Patent GB-733943-A (GB)
Title: Improvements relating to wired signal-transmitting systems
Inventor: GARGINI ERIC JOHN
IPC: H04N7/10
Filed: 1953-02-03
Abstract: 733,943. Distributing music &c. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Jan. 25, 1954 [Feb. 3, 1953], No. 2977/53. Class 40 (4). In a system for distributing sound programmes in which at least two cable channels are provided, a plurality of different sets of intelligence signals is transmitted over each channel (the sets being different for each channel), each receiver is capable of responding to any one of the sets of signals and includes switching means for connecting the receiver to any one of the cable channels, keying signals are transmitted over a phantom circuit of the channels, and means at each receiver selectively gates the sets of intelligence signals on the selected channel to a reproducing device in response to the received keying signals. In the arrangement described the distributing unit 1 (Fig. 1) comprises three sources of audio frequency signals 3, 4 and 5, which form one group, and three further sources 3a, 4a and 5a, which form a second group. The signals from these sources are applied to gates 6, 7, 8, 6a, 7a, and 8a, respectively, which are controlled by a pulse generator 9 producing rectangular pulses at say, 50 Kc/s., with the interval between pulses at least twice the pulse duration. Delay networks 10 and 11 ensure that the gates 6, 7 and 8 are open in turn cyclically, in time with 6a, 7a and 8a, respectively. The outputs from each set of three gates are combined in mixers 12, 12a, respectively, amplified and fed via D.C. restorers to the cable pairs 15 and 15a, respectively, of the 'star quad cable 16. The pulses from generator 9 are also fed over the phantom of pairs 15, 15a after amplification at 17. Each receiver unit 2 has a manual switch 18 for applying the signals from pair 15 or 15a to a high-speed gate 19. The pulses fed over the phantom are applied to an adjustable delay network 20 which is manually controlled to insert a delay of zero, that of delay network 10, or that of 10 plus that of 11, so that the gate 19 is opened to pass a selected one of the three sets of signals received over the pair preselected by switch 18. The output of gate 19 passes via low-pass filter 21 to loud-speaker 23, blocking capacitor 22 removing the D.C. bias. Preferred form of receiver, Fig. 2. The gating pulses are extracted from the phantom at the mid-points of chokes 30, 31, blocking condensers 32 to 35 being provided. The output from the adjustable delay network 36 is connected to the series combination of a capacitor 38 and a diode 39. The leak resistor 40 shunts the diode 39. The manual switch 42 (corresponding to 18 of Fig. 1) connects the selected cable pair to the anode of a diode 43 and the lower end of resistor 40; 44 is a lowpass filter. The capacitor 38 is maintained charged (by conduction of diode 39 during the negative gating pulses) to the potential difference between the mean and peak potentials of the pulses. This changes the level of the pulses so that a positive bias is maintained on the cathode of diode 43 in the intervals between gating pulses and is removed during the pulses to enable this diode to conduct.