Patent GB-864417-A (GB)
Title: Improvements in or relating to control arrangements for synchronizing impulse generators
IPC: H04N5/06
Filed: 1957-11-15
Abstract: 864,417. Automatic frequency control. FERNSEH G.m.b.H. [Nov. 15, 1957 ,Nov. 16, 1956], No. 35645/57. Class 40(5) Apparatus, particularly for television systems, for synchronising a second pulse train by frequency division of a master oscillation with a first pulse train of like frequency comprising means responsive to any phase difference between said pulse trains and operative to alter the effective number of pulses during each repetition of a predetermined time interval in a sequence of pulses derived from said master oscillator so as to reduce said phase difference, the operation of said means being such that, as said phase difference decreases, the effective number of pulses altered also decreases until, at synchronism, no alteration is made in the number of pulses. In the embodiment of Fig. 1 (for a television system) a pulse train of twice the line frequency is generated in master oscillator 1 which is applied to an additive mixer 2 the output of which is applied to a pulse generator 3, including a frequency divider to provide line synchronising pulses 3 at terminal 4 and local frame synchronising pulses at terminal 5. In order to synchronise the local frame synchronising pulses with external frame pulses, the local frame synchronising pulses are applied to terminal 6 and to pulse generator 7 to supply suppression pulses. The external frame pulses are applied to terminal 8 and to generator 9 generating gating pulses locked in phase to the frame pulses and have a duration exceeding that of the suppression pulses from generator 7, the relative phases of these pulses depending on the relative phases of the local and external frame pulses. The gating and suppression pulses are applied to mixer 10 the output of which comprises only the suppression pulses which do not coincide with the gating pulses. The suppression pulses at the output of mixer 10 (the number of pulses present vary directly as the phase difference between the trains of frame pulses) are applied to mixer 2 and alter the repetition rate of the master pulse train by suppressing some of the master pulses and thus bringing the local frame pulses into synchronism with the external pulses, the amount by which the master frequency is altered being reduced until the frame pulse trains come into synchronism, when the master frequency is unaltered. Alternatively, the alteration of the relative phases of the two trains of frame pulses is achieved by suppressing the individual pulses of a train of pulses derived from the train of master pulses, as by the connection 11 indicated in Fig. 1 directly to the divider circuits of generator 3. If the incoming external signal should fail the gating pulses generated from it will also fail and the suppression pulses are automatically eliminated to prevent them effecting the master pulses. A circuit for use as elements 7 and 10 of Fig. 1 is described (Fig. 2, not shown) and the suppression pulses are sinusoidal and generated by a start-stop oscillator, and the gating and suppression pulses are mixed and are applied to a valve acting as a limiter which ensures that only suppression pulses which do not coincide with gating pulses pass to the mixer 2. The automatic elimination of the suppression pulses in case of signal failure is ensured by a positive bias on a valve of the input stage to the oscillator generating the suppression pulses. In a further embodiment (Fig. 3, not shown) multivibrator circuits are used for frequency division of the master pulse train. In the embodiment of Fig. 4, the local frame pulses are applied to terminal 70 and the external frame pulses to terminal 71. The latter trigger a multivibrator 72 generating a rectangular pulse train each of duration equal to half the period of the triggering pulse, and the rectangular pulses are applied to a mixer 73 and combined with the local pulses. A limiter 74 provides a control pulse to one of gate circuits 75, 79. When a frame pulse occurs during the +ve portion of the multivibrator output gate 75 opens to allow pulses at terminal 77 (timed to occur between the pulses of the master train received at 81) to pass to mixer 76 while gate 79 is closed to block suppression pulses at terminal 80, gate 75 being closed and gate 79 opened when local frame pulses at 70 do not coincide with the +ve part of the multivibrator output. Therefore the output at terminal 82 will consist of the master train of pulses with its frequency varied according to whether gate 75 or 79 are open, both gates being closed if the pulses at 70, 71 are synchronised so that the frequency of the pulse train will be unaffected.