Patent GB-859058-A (GB)
Title: Improvements in systems for reproducing video records
IPC: H04N5/932
Filed: 1958-01-27
Abstract: 859,058. Television. AMPEX CORPORATION. Tan 27 1958 [Jan. 28 1957] No. 2698/58 Class 40(3). In a system for reproducing television signals from a magnetic tape record, the signal, which is subject to interference particularly during the blanking and synchronizing intervals, is applied to a circuit for reforming the blanking pedestals and synchronizing pulses free from interference. The invention is described as applied to apparatus as described in Specifications 798,927, 798,928, 798,929 and 798,930 where the operation of a switching circuit introduces short impulses into the signal and the use of frequency-modulation during recording results in undesirable frequency components which appear superposed on the blanking and synchronizing signals. The general procedure of the invention is to blank the signal throughout the whole of the horizontal and vertical blanking intervals under the control of the recorded synchronizing pulses, and then to reinsert synchronizing pulses derived from the recorded synchronizing pulses preferably through a gate circuit timed to pass only synchronizing pulses. The detailed operation is illustrated with reference to Fig. 6. The original signal appears through amplifier 51 and is first D.C. restored by clamp circuit 72 operated during the synchronizing signals. The synchronizing signals are separated by circuit 56 and the video component clipped at predetermined black and white levels in circuit 54 to limit interference. The signal is there-after blanked on a clamp 58 throughout the whole of the horizontal and vertical blanking intervals to establish blanking pedestals free from any interfering signals. A stretcher circuit 59 compensates for compression that occurs in other stages and the signal is then applied through gain-control and amplifier stages 61-63 to a mixer 64 where synchronizing pulses are reinserted on the blanking pedestals. The reformed signal appears through amplifier 67. Synchronizing pulses for application to mixer 64 are derived from the originals by applying the output of separator 56 to a gate stage 68. The gate receives gating impulses, which, in the case of the horizontal pulses, begin with the blanking interval and end one microsecond after the synchronizing pulse, and in the case of the vertical pulses last throughout the blanking interval. All undesired noise or frequency components except when the gate is on are thereby removed. The derived pulses are applied through amplifier 69 and circuit 65 to mixer 64 and are also applied via a further amplifier 71 to operate clamp circuit 72. The vertical gating pulses for control of gate 68 are produced from the original synchronizing pulses by a chain of circuits 74-81, and the horizontal gating pulses are produced by a free-running oscillator formed of circuits 82 and 83 which is synchronized from the pulses obtained at the output of amplifier 71. The vertical and horizontal pulses are combined in mixer 84 and applied to gate 68 via amplifier 85. The combined pulses are also applied to circuits 86 and 87 which operate to delay the rear edges of the pulses and particularly the horizontal pulses so that they extend throughout the whole of the blanking interval. The resulting pulse train is then applied via amplifier 88 to control blanking clamp 58. Detailed circuits for units 54, 56-58 and 74-88 in Fig. 6 are described with reference to Figs. 7-13 (not shown).