Patent GB-830684-A (GB)
Title: Method of multiple track recording
IPC: H04N5/92
Filed: 1956-03-15
Abstract: 830,684. Television. MINNESOTA MINING & MANUFACTURING CO. March 15, 1956 [May 9,1955], No. 8077/56. Class 40 (3). A band of television or like wide-band signals, such as radar signals, which are produced by repeatedly scanning a field of view, is recorded on a moving medium with the aid of apparatus having a cut-off frequency lower than the minimum frequency of the signals by a method in which the signals are sampled or modulated in such manner that they are compressed and translated into the band reproducable by the apparatus. The sampling or modulation frequency is made equal to the lowest frequency to be recorded plus the cut-off frequency. The resulting signal comprises two overlapping bands, each extending from zero to cut-off frequency. The first comprises an inverted band of the signals below the sampling or modulation frequency, being the sampling or modulation frequency minus the signal frequencies. The second comprises a non-inverted band of the signals above the sampling or modulation frequency, being the signal frequencies minus the sampling or modulation frequency. The invention is illustrated. as applied to the recording of television signals having a bandwidth of 3.6 Mc. by magnetic tape apparatus in which the cut-off frequency is 1.2 Mc. The signals from camera 1, Fig. 1, are applied directly to a first head 33 which records the lowfrequency part of the band up to the cut-off frequency, i.e. 1.2 Mc., on a first track, and are also applied to a second head 331, for recording on a second track, via a sampling modulator 25, which functions in accordance with the invention to translate and compress the 1.2-3.6 Mc. part of the band into the range 0-1.2 Mc. capable of being recorded by the head. To do this, the modulator is supplied with a signal at the lowest frequency to be recorded plus the cut-off frequency, i.e. at approximately 2.4 Mc. In practice the frequency of the signal is made equal to an odd multiple of one quarter of the line scanning frequency. The signal is derived from a generator 19 which is locked to the master oscillator 5 for the camera sync. gener- . ator 3. The modulator 251 may operate with a sine wave input, but is preferably supplied with a pulse input by applying the output of generator 19 via a class C amplifier 231. The operation of the modulator is such that alternate output samples are of opposite polarity, the interval between samples being equal to the duration of a sample. The signals in the band 1.2-3.6 Mc. are also preferably recorded on a third track by a third head 332 to which they are applied via a second similar sampling modulator 252 operating in phase quadrature with the first. A phase-splitter 21 supplies signals in phase quadrature from generator 19. A reference signal generator 27 produces a peak white signal during the flyback interval which is added to the signals in each head by means of circuits 31. A 1.2 Mc. reference signal is derived from the 2.4 Mc. signal by division in circuit 37 and is applied to a gate 39 which is opened only during the flyback intervals. The resulting short bursts of signal are then added to an associated television sound signal and recorded on a fourth track by a fourth head 49. In reproduction, Fig. 2, a converse method is employed, the low-frequency part of the band being derived directly from the first track by a head 51, whilst the 1.2-3.6 Mc. part of the band is reformed by applying the outputs of heads 511 and 512 engaging the second and third tracks to sampling demodulator 1131 and H32 which function in a similar manner to the sampling modulators. The resulting signals are combined in adder circuit 93. A 2.4 Mc. signal for the demodulators is derived by doubling in stage 75 the signal from a 1.2 Mc. oscillator 71 which is synchronized by the reference bursts recorded on the fourth track. These are separated from the output of a head 53 by filter 61 and rectified at 63 to obtain pulses which gate out the original bursts to synchronize oscillator 71 by way of discriminator 69 and reactance tube 73. The rectified bursts are also applied to gates 97 to separate out the peak white reference signals appearing from heads 51. An oscillator 103 is stabilized at the average frequency of the pulses from head 51 by means of a discriminator and reactance tube combination 99, 101 having a loop timeconstant long in comparison with flutter and skew variations, and produces reference pulses in circuit 105 for discriminator and control tube combinations 109, 111 individual to each head. These combinations have time-constants short in comparison with flutter and skew variations and control delay lines 87 following each head so as to maintain the outputs accurately in phase with each other. The delay lines are formed with saturable inductive elements so that the delay may be varied by a direct-current control signal. The terminating impedances 89 may also be varied. The rectified bursts may be taken over lead 115 to serve as reference pulses in a servo system controlling the tape drive mechanism. The delay line system may be replaced by the capacitor storage system described in Specification 730,997. Reference is made (without giving details) to a system in which the recording cut-off frequency is 0.9 Mc. The 1.8-3.6 Mc. portion of the band is sampled in accordance with the invention at a frequency of 2.7 Mc. and recorded by one head, whilst the 0-1.8 Mc. portion of the band is sampled at 0.9 Mc. and recorded by a second head. Specification 830,262 also is referred to.