Patent GB-943621-A (GB)
Title: Improvements relating to pulse separating circuits
IPC: H04N5/08
Filed: 1960-01-25
Abstract: 943,621. Television. TELEFUNKEN G.m.b.H. Jan. 25, 1960 [Jan. 23, 1959; Jan. 27, 1959], No. 2567/60. Heading H4F. In an arrangement for separating synchronizing pulses, or disturbing pulses, from a composite television waveform, the latter is supplied to the control grid of a valve having at least two control electrodes and to the anode of which keeping pulses are applied during sampling periods in which a predetermined level, pulses above which are to be separated, occur, the anode conduction during this period then providing a bias for the control electrode which, in the intervals between sampling periods, allows the valve to conduct to the second (screen) control electrode only in response to pulses which exceed the predetermined level. Fig. 1 shows the arrangement for separating synchronizing pulses 16 from a composite television waveform 2, in which the latter is fed to the control grid of valve 1, the anode of which is supplied with keying pulses 10 (derived from the scanning circuit during fly-back periods) occurring during the " back-porch " intervals so that the valve conducts to the anode during such intervals and by giving the time constants of the circuits associated with capacitors 8 and 9 appropriate values a bias voltage is developed at the control grid which, during the intervals between pulses 10, allows valve 1 to conduct to its screen grid only when the level of waveform 2 exceeds that determined by the bias voltage. Since the latter is determined by the " back-porch " level the synchronizing pulses (which exceed this level) are developed at the screen grid and are derived as pulses 14 of the output of a limiter 13. In the arrangement of Fig. 5, which is concerned with separating disturbing pulses, such as pulses 22, from the composite television waveform 2, the latter is supplied to the control grid of valve 34 (equivalent to valve 1 of Fig. 1) but in this case the anode keying pulses 21 are made to coincide with the synchronizing pulses 16 so that the bias developed at the control grid allows conduction to the screen grid only for pulses which exceed the level of the synchronizing pulses. The disturbing pulses 22 are thus developed alone at the screen grid and although some of the pulses 22 are superimposed on the synchronizing pulses 16 their energy is insufficient to materially affect the bias level which is mainly determined by the level of the synchronizing pulses. Fig. 6 shows an arrangement similar to Fig. 5 with the addition of a circuit 25 which produces a D.C. bias voltage for the cathode of valve 34 to inhibit operation of the latter when the keying pulses 21 cease to coincide with the synchronizing pulses 16, the latter being derived by known means or by an arrangement similar to Fig. 1. This facility is provided, when the separated disturbing pulses 22 are utilized to inhibit operation of a synchronizing pulse separator circuit (of conventional type or one similar to the arrangement of Fig. 1) to remove such disturbing pulses from its output, in order to prevent the synchronizing pulse separator from being inhibited for periods which include the occurrence of the synchronizing pulses themselves if, due to lack of coincidence between the keying pulses and the synchronizing pulses, the level at which valve 34 conducts to its screen grid departs from the required level. A combined circuit is described with reference to Fig. 8 (not shown), which includes a conventional synchronizing pulse separator (but which may take the form of Fig. 1) together with the arrangement of Fig. 5, the separated disturbing pulse output of which is employed to remove such pulses from the synchronizing pulse separator. The circuit, additionally, provides coincidence circuit means 25 (Fig. 5), for deriving the D.C. inhibiting bias for the disturbing pulse separator and also includes a phase comparison circuit in which the phase of the separated synchronizing pulses is compared with that of the keying pulses the comparison providing a control function which may be employed to correct the operation of the line-scanning oscillator. When the composite video signal supplied to the disturbing pulse separator includes the D.C. component (e.g. the composite video signal at I.F. frequency) the voltage variations at the anode of the separator may be employed as a gain-control voltage (Fig. 7, not shown).