Patent GB-798083-A (GB)
Title: Improvements relating to the generation of synchronising pulses, especially for television
Inventor: TROTT FRANK RUNCORN
IPC: H04N9/44
Filed: 1953-12-05
Abstract: 798,083. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Nov. 25, 1954 [Dec. 5, 1953; March 26, 1954; Nov. 18, 1954], Nos. 33886/53, 8823/54 and 33427/54. Class 40 (3). Line and field synchronizing pulses for the control of interlaced scanning are generated under the control of two separate control signals which are not locked to each other in frequency, the arrangement being such that the interval between successive field pulses always contains an integral number of line periods plus half a line period irrespective of variations in the frequency difference of the control signals such having the effect of varying the integral number only. In one embodiment (Figs. 1 and 2), the control signals comprise a source of line frequency pulses 1 (which is not locked to local mains but which may be crystal controlled) supplying pulse trains (a) and (b), the negative excursions of which correspond, respectively, to the end and middle of a line and a source 4 of field frequency pulses (c) which is locked to the local mains frequency. The pulses from source 4 (C, Fig. 2), which have a period T are supplied to an Eccles-Jordan trigger circuit 5 which generates the pulse trains (d) and (e) and these pulses are employed to " open " gates 2 and 3 to which the pulses (a) and (b) are fed. The outputs of these gates are then fed to further gates 8 and 9 respectively which are opened by the pulses (c) and supply the pulse trains (f) and (g) to a combining circuit 14 and then to a shaping circuit 15 producing the pulse train (h). This latter train is supplied to a multivibrator 17 having two stable conditions, one of which is larger than the other, and it is arranged that the first positive pulse (h) during each period T triggers the multivibrator from its long term state to its short term state, it being triggered back to its long term state by a subsequent pulse (h), thus producing the pulse train (k). This pulse train, which has a period T1, which may not be constant but which always contains an integral number of line periods, plus half a line period regardless of the actual relationship between the sources 1 and 4 may then be employed to control the production of field scanning waveforms whilst pulse train may be similarly employed for line scanning control. Source 1 may produce two 180 degrees phase displaced sinusoidal waves instead of pulse signals and in this case the end of a line may be represented by each positive-going crossing of the waveform axis by one wave and the middle of a line by the positive-going crossing of this axis by the other waveform. In a modification of the arrangement of Fig. 1, the outputs of the gates 2 and 3 are directly combined in 14, the output from which is combined with the pulse train (c) to produce a pulse train similar to (f), Fig. 2. This pulse train is then employed to effect one triggering operation of multivibrator 17 and.the reserve triggering is effected by the output of the combining circuit 14 (Fig. 3, not shown). An arrangement operating according to the same general principle as that of the embodiments of Figs. 1 and 3 and including complete circuit details is described with reference to Figs. 4 and 5 (not shown). Figs. 6 and 7 illustrate a further embodiment in which the line synchronizing pulses have a fixed relationship with respect to another reference frequency which is not locked to the mains frequency and is particularly concerned with maintaining a fixed frequency relationship between the field synchronizing pulses and the chrominance sub-carrier in a colour television system. In this embodiment a master oscillator 50 generates pulses (t) of twice line frequency (and which have a fixed frequency relationship with respect to the chrominance sub-carrier) from which in a binary counter 51 pulses (u) of line repetition frequency are produced. These pulses are differentiated by means of 52, 53 and the negative " spikes " clipped by clips 54 to produce pulses (v) which are fed to a coincidence " gate " 55. When the gate is " opened " by means of a pulse (w), pulses (x) are passed to a flip-flop circuit 56 which has an unstable state of about 100 Ás. in which the desired field pulses (y) are produced. These latter pulses are also fed back, via a device 57 which introduces a delay r of less than one line period, to the counter 51 which therefore effects division by an odd number as required by half-line interlace systems. Additionally, the pulses (y) are employed, after differentiation by means of 58, 59 and clipping of the positive " spike " in clipper 60, to trip a flip-flop circuit 61 to its unstable state (which lasts for about 20 ms.) and so terminate the pulse (w) fed to the coincidence gate 55. The commencement of pulse (w) then occurs at the end of the unstable state, gate 55 being " opened " on the occurrence of the next pulse (v). To ensure that the frequency of pulses (y) remains within a predetermined difference from mains frequency, the duration of the unstable state of 61 is controlled by the output of a frequency discriminator 62 in which the frequency of pulses (y) is compared with the mains frequency. In another embodiment a master oscillator 70 (Fig. 8) operating at the chrominance sub-carrier frequency, supplies signals to a frequency dividing circuit 72 which divides by an odd integer to produce pulse of half line frequency and the division ratio of which is varied in even integral steps in dependance on variations in mains frequency by means of a frequency discriminator 73 in which the mains frequency is compared with the field frequency which corresponds to the half-line frequency pulses from 72. The latter pulses are then employed to control, via a phase control circuit 77, a second master oscillator 74, operating at twice line frequency and from the output of which line and field synchronizing signals are derived by means of suitable dividing circuits 75, 76 respectively. Specifications 458,161, [Group XL], 694,503, [Group XL (c)], 794,908 and 798,084, [Group XIX], are referred to.