Patent GB-734056-A (GB)
Title: Improvements in or relating to multiplex television systems
IPC: H04N7/08
Filed: 1953-04-28
Abstract: 734,056. Multiplex television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. April 28, 1953 [May 1, 1952], No. 11708/53. Class 40 (3). [Also in Group XL (c)] The invention relates to a multiplex television system in which two information signals are transmitted by means of two different carrier waves the sidebands of which overlap at least in part. Only the upper sideband of the lower frequency carrier and the lower sideband of the upper frequency carrier are transmitted and with a phase relation such that interference in the receiver which is due to the overlapping of the sidebands and which occurs during one frame period is substantially neutralized visually by a similar interference of opposite sign in the following frame period. The necessary phase relationship is brought about either by selecting the frequency difference between the two carriers to be an old multiple of half the line frequency, provided that a complete image is composed of .an odd number of lines, or by causing one carrier to change in phase by 180 degrees at the beginning of each new frame. In accordance with the invention, the frequency difference of the two carriers is made at least equal to the highest frequency of the two information signals while one sideband comprises the sum of the two information signals plus a D.C. signal of constant amplitude and the other sideband comprises the difference of the two information signals plus a D.C. signal of constant amplitude. At a receiver, the two sideband signals are separated by means of band-pass filters and applied to detectors. The detected signals are then both added and subtracted to reveal the original information signals. The Specification includes a mathematical discussion by which it is shown that the system reproduces the original information signals correctly but with each combined with an interfering signal originating solely in the corresponding original signal. Due, however, to the aforesaid 'phase relation established between the carriers, the interfering signals reverse at the beginning of each frame and substantially cancel in their visual effect. In one embodiment, Fig. 1, signals S1 and S2 from two cameras I and II are limited by lowpass filters F1 and F2, each having a cut-off frequency fp and are then combined and subtracted by means of adding circuits Al, A2 and inverting circuit K1. Furthermore, adding circuits Al and A2 have supplied to them direct voltage A and B respectively derived from source G1. In a modulator M1 the output from adding circuit A1 viz. S1+S2+A, modulates a carrier-wave of frequency fd. In a modulator M2 the output from adding circuit B, viz. S1-S2+B, modulates a carrier-wave of frequency fd+fp. The two carrier-waves are derived from the source O1 which is controlled by means of the line or frame synchronizing pulses at L to ensure either that fp is an odd harmonic or half line frequency or that the fd+fp carrier is reversed in phase at the beginning of each frame. The output signals from modulators M1 and M2 are combined in adding circuit A3 and the output applied to the aerial Z through a filter F3 having a pass-band as shown in Fig. 2, whereby the transmitted signal comprises a carrier fd together with the upper side-band due to S1+S2+A and a carrier fd+fp together with the lower sideband due to S1 - S2+B. At a receiver for the system, Fig. 3, the signal is applied to two band-pass filters F4, F5 which have pass-bands as shown in Figs. 4 and 5 respectively. The output signals are then applied to two detectors D1 and D2 from which the output signals are respectively S1+S2+S1<SP>1</SP>+S2<SP>1</SP>+A and S1- S2+S1<SP>1</SP>+S2<SP>1</SP>+B where S1<SP>1</SP> and S2<SP>1</SP> are interfering signals originating from S1 and S2 respectively and the plus and minus signs indicate the phase changes occurring in successive frames. Adding circuits A4 and A5, together with inverting stage K2 add and subtract the two outputs whereby the signals finally applied to reproducing tubes BS1 and BS2 are respectively S1S1<SP>1</SP>+¢ (AB) and S2S2<SP>1</SP>¢ (A-B). The direct currents are made inoperative by appropriate biasing of the tubes and the interfering signals cancel in visual effect due to the change in phase in each frame. In an alternative transmitter embodiment, Fig. 6 (not shown), the S1-S2+B signal is first modulated on to a sub-carrier of frequency fp and then combined with the signal S1+S2+ A before modulation on to the carrier of frequency fd. The modulated wave is passed through a filter having a pass-band as shown in Fig. 2. In yet a further alternative transmitter embodiment, Fig. 7 (not shown), a signal S1 +¢ (A+B) is modulated by a signal 12 cos 2#fpt whilst a signal S2+¢ (A-B) is modulated by a signal 12 cos 2f#pt. The plus and minus signs again indicate the phase changes occurring in successive frames. The modulated signals are then combined and modulated on to a carrier of frequency fd, the modulated wave again finally being led through a filter having a pass-band as shown in Fig. 2. In both of these alternative transmitters the transmitted signal is identical with that produced by the transmitter of Fig. 1. The transmitted signal may be shifted in frequency by the use of additional modulating stages, and the receiver is preferably of the superhetrodyne type so that, if a plurality of transmitters each emit a pair of carrier waves as described, all these may be transformed to the same frequency band and only one set of band-pass filters F4 and F5 is required. Details of the filter characteristics are discussed. Figs. 8 to 10 (not shown), illustrate alternative filter characteristics. The two information signals may be those required for stereoscopic or twocolour television. Where it is required to transmit a third colour signal this may be transmitted in an adjacent frequency band. Figs. 11-13 (not shown), show a system for doing this.