Patent GB-798927-A (GB)
Title: Improvements in and relating to broad band magnetic tape recording and reproduction
Inventor: GINSBURG CHARLES PAUSON
IPC: H04N5/782
Filed: 1955-09-16
Abstract: 798,927. Magnetic recording and reproducing systems. AMPEX CORPORATION. Sept. 16, 1955, No. 26638/55. Class 40 (2). [Also in Group XL (b)] Television signals are recorded on a magnetic record medium as frequency modulations of a carrier wave, the centre frequency of which is near the maximum frequency which can be recorded on the medium (for a given relative velocity between the medium and the recording heads). In one embodiment (Fig. 1), the signals to be recorded are supplied via a modulation levelcontrol unit 125 to a, variable reactance tube 126 which frequency modulates an oscillator 127 operating at about 45 mc/s. and the output from which, after amplification in 128, is converted to a frequency of about 4 mc/s. in mixers 129 and the output of the latter supplied via amplifier 130 to four channel amplifiers 121 ... 124 which feed, via switch S2, individual magnetic transducer heads in housing 28. These heads, the construction of which is also described in Specification 798,928 (see Group XXXVII), are rotated at high speed by means of a motor 23 so as successively to "scan," transversely to its length, the magnetic recording tape 11, the latter being moved from left to right by means of a capstan 17 driven by a second motor M. To ensure a constant relative speed between the rotation of the heads and the movement of the tape and to compensate for mechanical deficiencies especially slippage of the driving belt 56, the speed of both motors is controlled by means of a photo-electric pulse generating device 24 operated by the shaft which rotates the heads. As shown in Figs. 8 and 9, this device comprises a circular member 61. rotated (with the heads) by means of shaft 31 and the periphery of which is provided with reflecting segments L1, L2 and non-reflecting segments D1, D2 so that, as the shaft rotates, light from a source 66 is reflected from segments L1, L2 on to a photo-electric cell 67 and produces a square-wave form signal of frequency dependent on the speed of rotation of shaft 31 and therefore dependent on the speed of the heads. During recording this signal is supplied, via a cathode-follower stage 71, represented in Fig. 1, by CF, to a filter 81 which converts it into a sine wave of the same frequency and this is supplied via amplifier 82 to a fixed transducer head 19 which records the signal as a control frequency on a marginal portion of the tape. The output of CF is also supplied to a divider 77 which reduces the square-wave frequency to the nominal frequency of a local source (e.g. electric mains) 93 and the output of the divider, after conversion to sine-wave shape in filter 78 supplied, firstly, via switch S1 (in the position shown) to a power amplifier 79, the output of which supplies driving power for the capstan motor M and, secondly, to a phase comparator 87 in which it is phase compared with the local source 93, the output of comparator 87 being employed to control the frequency of an oscillator 84 which supplies, via amplifier 83, the driving power for the head motor 23. During " play-back " (the tape having been rewound from spool 13 on to spool 12) the motor 23 is controlled as described above for the recording cycle, but the capstan motor M is now controlled by varying the frequency of an oscillator 96 in accordance with the output of a phase comparator 99 in which the phase of the output of filter 81 is compared with the already marginal recorded control signal, this being derived via transducer 19 and amplifier 103. The output of oscillator 96 is then supplied, with switch S1 in the opposite position to that shown, to amplifier 79 and thence to motor M. The recorded frequencymodulated television signals are derived via the rotating heads 28 and supplied., with switch S2 in the opposite position to that shown, to preamplifiers 131 ... 134, the outputs of " alternate " ones being combined in mixers 136, 137 and supplied via respective amplifiers 138. 139 to switching circuit 141, 142 controlled by a signal derived from the pulse generator device 24 so as to pass the outputs of amplifiers 138, 139 alternately to a limiter 143. The limiter output is then converted to an I.F. frequency in mixer 144 and., after further limitation in 148, is fed to a discriminator 149 which supplies the originally recorded television signals to the output circuit. In order to ensure that all information is recorded the width of the tape 11 is made greater than the circumferential spacing of the rotating heads (four heads spaced 90 degrees apart) and since this results in duplicate information being recorded on the marginal portions of the tape, erase heads 18, 20 are provided on each side, the former " clearing " the margin to allow the recording of the control frequency signal via head 19, and the latter " clearing " the opposite margin so that other information, e.g. sound signals, may be recorded via head 21. Because of the increased tape width, the switching operation of channel amplifiers 138, 139 must be arranged so that only the desired information is passed to the limiter 143. Such switching arrangements may be dispensed with and the combined outputs of the channel amplifiers supplied to the limiter 143 with some sacrifice in performance (Fig. 2, not shown). The connections to the operating coils of the (four) rotating heads may be made via individual slip-rings (Fig. 10, not shown) or the coils may be connected in series (Fig. 11, not shown) or in series-parallel combinations (Fig. 14, not shown) so that only two slip-rings are necessary. Instead of employing an (sine wave) oscillator such as 127 (Fig. 1), the frequency modulated carrier may be produced by controlling the frequency of a multivibrator by means of the signals to be recorded (Fig. 14, not shown). In all embodiments, the ratio of the frequency deviation corresponding to the maximum signal amplitude to the highest modulating frequency is of the order of 0.2. The Specification includes circuit details of the phase-comparators (Fig. 13, not shown), the frequency modulated multivibrator (Fig. 15, hot shown) and the switching circuits 141, 142 with reference to Fig. 16 (not shown). Specifications 798,929 and 798,930 also are referred to.