Patent GB-820374-A (GB)
Title: Transmission systems for coded signals containing d.c. components
IPC: H04N7/169
Filed: 1956-06-07
Abstract: 820,374. Secret transmission. ZENITH RADIO CORPORATION. June 7, 1956 [June 7, 1955 (2)], No. 17605/56. Class 40 (4). In a system in which the coding of a signal introduces a D.C. component which would be lost in transmission, thereby introducing distortion in the decoded signal at the receiver, the D.C. is transmitted as modulation of an alternating current whose frequency is outside the range of the frequencies to be transmitted. The invention is applied to the coding of the audiofrequencies in a subscription television system. At the transmitter the audio source 24, Fig. 2, after amplification, feeds a phase-splitter 26 whose two outputs feed a coder 28 which selects one or other according to which of two amplitudes is emitted by the pulse generator 38. The resulting output C thus consists of alternate portions of uninverted and inverted signals which output may contain a substantial D.C. component which is caused to modulate (at 32) a 31.5 kc/s. (= twice the line scan frequency) carrier and mixed with the A.C. component of the signal at 31 and caused to modulate a carrier at 36 for outward transmission. The television signal received from the pick-up device 12 is fed to a coder 14 which imposes or does not impose a delay thereon in accordance with the presence or absence of pulses received from generator 38 thereby varying their phase relationship with the sync. pulses, which are fed in subsequently at 15. to scramble the signal in known manner, the signal then being radiated in the usual manner. The device 38 comprises a counting- device responding to the line sync. pulses to develop a square-wave signal whose amplitude changes every 15 lines. During the frame flyback random pulses are applied to a bistable trigger whose condition then determines the initial phase of the square wave signal for the next frame. The device 38 is arranged to change its state during a line fly-back period and the 31.5 kc/s. oscillator is so phased, for co-operation with one type of a receiver, that one of its amplitude peaks never coincides with a line fly-back period. Fig. 1 illustrates an extreme case of distortion in which the changesover of device 38 coincide with zeros of the audio signal W. Wave X illustrates the wave as coded showing a substantial D.C. component 11. Wave Y represents the received coded wave without its D.C. component and Z shows the distorted waveform produced when the wave Y is "decoded." Curve E, Fig. 6, represents the received wave Y with the 31.5 kc/s. signal superimposed. In one type of receiver, Fig. 4, the received wave is fed to filters 66, 67 which separate out the audio signal G, Fig. 6, and the 31.5 kc/s. signal, the latter feeding a detector 71 producing both a positive and negative D.C output. The filter 66 feeds a phase splitter 68 producing outputs G, H and these are combined with the detector outputs to produce waves J, K. These are selected in turn in the decoder 69 to produce the required output wave L. The video unscrambling and decoding are controlled by a source 38<1> synchronized and phased with the source 38 at the transmitter by signals sent during the frame fly-back period. In a modification, Fig. 5, not shown, the distorted waveform previously described is produced as shown at M, Fig. 6, and the decoder control circuit (as 38<1>, Fig. 5) produces a square wave N whose amplitude is controlled in accordance with the amplitude of the detected 31.5 kc/s. signal. The two signals M and N are then added to produce the undistorted signal L. In a further modification the signal E, Fig. 8, is fed to a phase-splitter to produce signals E and E<1>. These signals are switched alternately by the decoder control circuit to produce the waveform F. It will be seen that the required signal is formed by the envelope of the maxima and minima of alternate portions of the curve F, and that these may be sampled by an appropriate series of regularly-spaced pulses P. These pulses may be developed by a generator synchronized by the line-scan pulses. Curve Q shows the output of the sampler which is fed through a low-pass filter to produce the required audio signal R.