Patent GB-1155347-A (GB)
Title: Apparatus for Recording Video Signals on Magnetic Tape.
Inventor: BELL NORTON WILLIAM
IPC: H04N5/91
Filed: 1967-08-22
Abstract: 1,155,347. Magnetic video tape recording. BELL & HOWELL CO. 22 Aug., 1967 [22 Aug., 1966], No. 38549/67. Heading G5R. [Also in Division H4] To balance out delay distortion in video signal magnetic tape recording and playback apparatus, a composite video signal including a pattern of horizontal and vertical synchronizing pulses defining the commencement of respective horizontal scanning lines and vertical line sequences is recorded on a magnetic tape driven past a rotating transducer head scanned across the tape at an angle to its direction of movement, and the recorded signals are played back by moving the tape in the same direction as during recording past a position at which it is scanned by a rotating transducer head moving along each recorded track in the direction opposite to that of head movement during recording. Preferably the played-back signals are reproduced by applying them to modulate a scanning beam deflected in a series of horizontal lines synchronous with the horizontal synchronizing pulses in the played-back signal, the direction of scanning of each line being opposite to the direction of line scanning during development of the initial video signal. As shown in Fig. 1, magnetic tape 10 is drawn by a capstan 16 (acting also as a guide roller) from a supply reel around a guide roller 14 and around a fixed drum 12 having a slot 20. The tape is scanned through the slot 20 by magnetic heads 22 and 24 rotated by a motor 28. Reversal of rotation of the heads during playback, as required by the invention, is provided for by a switch 34, which governs the direction of rotation of the motor 28. The motor 28 may be a synchronous motor including a split phase starting winding of which the phase is reversed by the switch 34 to reverse the direction of rotation. Control of proper tracking between the heads and successive tracks on the tape is provided by servo means controlling the motor 18 in a conventional manner. However, because the tape is moving in the same direction both on recording and on playback the foward motion of the tape subtracts from the angle of the track when the heads are rotated in one direction and adds to that angle when the heads are rotated in the reverse direction. This difficulty is overcome by providing guide roller 14 with flanges and making it adjustable along its axis so as to adjust slightly the path of the tape as it passes around drum 12. A complete recording, playback and reproducing arrangement is shown in Fig. 2. The output, from a video receiver 36 for receiving a broadcast television signal, or from a camera, consists of a composite video signal including horizontal and vertical synchronizing pulses together with amplitude-modulated video information. The signal is frequency-modulated at 38, amplified at 40, and fed to the heads 22, 24 through switch 42 for recording. At the same time the signal from receiver 36 may be applied through a switch 44 in the record position to a sync. separating circuit 46. The video information signal is amplified by a video amplifier 48 and applied to modulate the beam of a cathode-ray picture tube 51 in conventional manner. The horizontal and vertical synchronizing pulses are applied to horizontal and vertical scan generators 50 coupled by switches 52 and 54 respectively to tube 51 so that when the switches are in the "record" position, as shown, the normal left-to-right horizontal line scan and top-to-bottom vertical scan are produced. Playback.-It is assumed that the speed of motor 28 has been selected so that one complete frame of information is laid down in a single slant track scanned by one of the heads 22, 24. During playback, all the information in a single track is reproduced in reverse time sequence. This time reversal requires reversal of both the horizontal and vertical scans of tube 51, which is accomplished by switching switches 52 and 54 to the playback position, whereupon sawtooth signals of reverse polarity are applied to the deflection systems of the tube 51. (Switches 42 and 44 are in the playback position). Alternatively, where each pass of a scanning magnetic head records the video information making up a group of lines less than a complete frame, (e.g. one quarter), the vertical scan must be reversed during the reproduction of each group of lines, but the successive groups of lines must be positioned vertically in the order in which they normally occur. The waveform of the vertical scan signal has its slope reversed during each quarter interval of the vertical sweep, in this case, but the average slope is the same as in the normal scan. Where relatively narrow tape is used, and the magnetic heads are rotated substaintially transversely to the direction of tape movement, a single track on the tape may correspond to only a single line of video information. In this case, only the horizontal scan is reversed during playback while the vertical scan is normal.