Patent GB-778406-A (GB)
Title: Improvements in or relating to multiplex television transmission systems and transmitters and receivers for use in such systems
IPC: H04N11/02
Filed: 1953-10-16
Abstract: 778,406. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Oct. 16, 1953 [Oct. 21, 1952], No. 28618/53. Class 40 (3). In a colour or stereoscopic television transmission system signals representative of different aspects of the object are transmitted as modulations of different envelopes of an auxiliary carrier wave which, at the receiver, is divided into positive-going and negative-going parts from which the modulating signal are recovered. The auxiliary wave is produced by causing each aspect signal to modulate a carrier wave, the frequency of which is equal to the bandwidths available for transmission and also is so related to the frequency and phase of the line-scanning employed that the maxima of the resultant modulation product falls which occur during one line scan fall between the maxima which occur during the same line scan in the succeeding frame. This arrangement permits a smaller bandwidth to be employed than conventional transmission systems. In one embodiment (Fig. 22), which is concerned with a three-colour television system, different component colour signals produced by cameras 1, 2 and 3 are supplied via L.P. filters (the characteristics of which are shown in Fig. 1), F 1 , which has a cut-off frequency afp, where fp is the transmitted band width, a being less than one, and F 2  and F 3'  which have cut-off frequencies equal to the transmitted bandwidths fp, to modulators M 1 , M 2 , M 3 , in which they are caused to modulate auxiliary carrier waves of frequency fp (called " dotting oscillations ") produced by oscillator O, the carrier supplied to M 1  and M 3  having the form P 1 =(a+2a cos #pt) and that supplied to M 2  having the form P 2 =(-b+2b cos #pt), the second harmonic being included in each, or one, carrier wave if high definition is required in the signal modulated thereon. To ensure correct " dot-interlace " in the reproduced picture, the frequency fp is either made equal to an odd multiple of half the frame frequency or, the phase of P 1  and P 2  is varied at the beginning of each frame period under the control of synchronizing pulses supplied to the oscillator at 4. The output of modulator M 3  is then supplied, via a L.P. filter F 4  (cutting-off at ¢fp), to a further modulator M 4  in which it modulates an oscillation P 3  (desired from oscillator O) having the form (¢+cos ¢#pt+ cos #pt+ cos 3#pt/2), the resulting modulation products (which have the form shown in Fig. 11, where S 1'  S 2  and S 2  represent, respectively, the signals produced by cameras 1, 2 and 3) being combined with the outputs of M 1  and M 2  and supplied via filter F 5  having a characteristic as shown in Fig. 1, to a modulator M 5  in which the composite signals modulates a high-frequency carrier for transmission by aerial 5. At a receiver suitable for the reception of signals transmitted by the arrangement of Fig. 22, the demodulated high-frequency carrier derived by detector D (Fig. 23) is supplied to a device C 1  which separates the positively and negatively going portions, supplies the latter, via a delay device V 1 , to a picture reproducing tube BS 2  (suitable for reproducing the colour aspect represented by signal S 2 ) and supplies the positively going portion (Fig. 12a) to the combination of a delay device V 2  (introducing a delay Tp (see Fig. 12b) and a polarity inverter K 2  in parallel with an amplifier or attentuator A, the resultant output being a signal having positively and negatively going parts representative of S 3  (Fig. 13). The positively going part of this signal may be separated out and supplied to the tube which reproduces the colour aspect represented by S 3  or, to reduce interference due to rapid variations of the signal S 1  and as shown in Fig. 23, the positively and negatively going part of this signal (Fig. 13), are separated out by a device C 2 , the former part being supplied to a device V 3 , introducing a delay Tp, and the latter part to a polarity reverser K 3'  the output from which is combined with that from V 3  to produce the signal S 3  which is supplied to tube BS 3  to reproduce the colour aspect represented by signal S 3 . To produce signal S 1  the polarity of signal S 3  is reversed in K 1  and combined with the output of device V 3  and the resultant signal (S 1 ) supplied to tube BS 1  to reproduce the colour aspect represented by this signal. The three colouraspect images are then combined optically by known means to produce a composite colour picture. In a second embodiment, the output signals S 1 , S 2 , S 3  from the cameras 1, 2, 3 (Fig. 31) are supplied via respective filters F 1 , F 2 , F 3 , the latter (F 3 ) having a cut-off frequency ¢fp and the other two cut-off frequencies fp, to respective modulators M 1 , M 2'  M 3  in which they modulate respective " dotting oscillations " P 1 , P 2 , P 3  (as defined above) the signal S 1  (i.e. the input to M 1 ) being combined with a directcurrent voltage derived from source G and having an amplitude S 3 m equal to, at least, the maximum value of the signal S 3 . The output from modulator M 1 , is, after transmission through a L.P. filter F 4 , again modulated (in M4) on to oscillation P 3  which, in this case, is 180 degrees out of phase with the P 3  oscillation supplied to M 1  and the output from M 4  (S 1  +S 3 m, Fig. 26) combined with the output of M 3  (Fig. 25) to produce the waveform shown in Fig. 27 which when combined with the output of M 2  produces the signal waveform shown in Fig. 28. The latter signal is then passed through filter F 5  (having a characteristic as shown in Fig. 1) and modulated on to a carrier wave in M 5  for transmission from aerial S. In a receiver (Fig. 32) for such a transmitted signal the waveform (Fig. 28) derived at the output of detector D is supplied to a first device C 1  which splits it into a negatively going part, consisting of signal S 2  which is supplied to the related reproducing tube BS 2 , and a positively going part which is supplied to a second device C 2  which splits' the latter into an upper part S 1  and a lower part S 3'  the former being supplied directly to the related reproducing tube BS, and the latter, via a L.P. filter F 6  having a characteristic as shown in Fig. 1 and a cut-off frequency ¢fp, to its related reproducing tube BS 3 . By a suitable choice of the ratios between the amplitudes of the signals S 1 , S 2'  S 3  a black/white image differing from conventional black/white images only in having less contrast and having a dotted structure may be reproduced by conventional black/white television receivers. French Specification 652,000 is referred to.