Patent GB-757238-A (GB)
Title: Improvements in or relating to multiplex television signalling systems
IPC: H04N11/18
Filed: 1953-12-30
Abstract: 757,238. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Dec. 30, 1953 [Jan. 2, 1953], No. 36185/53. Class 40 (3). Relates to a multiplex system for the transmission within a common frequency band of three signals I, II and III representing three different aspects, e.g. colour components, a television image the frame scan of which consists of an odd number of lines in two interlaced fields. Signal I of larger bandwidth is transmitted during all line scans and signals II and III of smaller bandwidth are transmitted alternately at line frequency as modulations of an auxiliary carrier. In accordance with the invention, the phase of the auxiliary carrier is shifted by an odd number times # radians in the course of or after every two frames. In such a system, due to the use of a common frequency band, signal I is interfered with by signals II and III and vice versa. Owing to the choice of an odd number of lines for the frame, interference in a given line of signal I which originates from, say, signal II and vice versa occurs every two frames. By shifting the phase by # radians the interference cancels visually in the images reproduced from the signals. In one way of carrying out the invention, Fig. 6 (not shown), the auxiliary carrier is merely arranged to have a frequency which is an odd multiple of one quarter of the line frequency. By such choice of frequency the phase of the auxiliary carrier changes continuously so as to have shifted by # radians after two frame periods. Furthermore, if the number of lines in the image frame is a multiple of eight plus or minus three, such as in the American System (525 lines), the British System (405 lines) and the French System (819 lines), the interference effects on adjacent lines displaying the same aspect, e.g. colour, of the image are also in phase opposition. This effect considerably improves the image quality. If the number of frame lines is a multiple of eight plus one, such as in the European System (625 lines), the interference effects on adjacent lines occur in phase, and according to modification, Fig. 4 (not shown), phase opposition is brought about by additionally shifting the phase of the auxiliary carrier by # radians after each field. In a second way of carrying out the invention the auxiliary carrier is arranged to be an odd multiple of half-line frequency. With this choice of frequency the subcarrier is normally in phase after two frames and the desired phase opposition is brought about by an added shift of # radians. If, however, the interfering effects on adjacent lines are to be in phase opposition, the phase shift is applied after three out of every four fields which still results in a phase shift of an odd multiple of # radians over two frame periods. This aspect of the invention is illustrated in Fig. 7 in which signal I generated by a first camera is applied through low-pass filters F1 directly to adder A whilst signals II and III from two further cameras are applied through switch means S1 to modulate an auxiliary carrier in modulator M1, the output of which passes through band-pass filter F2 to adder A. Switch means S 1 changes over at line frequency under the control of line synchronizing pulses from source L. The auxiliary carrier is generated by oscillator 03 synchronized from the line pulses and arranged to produce output in phase opposition at points a and b which are connected to the modulator through two switches S4 and S5. S4 is changed over to change the phase of the auxiliary carrier by # radians at field frequency under the control of field synchronizing pulses from source R, and S5 is changed over at half frame frequency by field synchronizing pulses from source B so as to offset the phase change introduced by switch S4 upon every fourth switching operation. Fig. 5 illustrates a receiver which may operate with any of the transmissions described. The received signal is detected at DT and the broad-band signal 1 applied to a cathode-ray tube BS1 reproducing the first aspect of the image. The auxiliary carrier is selected by filter F4, detected at D and applied to cathode-ray tubes BS2 and BS3 reproducing the second and third aspects of the image through switch means S3 which is changed over at line frequency in synchronism with the corresponding change over of the transmitter modulation under the control of the line synchronizing pulses. Where the invention is applied to colour television the outputs from DT and S3 are applied to the control electrodes of a three-colour cathode-ray tube.