Patent GB-562707-A (GB)
Title: Apparatus for reducing echo effects in picture transmission systems
IPC: H04N7/06
Filed: 1943-01-08
Abstract: 562,707. Television. PHILCO RADIO & TELEVISION CORPORATION. Jan. 8, 1943, No. 390. Convention date, March 6, 1942. [Classes 40 (iii) and 40 (v)] Distortion of the received picture in a radio television system, due to delayed reception of the synchronizing signals over a path involving reflection, is substantially eliminated by periodically reversing or changing the phase of the carrier wave. These echo signals normally give rise to light or dark patches on the picture, but by the present system light and dark echoes occur alternately in successive lines or frames and in effect cancel each other out. The system may be used with single-carrier transmission, or with alternate-carrier transmission in which the synchronizing signals are sent on a carrier differing in frequency from the picture carrier. Referring to a single-carrier system, the position of the reflecting object may be such that the delayed synchronizing signal arrives in the same carrier phase as the momentary picture signal with the result that a dark band appears on the picture as shown in Frame 1 on Fig. 3. If the phase of the synchronizing signal is reversed at the end of that frame, the succeeding frame (No. 2) will contain a light band, and distortion is eliminated by the rapid succession of frames. When a reversal of phase of the synchronizing signal is effected after each line in an interlaced system, the successive frames appear as in Fig. 5. Various sequencies of phase reversal may be employed, Fig. 7 (not shown), the essential feature being that if in one line the synchronizing signal and the picture signal are in the same carrier phase, in the next line they are in phase opposition. The period of reversed phase may include the whole or part of the blanking signal. Three or more phases of carrier may be employed, Fig. 11 (not shown). With an alternate-carrier system, the echo signals appear as alternate light and dark bands in each line, as shown in Frame 1 of Fig. 4 ; and the result of reversing the phase of the synchronizing signal carrier at the end of each frame is to reverse the position of the bands as shown in Frame 2. Similar change-over effects are produced if phase is reversed at the end of each line. A transmission system of the alternate-carrier type is shown in Fig. 8, in which an oscillator 19 feeds a frequencymultiplier 20, and the resulting wave passes through a phase-reverser 21, modulator 23 and side-band filter 24 to the aerial 25. The complex picture signal passes through a stage 26 to amplitude-modulate the carrier, and the synchronizing signal is isolated and applied to a frequency-shifter 29, e.g. a thermionic reactance, to give a different carrier frequency during the synchronizing signals. The blanking signal is also separated and controls a device 22 by which the phase of the carrier is reversed periodically at 21. The phase-reversing circuit 21 and the associated control circuit 22 are shown in Fig. 9. The carrier wave from 20 is applied in push-pull to two valves V1, V2 which have a common anode circuit 31 feeding a further valve stage V3. The blanking signal from 27 is applied through a device 39, which rejects alternate pulses, to a valve V4 having load resistances 37, 38 in its anode and cathode leads. The grids of valves V5, V6 are coupled respectively to the anode and cathode of the valve V4 and resistances 34, 36 connected in their cathode leads cause one or other of the carrier valves V1, V2 to be operative. In this way, the phase of the carrier output is periodically reversed for the duration of every second blanking signal. The invention may be used in television systems in which the picture signals are transmitted by frequency, instead of amplitude, modulation. Specification 551,798 is referred to.