GB789673A - Improvements in or relating to television systems - Google Patents

Abstract

789,673. Television. MULLARD RADIO VALVE CO., Ltd. Feb. 14, 1956 [Nov. 18, 1954], No. 33434/54. Class 40 (3). [Also in Group XL (c)] In a television system utilizing a main vision signal v, v<SP>1</SP>, such as a luminance signal, modulated on a main carrier V, a pair of quadraturemodulated signals c such as colour signals, on a sub-carrier C and a sound signal on a carrier S, there is transmitted a reference carrier signal R, the frequency difference between which and the sound carrier S is employed at a receiver to produce a phase-reference signal for the synchronous demodulation of the subcarrier. In one arrangement having frequencies selected for example as shown, the carriers C, Sand R are equally spaced. After demodulation at a frequency centred on the main carrier, the sound and reference carriers appear at 3.5 and 4.3 Mc/sec. respectively. These are then selected and applied to a square-law detector wherein a difference frequency of 0.8 Mc/sec. is produced and combined with the original 3.5 Mc/sec. to produce a lower side-band at 2.7 Mc/sec. corresponding to the frequency at which the sub-carrier appears and serving as the phase-reference signal. The signals are produced at the transmitter starting with signals of 2.7 Mc/sec. and 3.5 Mc/sec. The 2.7 Mc/sec. signal which provides the subcarrier is divided down and locked to the mains supply frequency. The 2.7 Mc/sec. and 3.5 Mc/sec. signals are then fed to a square-low mixer to obtain 4.3 Me/sec. and this, together with the 3.5 Mc/sec. are combined with a 4.5 Me/sec. carrier to produce the sound and reference carriers at 41.5 Mc/sec. and 40.7 Mc/sec. respectively. In an alternative arrangement, the reference frequency carrier R is located at 40.6 Mc/sec. so that the difference between carriers S and R equals 0.9 Mc/sec. which may be multiplied by 3 to obtain directly the phase-reference signal at the 2.7 Mc/sec. of the sub-carrier. It is stated that the difference between the reference and sound carriers may be made any integral multiple or sub-multiple of, or equal to, the sub-carrier frequency. In another arrangement, Fig. 2 (not shown), the reference carrier R is located between the sound carrier and the sub-carrier at a frequency of 41.9 Mc/sec. In yet another arrangement, Fig. 3 (not shown), the subcarrier is located outside the band of the main vision signal between the reference and sound carriers. The invention is stated to be applicable to systems where the sub-carrier is subjected to periodic 90 degree phase shifts to reduce cross-talk. The reference carrier is subjected to similar phase shifts so that the phase reference signal derived at the receiver undergoes corresponding changes. Where the sub-carrier is locked to the mains frequency, the effects of variations in the main frequency may be compensated by applying corresponding variations to the reference carrier so to maintain the desired multiple or sub-multiple relationship between the carriers concerned.