Patent GB-794724-A (GB)
Title: Improvements relating to synchronizing and blanking signal generators for television
IPC: H04N5/06
Filed: 1954-09-24
Abstract: 794,724. Television. SOC. NOUVELLE DE L'OUTILLAGE R.B.V. ET DE LA RADIOINDUSTRIE. Sept. 24, 1954 [Oct. 6, 1953], No. 27670/54. Class 40 (3). In a circuit for generating television synchronizing and blanking waveforms wherein a master oscillator operating at twice line frequency feeds in parallel a multi-tap delay line and a frequency divider formed of a plurality of binary stages, the leading and rear edges of the pulses forming the waveforms being determined precisely by utilizing the outputs from the delay line taps which are gated as required under the control of a gate pulse generator actuated by the frequency divider, the gate pulse generator is an additive or " and " network formed of unidirectional elements, e.g. crystal diodes, connected in appropriate combinations to the stages of the counter. In a first embodiment, Figs. 2-4, see below, the invention is described as applied to the French 819 line system. In a second generally similar embodiment, Figs. 9-11 (not shown), the invention is described as applied to the 525 line, 60 frame system. Reference is also made without giving details to the 625 line, 60 frame system. Figs. 2 and 3 show the details of the blanking and synchronizing waveforms for the French 819 line system. The line synchronizing pulse AB is of 2.5 microseconds duration and occupies a position 0.5 microsecond from the front edge of the 8 microseconds blanking interval DE. The frame synchronizing pulse GC is of 20 microseconds duration and occupies a position six half lines from the front edge of the frame blanking interval IJ which is of 41 lines duration. In Fig. 4 master oscillator 10 operates at twice line frequency, i.e. 40,950 c/s., and supplies a pulse output through a wave-shaping circuit 12 to a multi-tap delay line 14 and a frequency divider 16. The output from tap D of the delay line determines the leading edge of the line blanking wave whilst that from tap E occurring 8 microseconds later determines the rear edge. Since the camera may be operated remotely from the transmitting apparatus, the actual blanking intervals appearing in the video waveform may be subject to delay, and in order to ensure accurate positioning of the blanking intervals with respect to the synchronizing signals, which are inserted at the transmitter, taps D and E are ganged together and made adjustable along the line. The leading and rear edges of the line synchronizing pulse are determined by the outputs from taps A and B respectively, tap A normally being spaced 0.5 microsecond in time from tap D. Since the front and rear edges of the frame blanking waveform coincide with line blanking, the output from tap D serves also to determine these instants. The leading edge of the frame synchronizing pulse coincides alternately with a line synchronizing pulse and the half-line position and may be determined therefore by the output from tap A. The rear edge of the pulse occurs 20 microseconds later and is determined by the output at a tap C due to the next following pulse into the line, tap C being chosen such that half the line duration minus the delay between taps C and A is equal to 20 microseconds. The frequency divider 16 comprises a chain of ten binary trigger stages, Figs. 5 and 7 (not shown), with feedback from an output circuit 86 to stages 1, 3, 4, 7 and 8 in order to reduce the count from 1024 to 819. The divider provides outputs therefore at the frame frequency of 50 c/s. The output from the final stage is derived through network 120 and further outputs at selected instants before the final stage output are provided on leads 61 and 62 by a gate pulse generator 18 comprising combinations of rectifiers, Fig. 6, connected to selected groups of counter stages. The output on lead 61 is employed for gating the front edge of the frame blanking waveform and occurs 41 lines before the final stage output, i.e. at count 430. The output on lead 62 is employed for gating the front edge of the frame synchronizing pulse and occurs six half lines after that on lead 61, i.e. at count 436. The two counts have binary digits 1, 3, 6, 7, 8, 9 and 10 in common. Thus the network of rectifiers consists of a common part 63 together with a part 64 and 65 individual to each count. Each rectifier is connected so as to be cut-off when the associated binary stage is in the desired condition, a positive output appearing on the common lead when all the rectifiers are cut-off. A negative output is arranged to appear on lead 62 by the inclusion of phase inverter 66. The output of the master oscillator through wave-shaping circuit 12 also drives a divide-by-two stage 102 to provide an output at line frequency. The line blanking waveform is produced by bi-stable multivibrator 22 and appears at output S2. The upper stage of the multivibrator is triggered by the pulses from tap D on the delay line, alternate pulses being selected by passing the output through a gate stage 91 controlled by divider 102, and the multivibrator is reset by the output from tap E which is applied through rectifier 104 and network 106 to the lower stage. The frame blanking waveform is produced by a similar multivibrator 23 and appears at output S3. The output from tap D determines both the front and rear edges in this case, the upper stage of the multivibrator being controlled by gate 96 in response to the frame frequency output on lead 61 and the lower stage being controlled by gate 100 in response to the counter output 41 lines later through network 120. The complete synchronizing waveform is produced at output S1 by multivibrator 21. The line synchronizing pulses are produced by the outputs from taps A and B, the output from tap A being applied to the lower stages of the multivibrator through gate 98 which is controlled to transmit alternate pulses by the output from divider 102 applied through a normally open gate 92, and the output from tap B being applied to the upper stage of the multivibrator through a normally open gate 94. The frame synchronizing pulse is produced in response to the output from gate pulse generator 18 on lead 62. This closes gates 92 and 94, and opens gate 98 thereby allowing the next following pulse from tap A to trigger the lower stages of multivibrator 21 whilst the inset takes place in response to the pulse appearing 20 microseconds at tap C and applied through network 108 to the upper stage.