736,126. Distributing music &c. ELECTRIC & MUSICAL INDUSTRIES, Ltd. April 2, 1954 [April 10, 1953], No. 9727/53. Class 40 (4). In a picture-signal distributing system employing repeaters, distributed signals are returned to a monitoring location from the output side of one or more repeaters. The system shown in Fig. 1 is in general of the type described in Specification 704,141, the signals fed from the distributing station 1 over the line 2 to subscribers 3<SP>1</SP> ... 3n and 4<SP>1</SP> ... 4n consisting of a first carrier wave of 5.4 Mc/s. modulated with picture signals, and a second carrier wave of 1 Mc/s. modulated with synchronized scanning information. The repeater 5 consists of a main amplifier 6 passing all frequencies trans mitted over the line 2, which has band stop filters 7, 8 (attenuating 200 to 600 kc/s.) at its input and output sides. The auxiliary amplifier 9 which amplifies in the opposite direction has a pass range of 200 to 600 kc/s. and has band-pass filters 11', 12 with the same pass range at its input and output sides. Part of the output of the amplifier 6 is fed to picture signal amplifier and detector 13 and to a further amplifier 14 which amplifies the second carrier wave modulated with scanning information. The amplifier and detector 13 produces picture signals demodulated from the 5.4 Mc/s. carrier, which are amplified at 15 and fed to modulator 16. The output of amplifier 14 is applied in parallel to the frequency divider 17, which divides by four and produces an output of 250 kc/s. which is fed to the modulator 16, and to the detector circuit 18, the output of which is a waveform comprising line and frame frequency interruptions of different amplitudes and durations which is applied to pulse generator 19. This generator responds to the frame interruptions and produces a gating pulse having a duration of, say, ten line periods which is applied via delay network 20 to gate 21, thus opening the gate for a predetermined ten line periods, so that signals from modulator 16 pass via amplifier 9 back to the station 1. The duration and delay of the gating pulses produced at the different repeaters are arranged so that at each repeater a different section of the picture is sampled, the sections being successive and contiguous. Thus, in the normal case, a complete sequence of picture signals is continuously returned and is applied to the monitor cathode-ray tube 22 via band-pass filter 23 (200-600 kc/s.). In order to compensate for different relative delays in the signals returned from different repeaters, the frame frequency pulses used to control the frame time base 24 of the monitor tube 22 are applied to a series of gating pulse generators 251 ... 25k connected in tandem. These generators produce a sequence of gating pulses, corresponding respectively to those generated at the repeaters, which are applied to gates 271 ... 27k. Line pulses for controlling the line time base circuit of tube 22 thus pass through these gates to the corresponding adjustable delay networks 28<SP>1</SP> ... 28k, the outputs of which are combined and fed to the line time base circuit 26. Alternative arrangement (Fig. 2). The distributed signals pass to the end of each of the lines 21 ... 2e where the final repeater 5k is provided with a monitoring signal generator 29. This comprises a picture tube 30, and a pick-up tube 31 which generates signals representative of the image produced on tube 30. Masks 30a are so arranged in front of the tubes 30 that each pick-up tube 31 generates signals corresponding to a different section of the picture. Keying pulses are sent over the lines, either by the operator as desired, or automatically at regular intervals, which cause all the repeaters 5<SP>1</SP>... 5k to function in the reverse direction and return signals from the monitoring signal generators 29. A repeater, as shown in Fig. 3, includes a flip-flop circuit 44 in the normal condition of which valves 32 and 35 conduct, so that signals are passed from left to right through valve 32, amplifying stages 39, and valve 35. The keying pulses, which are in the frequency spectrum of the scanning information carrier, are amplified by amplifier 46, separated by 47, and applied to the flip-flop 44. Valves 32 and 35 are thus cut-off and 33 and 34 conduct, so that the repeater amplifies signals from right to left. Switch 48 (Fig. 2) also reponds to the keying pulses to switch the output of tube 31 to the valve corresponding to 33 in repeater 5k.