Patent GB-566669-A (GB)
Title: Apparatus for reducing echo effects in picture transmission systems
IPC: H04N11/00
Filed: 1943-05-04
Abstract: 566,669. Television &c. PHILCO RADIO & TELEVISION CORPORATION. May 4, 1943, No. 7033. Convention date, May 5, 1942. [Classes 40 (iii) and 40 (v)] Relates to the cancellation of echo signals, due to delayed reception of synchronizing signals in a radio television or picture transmission system, by periodically changing the phase of the carrier wave as described in Specifications 562,707 and 562,794 ; and consists in effecting the desired phase change by changing the carrier frequency a predetermined amount for a predetermined time, preferably for a fraction of the blanking interval. Fig. 1 shows the same line in two successive frames of the signal, the upper part showing amplitude variations and the lower part frequency variations, the section 1 being the line-synchronizing signal, 2 and 3 portions of the blanking signal, and 4 the video signal. During a portion of the blanking period 3, the frequency is raised for a period tx an amount such that the phase is advanced 90 degrees ; and in the following line the frequency is correspondingly reduced so that the phase is retarded by 90 degrees. The echo signals shown shaded in the Figure consequently cancel each other. Fig. 3 shows a suitable lay-out in which the carrier source 5 is connected through an optional frequency-multiplier 6, amplifiers 7, 10, modulated stage 8 and side-band filter 11 to the aerial 12. The mixed video, synchronizing and blanking signal is applied from 13 through an amplifier 14 and modulator 9 to the modulating stage 8. The synchronizing signals 24 are separated from the mixed signals at 15 and are applied through an amplifier 16 to a frequency shifter 17 which operates on the carrier source 5 and may be of the kind described in Specification 490,625. The line synchronizing signals are separated from the frame synchronizing signals at 18, are halved in frequency at 19 and are used to control a sine-wave generator 19a, which in turn controls a square pulse generator 20. The square pulses are converted at 21 into alternate positive and negative narrow pulses 22 which are amplified at 23 and applied through a gain control device 26 to the combining amplifier 16, causing a combined signal 25 of the same form as is shown in the lower part of Fig. 1 to be applied to the frequency shifter 17. Frame synchronizing signals derived from 18 are applied through a stage 27 to the amplifier 23 to cut out that amplifier during the frame signal intervals. Owing to the frequency selectivity of the receiver, the changes in frequency during the intervals tx, t<1>x would cause respectively a rise and a fall in the received signal. This is compensated by arranging that the transmission amplitude falls during the interval tx and rises during the interval t<1>x, as indicated in the upper curves of Fig. 1. To effect this, the pulses 22, Fig. 3, are fed from the gain control 26 through a second gain control to the signal amplifier 14 so that they act through the modulator 9 to produce the desired amplitude changes. The lay-out described utilizes different carrier frequencies for the synchronizing and the video signal, but the echo cancellation system is applicable to systems employing the same carrier frequency for both signals. The 180 degrees phase change may be effected in a single step instead of the two steps described above ; or it may be effected by three-phase or polyphase methods described in Specification 562,707. Specification 485,924 also is referred to.