Patent GB-691952-A (GB)
Title: Improvement in cross talk eliminating apparatus in a time division multiplex system
IPC: H04J3/04
Filed: 1951-05-01
Abstract: 691,952. Colour television. RADIO CORPORATION OF AMERICA. May 1, 1951 [May 8, 1950], No. 10232/51. Class 40 (iii). [Also in Group XL (c)] In a time-division multiplex system, particularly for use in dot-sequential colour television, one channel, e.g. the blue-signal channel, is so restricted in band-width as to avoid cross-talk in this channel from the remaining signal channels, while the remaining channels have band-widths which extend into a region of the frequency spectrum wherein cross-talk occurs, cross-talk between these remaining channels being removed by so inter-connecting them as to feed into each that portion of the signal carried by the other channel lying within the cross-talk region, suitably adjusted as regards amplitude and phase. In the transmitter illustrated in Fig. 1, the outputs from "red" and "green" cameras 2, 14 are applied through low-pass filters 4, 16, having pass-bands extending into the " cross-talk " region, to respective adder circuits 6, 8. The output from filter 4 is also applied to a band-pass filter 10 which passes signals lying within the cross-talk region only, the output from filter 10 being added in circuit 6 to the direct output from filter 4. The output from filter 10 is also applied to a phase-shifter 12 which imposes a uniform 120 degree phaseadvance upon all signals lying within the passband of filter 10, the output from this phaseshifter being applied to the adder 8 in the green channel. The output from filter 16 is similarly coupled to adders 6 and 8 through corresponding filter and phase-shifting circuits 18 and 20, the phase-shifter 20 imposing a 120 degree phasedelay upon signals applied thereto. In addition, the output from the blue camera 22 is applied through a low-pass filter 26 which passes only components lying within the cross-talkfree region. For certain applications, to be described later, the blue channel may also include a circuit 24. The outputs from circuits 6, 8 and 26 are sampled cyclically at a high rate by a device 44 shown as a rotary distributer, the output from the sampling device being subsequently filtered and modulated upon a carrier wave for radio transmission. The apparatus described may be used in a number of ways, the pass-bands of the filters 4, 10, 16, 18 and 26 depending upon the manner in which the apparatus is used, the sampling rate of the device 44 and the cut-off frequency of the transmitter. Thus, horizontal interlacing of the red and green components by periodically altering the phase of the sampler with respect to the frame frequency may or may not be used. Also, by use of the circuit 24 an additional interlacing in the blue channel may be effected so as to effectively increase the band-width of the " blue " signal that can be transmitted without extending into the cross-talk region. The circuit 24 comprises a low-pass filter 38 and a sampling device 40 operating at twice the maximum frequency of the cross-talk-free region, the phase of the sampling changing by 180 degrees in successive line periods. The receiver for the transmitter of Fig. 1 comprises a sampling device operating in synchronism with sampler 44 and appropriate low-pass filters in the output channels therefrom. The blue channel also includes a further sampling device operating in synchronism with sampler 40 if horizontal interlacing is employed in the blue channel. The invention may be modified so that the interconnections between the red and green channels are effected at the receiver or in part at the receiver and in part at the transmitter. In the case where the crosscoupling is effected at the receiver, interconnections similar to those shown in Fig. 1 are arranged between the red and green channels after the sampler at the receiver. Further sampler devices are preferably arranged in the green and red channels before the signals carried by these channels are applied to the image reproducing device. In a second embodiment, Fig. 6 (not shown), a four-channel time-division multiplex system is provided for transmitting the concomitant sound signal together with the three colourcomponent signals. The red, green and blue channels each carry signals covering the same frequency band and extending into the crosstalk region. The cross-talk-free region, in the example given, covers 0.8-1.25 Mc/s., and the sound signals are frequency-changed to lie within this band. The three colour-component channels are interconnected in a manner similar to that described with reference to Fig. 1 and the outputs from the four channels are applied to respective terminals of a four-way sampling device to effect time-division multiplexing of these signals. A corresponding sampling device is provided at the receiver, the sound-signal output therefrom being applied to a suitable frequency changer in the sound channel.