Patent GB-1104177-A (GB)
Title: Television recording and reproducing apparatus
IPC: H04N5/91
Filed: 1965-05-31
Abstract: 1,104,177. Recording television signals. WINSTON RESEARCH CORPORATION. 31 May, 1965 [12 June, 1964], No. 23017/65. Heading G5R. [Also in Division H4] In an apparatus for recording television signals along a longitudinal track on a magnetic tape the television signals are applied to a blocking circuit which suppresses the signal during a part of the horizontal blanking interval which includes the horizontal synchronizing pulse and in place thereof there is substituted a secondary synchronizing signal in the form of one cycle of an approximately sinusoidal wave. In addition a further amplitude modulated pulse conveying sound information may be inserted in the interval following the synchronizing signal. The final recorded signal takes the form shown in Fig. 4H. The black level is retained at the beginning and end of the blanking interval, whilst the synchronizing and sound signals appear superposed on a predetermined reference level intermediate black and white. In reproduction the signal is applied to further blocking circuits to separate the synchronizing and sound signals from the picture signals. The sound information is extracted by a low-pass filter having a cut-off frequency less than horizontal scan frequency, whilst the synchronizing signal is employed to generate a conventional synchronizing pulse. For this purpose a trigger circuit is conditioned just before the synchronizing signal and is arranged to respond to the positive to negative transition. The trigger circuit, or a generator driven thereby, furnishes synchronizing pulses with a slight delay which is offset by a corresponding delay of the picture signal component. In accordance with a feature of the invention there is provided in the recording and reproducing channels preemphasizing noise reducing circuits (not described in detail) which have non-linear amplitude response characteristics such that the emphasis depends both on amplitude and rate of change whereby low-amplitude high-frequency signals are given more emphasis than high-amplitude signals. The channel to the recording head also includes an equalizing stage, Fig. 7 (not shown), which emphasizes high and low frequencies to compensate for head deficiencies. The reproducing head channel includes an equalizing and peaking circuit, Fig. 9 (not shown), in which the entire frequency spectrum of the reproduced signal is applied to the grid of differential amplifier (160) via a delay line (152), whilst the high- and midfrequency components with adjustable delay and amplitude are derived via transistors (162), (164) and (166) and applied to the cathode. The amplifier output is applied to adjustable peaking circuit (78) via impedance matching transistors (174), (176). The blocking or gate circuits for separating the picture component from the synchronizing and audio signal are shown in Fig. 10 (not shown) at (88) and (99) respectively. The reproduced signal is applied via filter (186) and emitter follower (188) to point (204) and thence via impedance matching stage (190) to the gates. The gates are opened at appropriate times by pulses from source (222). The signal at point (204) is D.C. restored during an interval immediately preceding the synchronizing signal by clamp circuit (196), (198) actuated by pulses from a source (208). The clamp restores to a potential in part determined by potentiometer (194) and in part by a potential related to the average brightness of the picture signal which is obtained by integrating the output of gate (88) by circuit elements (200), (202). The blocking or gate circuits for the signal to be recorded, Fig. 6 (not shown), are similar apart from the D.C. restoration feature. The pulses to actuate the blocking or gate circuits for the signal to be recorded are derived from the horizontal scanning circuit of an associated cathode-ray reproducer, the required timing being established by delay lines or trigger circuits. The pulses to actuate the signal reproducing circuit are obtained by a closed loop system synchronized by the reproduced synchronizing signals and introducing a one line delay. The secondary synchronizing signal is established by a circuit, Fig. 8 (not shown), in which the original synchronizing pulse is applied through transistor (130) to a shortcircuited delay line (132) so as to introduce a delayed pulse of opposite polarity, and the resultant combination of positive and negative pulses is shaped to approximate a sine wave by the circuit of transistor (138).