Patent GB-704803-A (GB)
Title: Improvements in and relating to high definition television systems and methods
IPC: H04N11/24
Filed: 1951-10-25
Abstract: 704,803. Television transmission systems. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 25, 1951 [Oct. 31, 1950], No. 24986/51. Class 40 (3) In order to. reduce the effective bandwidth of a television signal, the frequency range occupied by the signal, Fig. 5a, is split into three bands A, B, C, Fig. 5b, the upper band C being transposed in frequency to produce a band C', Fig. 5c, lying within the central band B and being transmitted together with the lower band A in alteration with the central and lower bands, A and B, Fig. 5d and 5c. At a co-operating receiver, means is provided for transposing band C<SP>1</SP> back to band C to enable the full frequency range of the signal to be reproduced; however band C' is frequency-interlaced with band B as described in Specification 704,791 (i.e. the spectral components of band C' which are spaced by the line frequency are interleaved in frequency with those of band B) so that the signal may be reproduced on a conventional receiver the portion C' of the signal not producing any noticeable visual effect. At the transmitter, Fig. 1, the signal generated by camera 15 is applied to the three filters 18, 19, 20 passing bands A, B and C respectively. Band A is passed by the amplifier 22 to the blanking-and-synchronizing-pulse mixer 24 whence it is amplitude-modulated on to a carrier in a conventional transmitter. Band B is applied to an amplifier 31 and band C, after conversion to band C', is applied to an amplifier 41, the two amplifiers being keyed alternately by square-wave signals generated at 54 at the picture frequency ; thus each amplifier passes signals in alternate frames. These signals are then combined with that from amplifier 22 and applied to the transmitter unit. To effect the transposition of band C, the output from filter 20 is heterodyned in frequency-changer 33 with the output from oscillator 34 which operates at 875 times half the line frequency so as to produce frequency-interlacing. The output from frequency changer 33 is applied to filter 37 which selects the lower side-band of the resultant signal and applies it to an adder circuit 38 where it is combined with a signal of half the oscillator frequency derived from frequency-divider 39. The combined signal, comprising band C' and a " retransposition " signal for use at the receiver and lying close to the upper limit of C', is applied to keyed amplifier 41 and thence combined with the outputs of amplifiers 22 and 31. The output from frequency-divider 39 is also applied to a 875:1 frequency-divider 51 which consequently delivers a signal at one quarter the line frequency to a times-eight frequency multiplier 52, the output signal at double the line frequency being used to control the operation of the synchronizing-and-blanking-signal generator 50. Signals from generator 50 are applied to mixer 24 and camera 15 and also to control the operation of the square-wave generator 54. At the receiver, Fig. 3, the demodulated signals are applied in parallel to filters 73, 90 and 93 passing bands A, B and C' respectively. Band A is amplified at 74 and passed directly to the reproducing cathode-ray tube 76, The output from filter 90 (which passes band C' as well as band B) is applied to a keyed amplifier 91 which is operative only when band B is being received, and thence, during the operative periods, to amplifier 92 and cathode-ray tube 76. A narrow bandpass filter 95 selects the " retransposition " signal and applies it to frequency doubler 96, the output from which is heterodyned in frequency converter 98 with the output from filter 93. A further filter 100 selects band C from the sidebands of the resultant signal and applies it to amplifier 102 whence it is applied to the amplifier 92. Frequency converter 98 also develops a D.C. output signal when a signal is being applied thereto from frequency doubler 96, i.e. during the periods when band C is being received. This signal is applied to keyed amplifier 91 and controls its operation so that the amplifier is non- operative when band C' is being received. The transmission of bands B and C' may be alternated at other than the picture frequency. It is preferred that the cycle of operation be accomplished in an odd number of line periods; thus, since the transitions must occur during blanking intervals, the two bands must be transmitted for different successive periods. Thus in an arrangement wherein the complete switching cycle occupies five line periods, bands A and B and bands A and C' may be transmitted for alternate periods of respectively three line periods and two line periods. Figs. 6 and 7 (not shown) illustrate a modification of the arrangement illustrated in Figs. 1 and 3. At the end of each frame synchronizing pulse a short burst of oscillation is superimposed on the video signal, the oscillations being of different frequencies in alternate synchronizing pulses. At the receiver the frame synchronizing pulses are applied to two circuits tuned to respective ones of the oscillation frequencies. The pulses produced alternately in the two circuits in response to the signals applied thereto are applied to a bi-stable trigger circuit which is used to control the keying of the amplifiers translating the signals corresponding to the bands B and C. In a second embodiment of the invention wherein four frames are required to complete the cycle of operation the oscillator 34 operates a frequency equal to an odd multiple of one-quarter the line frequency and lying close to the lower limit of band C, as shown, in Fig. 9. The oscillator output is heterodyned with the signals of band C to result in a signal occupying band C', together with a signal component of the oscillator frequency. This is applied direct to keyed amplifier 41 ; since the signal already includes the retransposition signal, circuits 38 and 39 are dispensed with. As previously, synchronizing-signal generator 50 is controlled by the oscillator output through suitable frequency dividing and multipling circuits ; an output from circuit 50 is applied to a square-wave generator which in this instance develops keying pulses at one half the picture frequency, i.e. at one quarter the frame frequency. At the receiver the signals in the channel carrying band C' are applied to a diode rectifier whence, owing to the simultaneous presence of the " retransposition " signal, signals of band C are developed. The presence or absence of the " retransposition " signal is used to control the channel keying. The invention may be applied to a frame-sequential colour television system and a modification of the embodiment of Figs. 1 and 3 for colour transmission and reception is described.