Patent GB-784102-A (GB)
Title: Play-back system for recorded television images
IPC: H04N5/919
Filed: 1955-05-31
Abstract: 784,102. Television. MINNESOTA MINING & MANUFACTURING CO. May 31, 1955 [June 11, 1954], No. 15514/55. Class 40 (3). In an arrangement for recovering television signals which are recorded on a record medium in the form of samples of the original signal the samples being taken at a rate which is an odd multiple of one-half of the frame frequency signal samples derived from the record (by means of suitable transducers) are delayed for the period of one frame and then combined with undelayed derived signal samples to produce a composite signal in which the samples derived from one frame are interleaved with the samples derived from an immediately preceding frame. This arrangement allows the use of half the number of transducers and of a sampling frequency of one half that otherwise required for the reproduction of the same definition (excepting the extreme case of rapidly moving objects) and reduces errors resulting from misalignment of the transducers, variations in record speed and other mechanical factors. The invention is described as employing a moving magnetic tape 1 (Fig. 1), a plurality of tracks on which television signals are recorded as modulations of different phase displaced trains of a common carrier frequency as described in Specification 732,589. In the present arrangement however, the H.F. components are recorded separately from the L.F. components, the latter being recorded on duplicate tracks to prevent loss of L.F. information due to flaws &c. in the tape. The H.F. information is translated by means of transducers 31 ... 3n and, after amplification in preamplifiers 5, supplied to detector circuits 31 the outputs from which are fed, via filters 33, to sampling circuits 27. In these latter circuits the input signals are sampled under the control of sampling pulses derived from a common unmodulated timing wave via transducer 3 and supplied to the circuit 27 from appropriate tappings on a delay line 21, the sampling pulses having twice the frequency of the timing wave and a length equal to one half cycle of this wave. The outputs from the various sampling circuits are fed to a common channel 29 from which, via a H.P. filter, the succession of signal samples is fed to an adding circuit 72 over two paths, one of which includes a storage device 73 and, additionally, the adding circuit is supplied with the low-frequency components which are derived from duplicate tracks, on which they are recorded as frequency modulations of the common frequency, via transducers 3<SP>1</SP>. The storage device, which may comprise a cathode-ray storage tube (Fig. 3, not shown), or a magnetic storage drum arrangement (Fig. 4, not shown), operates to delay the input signals for the period of one frame so that, since the sampling frequency is an odd multiple of one half the frame frequency, the output signals from adding circuit 72 during one frame period consists of a series of interlaced dot samples of the signal samples of two successive frames, thus giving an increase in definition by a factor of two. The output frame 72 is then supplied to a conventional television transmitter via a harmonic filter 71 which comprises a number of " slot " filters tuned to successive harmonies of the carrier frequency, from the fundamental up to and including the nth which is the sampling frequency, the purpose being to eliminate dot structure and remove any spurious frequencies introduced by unbalance between the various channels. If, however, due to low-frequency disturbance in the scanning of the storage device, dots representing successive frames overlap, there may result in the output of adder 72 a component of the sampling frequency which, if removed by the nth harmonic filter in 71, would also result in the loss of picture information. To avoid this the nth harmonic filter is omitted from 71 the output from which is supplied (to the transmitter) via a " dot-connector " 124 (described in detail in connection with Fig. 3, not shown) the effect of which is to prevent the signal from falling to zero between samples. Specifications 727,542, [Group XIV], 730,997 and 776,804 also are referred to.