Patent GB-683926-A (GB)
Title: Improvement in colour television apparatus including an arrangement for eliminating adjacent channel interference
IPC: H04N11/12
Filed: 1950-10-19
Abstract: 683,926. Colour television ; impedance networks. RADIO CORPORATION OF AMERICA. Oct. 19, 1950 [Oct. 28, 1949], No. 25532/50. Classes 40 (iii) and 40 (viii). In a colour television system, so designed that its use is compatible with standard black-andwhite transmitting or receiving apparatus, the output signals from three respective colour cameras are sequentially sampled at a high rate to give time-division multiplexing of the three-colour channels and the pulse signals delivered at the output of the sampling commutator are passed through filter circuits such that each pulse is converted to a sinsusoidal waveform having a period equal to that of the commutator and so biased by a unidirectional signal that each cycle of the waveform has a maximum and two zero values spaced uniformly apart by one-third of the commutator period. Thus, at the receiver the maxima of one sinusoidal component waveform coincide with the zeroes of the other two component waveforms and so by sampling the maxima of the successive sinusoidal waveform to effect the separation of the received signals into their respective colour channels, " cross-talk " between the colour channels is avoided. to this end a circuit is provided in the output stage of the transmitter which converts each of the pulse trains delivered by the commutator into a signal comprising a sine-wave signal of the same frequency (e.g. 3.8 Mc/s) plus a unidirectional signal of a magnitude equal to half the sine wave amplitude. A family of such signals is illustrated in Fig. 1, the zeroes being set at 120 degrees and 240 degrees with respect to the maxima of the signal. As indicated, zero amplitude of the sine-wave signal does not necessarily correspond to zero-amplitude video signal from the corresponding camera, the zero-amplitude output being chosen to cause the effects of the commutator to disappear at the intensity level at which they are most noticeable. Fig. 4 illustrates a circuit for effecting the required transformation of the pulse signals together with a switching arrangement which simulates the preceding circuits. The capacity of condenser C<2> is twice that of condenser C<1> and each component sub-circuit, viz L, C<1>, R<1> and C<2>, R<2> has the same attenuation constant. Momentary closing of the switch SW represents the application of a pulse to the circuit from the commutator and curve 1, Fig. 3, illustrate the type of response curve that the circuit gives. Curves 2 and 3 show the responses due to the individual sub-circuits. If desired, more than one circuit of the type illustrated in Fig. 4 may be used, the circuits being arranged in cascade. In addition, the receiver may comprise a " cross-talk balancing unit " which is used to reduce any " cross-talk " which may exist between the colour channels, particularly due to the fact that the sampling pulses both at the transmitter and receiver are of finite duration and are not infinitesimally short as assumed in the simplified theory. The balancing unit is illustrated in Fig. 7 and comprises a length of transmission line 150 (artificial or otherwise) a quarter-wavelength long at the commutation frequency (3.8 Mc/s), connected in parallel with the input to the commutator and short-circuited at its far end. Waves reflected from the far end of the line are of reversed phase and are displaced by a half-wavelength with respect to the direct input to the commutator and by suitable adjustment of the resistors R<1> and R<2>, the resultant waveform may be made substantially free from " cross-talk ". The resistor R<1> behaves principally as an amplitude control while resistor R<2> acts as an impedance-matching termination for the line 150. The system according to the invention is completely compatible with standard black-andwhite transmissions, i.e. the receiver will receive conventional transmissions in black-andwhite, while a conventional receiver will receive and display the signals received from the transmitter as a black-and-white picture. Moreover, by virtue of the way " cross-talk " is avoided, the complete three-colour transmission may be contained within the standard width frequency band for black-and-white transmission with very little loss of picture definition.