Patent GB-858145-A (GB)
Title: Improvements in or relating to electrical systems for reproducing recorded signals
IPC: G11B15/14
Filed: 1957-09-17
Abstract: 858,145. Television; thermionic valve distributers; generating impulses photo-electrically. AMPEX CORPORATION. Sept. 17, 1957 [Oct. 8, 1956], No. 29311/57. Class 40(3). [Also in Group XXXV] Television: thermionic valve distributers. Relates to magnetic tape apparatus for the recording and reproduction of television signals in which a number (four as illustrated) of magnetic heads 23, Fig. 2, are carried by a wheel 22 so as to be traversed successively across a moving tape 11 whereby they sweep out spaced tracks as indicated at 301 in Fig. 7. During recording the video signal is applied simultaneously to all the heads, only those engaging the tape at any instant being effective however, and the dimensions of the apparatus are such that a succeeding head crosses line 302, Fig. 7 at one side of the tape before the preceding head has crossed line 301 on the other side. In reproduction therefore the heads produce time spaced but partially overlapping portions of video signal. In accordance with the invention, these portions are switched into a common channel so as to reform the complete video signal by means of a thermionic distributer the switching action of which is arranged to occur during the periods of overlap. The operation proceeds under the control of a reference wave A, Fig. 6, at the frequency of rotation of wheel 22, 240 c/s. in the example illustrated, the wave being derived directly from the wheel by photo-electric means (see below). The wave is employed to produce a first series of 240 c/s. square waves F-I of which the second pair are displaced by one quarter of a period with respect to the first pair, and is also doubled in frequency as shown at C and employed to produce 480 c/s. square waves D and E. Phasing means are included to permit the waves to be timed accurately with respect to the traversing actions of the heads, and the arrangement is such that each half period of waves D and E coincides with the effective period of traverse by a head. The distributer, Fig. 4, comprises four gate stages V1-V4, one for each head, the head signals being applied to grids 1 and the gate anode outputs being commoned at 165. Waves F and G appear from circuit 131 and are applied to grids 1 of stages V1 and V3 respectively, waves H and I appear from circuit 136 and are applied to grids 1 of stages V2 and V4, whilst waves D and E appear from circuit 146 and are applied to the commoned suppressor grids 3 of stage V1, V3 and V2, V4 respectively. Pairs of D.C. restoration diodes 211 are provided in the outputs from circuits 131 and 136 to clamp the positive going portions of the waves to the correct bias level for grids 1 of the gate stages. The gates conduct only in response to simultaneous appearance of positive portions of the waves on their grids with the result that the gates conduct in the sequence V1, V2, V3, V4, V1 &c: in synchronism with the appearance of the television signal portions from the heads. These signal portions appear as indicated superposed on the grid 1 waves G-I, the switching from one gate to the next occurring during the periods of overlap. In a modified apparatus, Fig. 5 (not shown), the 480 c/s. waves D and E are controlled by horizontal synchronizing pulses separated from the reproduced signal so that their transitions are timed by the synchronizing pulses. By this means the switching action of the distributer is arranged to occur at fixed times in relation to the television lines. Switching transients imported into a reproduced picture appear as a stationary pattern. By appropriate phasing the switching may be arranged to occur during the back porch period of the flyback so that no pattern appears in the picture. A complete recording and reproducing apparatus is described with reference to Fig. 1 (not shown). The television signal is modulated on to a 4À5 mc/s. carrier by frequency modulation before recording and the reproduced signal is subsequently demodulated. A delay line is included in the circuit to each head to correct for slight angular misalignment of the heads which results in incorrect relative timing of the reproduced signal portions and horizontal displacement of groups of lines in a reproduced picture. The tape transport is controlled by a motor driven during recording by wave C, Fig. 6, originating from wheel 22. Wave C is also recorded along one edge of the tape and serves during reproduction to provide a reference signal in an automatic control system (see Group XXXV) to ensure that the tape is played back at the correct speed. Generating impulses photo-electrically. A lamp 43, Fig. 2, is arranged to project light through a lens 44 on to one face of wheel 22 and operates in conjunction with a photoelectric cell 45 responsive to reflected light. One segment of the wheel is darkened and another segment is made reflecting with the result that rotation of the wheel results in the production of current impulses related to speed of rotation. Specification 798,930 is referred to.