Patent GB-883900-A (GB)
Title: Improvements in and relating to the recording of television signals
Inventor: SILVIE WILLIAM
IPC: H04N5/919
Filed: 1957-03-25
Abstract: 883,900. Television. BRITISH BROADCASTING CORPORATION. March 24, 1958 [March 25, 1957], No. 9774/57. Class 40(3). In apparatus for recording television signals on a medium, such as a magnetic tape, the signals are divided into a low-frequency band A and a high-frequency band by means of a low-pass filter 11 and subtracting amplifier 12, Fig. 2, the low-frequency signals being recorded on a first track by a head R1 and the high-frequency signals being applied to two modulators 14, 15 operating synchronously in phase quadrature at approximately the centre frequency of the highfrequency band and from which the outputs B and C are recorded on two further tracks by heads R2, R3. By choice of appropriate frequencies the invention provides signals of approximately the same band width for the three tracks so that the recording heads may be alike. As illustrated, -the low-frequency band extends from 0 to 1.2 mc/s and the high-frequency band extends from 1.2 to 3.0 mc/s. The modulators operate at 2.1 mc/s and the outputs are restricted to 900 kc/s  by low-pass filters 18, 19. The carrier oscillations for the synchronous detectors are supplied by an oscillator 16 whose output is applied directly to detector 14 and via a 90 degree phase shifter 17 to detector 15. Reference is made to the use of an oscillator operating at an odd multiple of half the line frequency. The oscillator is locked in frequency and phase with the television line synchronizing pulses by means of a discriminator 28 to which the oscillator supplies a 10 kc/s signal via frequency dividers 29, 30 and the television signal supplies a 10 kc/s signal via synchronizing pulse separator 26 and band-pass filter 27. The discriminator output controls the oscillator through a reactance valve 31. A reference oscillation D at 1.05 mc/s is obtained from frequency divider 29 and applied to head R2 for recording on the second track. Accompanying sound signals are applied to a modulator 33 to frequency modulate a 1.05 mc/s. carrier E which is applied to head 23 and recorded on, the third track. In apparatus for reproducing the recorded   signals, Fig. 3, heads R<SP>1</SP> 1 , R<SP>1</SP> 2  and R<SP>1</SP> 3  derive signals from the three tracks, the lowfrequency signals A<SP>1</SP> being applied directly to an output terminal 20 whilst the highfrequency signals B<SP>1</SP>, C<SP>1</SP>, are first re-established by balanced demodulators 21, 23 operating in phase quadrature. An oscillator 23 furnishes the necessary carrier at 2.1 mc/s and is controlled by the recorded reference oscillation D<SP>1</SP> which is extracted from the output of head R<SP>1</SP> 2 , by filter 34 and applied to control the oscillator via a frequency multiplier 35. The arrangement eliminates errors due to differences in the speed of the medium during reproduction. The signals from head R<SP>1</SP> 3  are also applied to a frequency modulation detector 37 and. low-pass filter 38 to derive the sound signals. The invention may be applied to colour television where a 0-3 mc/s luminance signal is recorded on three tracks as described above and two chrominance signals extending to 340 kc/s and 1 mc/s respectively are recorded directly on two further tracks. Signals in the band 0-100 kc/s may be separated out and dealt with as described in Specification 883,062. The signals may be applied to frequency modulate a 1.05 mc/s carrier from the output of divider 29 which may then be added to head R2 or R3 for recording.