Patent GB-767021-A (GB)
Title: Improvements in or relating to multiplex transmission systems
IPC: H04N11/14
Filed: 1954-02-09
Abstract: 767,021. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Feb. 9, 1954 [Feb. 9, 1953], No. 3790/54. Class 40 (3). [Also in Group XL (c)] In a television system for transmitting three signals, e.g. colour component signals, relating to an image scanned line-by-line, two of the signals are of narrow bandwidth and are transmitted as modulations of a sub-carrier falling within the frequency band of the third signal, the sub-carrier being amplitude modulated by one of said two signals and angle-modulated, i.e. frequency and/or phase modulated, by the other. At a receiver, the whole of the received signal is applied to one image-reproducing means, whilst a band-pass filter selects the subcarrier and applies it to amplitude- and anglemodulation detectors to derive the two narrowband signals for application to two further image-reproducing means. In order to eliminate interference between the two narrow-band signals and the third signal arising out of the use of a common frequency band, it is arranged that the sub-carrier differs in phase by # radians during successive scannings of the same line of the image, this resulting in visual cancellation of the interference. In a first system, Fig. 4, utilizing frequency-modulation, pick-up tubes I, II and III produce the wide-band and two narrow-band signals respectively, the output from I going via filter F2 to an adder A whilst that from III frequency-modulates a sub-carrier from oscillator O1 in modulator FM, the modulated sub-carrier then passing to an amplitude modulator receiving the signal from II and finally passing through a narrow-band filter F3 to the adder. The oscillator O1 is controlled by line synchronizing pulse L to be an odd multiple of half the image frequency, corresponding to one half the line frequency where there is an odd number of lines in an image. This choice of sub-carrier frequency ensures the necessary phase change to effect interference cancellation but necessitates the removal by a stage G of the D.C. component from the signal employed for the frequency modulation. To permit the D.C. component to be restored at a receiver it is signalled separately as amplitude variations of a pulse superposed on the back part of the line blanking interval. To this end the D.C. component is selected by low-pass filter LP and applied to a circuit GC receiving suitable pulses L7. The resulting pulse output is then combined with the original signal in adder A. At a receiver, Fig. 3, the whole of the output from a detector DT is applied to a first cathode-ray tube BS1, and a band-pass filter F1 selects the sub-carrier for application to amplitude- and frequency-modulation detectors AD and FD, the derived signals from which are then applied to cathode-ray tubes BS2 and BS3. The pulses conveying the D.C. component are separated from the detected signal by a gate stage GCR operated by suitably timed line frequency pulses L2. The gate output is fed to a pulse detector PD and is then combined with the output of the frequency-modulation detector in stage AR. In a second system utilizing frequency-modulation without the need to remove and reinsert the D.C. component, the sub-carrier frequency is chosen arbitrarily, preferably as close as possible to the top of the band of the wide-band signal, and the phase of it is controlled so that at the beginning of the scanning of each line of the image it is displaced in phase by # radians relative to the immediately preceding scan of the line. This may be achieved by controlling the starting phase for each line by the line synchronizing pulses and by shifting the starting phase by # radians after every frame under the control of the frame synchronizing pulses. If the number of lines per frame is odd, it is merely necessary to start the oscillator at the beginning of each line with a phase difference of # radians in relation to the preceding line. Phase modulation may be used in the arrangements described above by replacing the frequency-modulator FM in Fig. 4 by a phasemodulator. The D.C. component is automatically removed and stage G may therefore be dispensed with. The receiver may use a phase-modulation detector or a frequencymodulation detector followed by an integrating network. In a further system, Fig. 5, permiting frequency-modulation without the necessity of removing and reinserting the D.C. component, the sub-carrier signal is arranged to be changed in frequency in discrete steps in dependence upon the amplitude of the applied image signal. The arrangement is generally similar to Fig. 4 except that the sub-carrier is derived from oscillators O1-O3 whose frequencies differ and which are arranged in ascending order of frequency. The oscillators' outputs are applied to gate stages B1-B3 which receive the video signal from camina III and allow the passage of oscillations only in response to a video signal exceeding a threshold level, the value of which is made greater for successive gates. The gate outputs are then applied through limiting amplifiers V1-V3 to a common adding stage A1, whilst each amplifier when producing an output is arranged to suppress the output from the preceding amplifiers by means of cut-off bias derived by detectors D1 and D2. By this means only one oscillator is effective at a time, the frequency selected depending on the amplitude of the video signal. The arrangement in practice is extended to employ more than three oscillators. The frequency of each of the oscillators is selected to be an odd multiple of half the line or image frequency as described above in connection with the first system or is changed in phase periodically as described in connection with the second system. Where asymmetrical side-band transmission of the sub-carrier is employed interference thereby produced between the two signals modulating the sub-carrier may be reduced by reversing the polarity of one of the signals during alternate images. Frequency and phase modulation may be employed together to obtain special effects by employing one type of modulation for part of the band of the modulating video signal and the other type of modulation for the remainder of the band.