circuit 7 is not used as the frame suppression pulse source as described in connection with Fig. 1 but functions as a keying pulse source which operates two stepped wave generators 18 and 60. The pulses of Fig. 2(c) have a duration greater than that of the frame suppression pulse required and start and finish at 0.25 and 0.75 of a line period in successive frames as shown in Figs. 4(m) and (n). The switching and differentiating circuit 61 which is similar to circuit 8-17 of Fig. 1, has pulses of single line frequency applied to it and the output of the stepped wave generator 60, Fig. 4(p), has steps each of line period duration and synchronized with the line periods. The output of generator 60 is fed to a circuit 62 which is similar to the combined amplifier 20-22 and D.C. restorers 25-30 of Fig. 1, and then to two combining circuits 63, 64 which are similar to the circuit 31-35 of Fig. 1. The output of circuit 63, Fig. 4(q) has a most positive level at point 65 and that of circuit 64, Fig. 4(n) at point 66. The two outputs are applied to trigger and reset the trigger pair circuit 67 to produce frame suppression pulses as shown in Fig. 4(s).