Patent GB-689534-A (GB)
Title: Improvements in multiplex communications system
IPC: H04N5/60
Filed: 1951-06-07
Abstract: 689,534. Multiplex radio signalling: MULTIPLEX DEVELOPMENT CORPORATION. June 7, 1951 [June 23, 1950], No. 13545/51. Class 40 (v). A broadcast F.M. transmitter is modulated directly by a first audio signal source, the modulation depth being limited to produce a predetermined maximum frequency deviation, and also by a super-audible subcarrier modulated by a second signal source, the modulation depth produced thereby in the main carrier being substantially less than that due to the first source and the total frequency deviation of the main carrier being limited to a predetermined maximum value. A broadcasting network is described having a chain of three separately located F.M. broadcasting stations having different carrier frequencies and with radio relay facilities to enable main programme sources at any of the locations to be used to modulate any of the transmitters. Independent of the source of the main programme, subcarrier signals may be superposed on the transmitter carriers from either of the terminal stations of the network and relayed to the other stations for transmission, the direction of transmission of each programme signal along the chain being selectively controlled from a central point. The subcarrier signals are intended for reception only by authorized receivers of special type, which are controlled by special signals which mute the receivers, for example, during announcements between musical items. Terminal station, Fig. 5. The main broadcast programme at a terminal station is derived from audio source 100, connected through switch 101, pre-emphasis circuit 30, low-pass filter 31, limiter 115 and mixer 116, to the modulator 32 of the transmitter 33. The output of a first 30 Kc/s. subcarrier source and modulator 40, after passing through band-pass filter 41, is also applied to the mixer 116 to modulate the transmitter 32, 33. The signal fed to the modulator 40 is either music from source 119 or speech from source 130, depending on the position of the switch 120, and passes through pre-emphasis circuit 123, lowpass filter 124 and limiter 125. Switch 120 also controls the frequency of an audio-frequency oscillator 153, so that in the music position it generates a note of 85 c/s., for example, and in the speech position a note of 87.5 c/s. When contacts 189, 190 are closed, this note frequency or amplitude modulates a further " security subcarrier source 154 which may be set by switch 155 to one of four frequencies between 30 and 70 Kc/s, The third pair of contacts of switch 120 is arranged so that as the switch is operated in either direction, relay 168 is de-energized momentarily to close contacts 170. This energizes a latch magnet 172 to release clutch disc 174, driven by motor 178 for one revolution. The coupled disc 180 is thus rotated and pulsing cam 185 closes contacts 189, 190, and 186, 188 to connect the output of subcarrier source 154 through band-pass filter 191 to the mixer 116, to transmit one signal pulse. This occurs every time the music/speech switch 120 is operated and controls the speech muting devices at the special receivers. The subcarrier source 154 is also modulated by 80 c/s. decoy pulses from a source 205 randomly keyed by switch 203 operated by cam 200 on a disc 195 driven through a slipping coupling 196 to gear box 197 connected to the motor 178. In order to control the direction of transmission of the main programme along the relay chain, a switch 215 is provided which in its two positions, south/ north and north/south, causes the direction control source 216 to produce different audio frequencies, for example 80 and 87 c/s. respectively. This source modulates a further subcarrier source 218 operating at 18 Kc/s. and feeding into the low-pass filter 31. Direction control of the subcarrier programme from source 40, is similarly provided by switch 225 and two-frequency audio source 227, producing for example, 25 or 20 c/s., and feeding into low-pass filter 124. A further subcarrier of 55 Kc/s. modulated by a business message source may modulate the main carrier, Fig. 6 (not shown). Intermediate station, Fig. 7. At an intermediate station a directional receiver 17, 18 receives the transmission from the adjacent southward station. The main programme is passed through audio amplifier 260, low-pass filter 19 contacts 279 of relay 20, if it is ener. gized for south to north transmission, amplifier 263 and pre-emphasis circuit 21 to the modulator 23 of transmitter 24, 25. Relay 20 may be energized by local or remote control. In the latter case, the received 18 Kc/s. subcarrier is selected by band-pass filter 270, demodulated in circuit 271 and the 80 c/s. control modulation fed through amplifier 272 to the tuned relay 273 to close contacts 274, 275. The coil 277 of relay 20 is thereby energized as required and a signal lamp 291 lighted. In the case where the 30 Kc/s. subcarrier programme is originating at a northward station and is to be re-transmitted and relayed southward, the directional receiver 42, 43 is used. The subcarrier output is fed through amplifier 300, high-pass filter 46, amplifier 301 and contacts 302 of relay 47 to the modulator 23 of transmitter 24, 25. Relay 47 is remotely controlled by demodulating the subcarrier from amplifier 301 in circuit 310, the output from which is passed through amplifier 311 to the tuned relay 312, which responds to the 25 c/s. control modulation to energize relay 47. A signal lamp 304 is lighted. Other tuned relays 405A, 423A and switching relays 45, 49 respond to the other control frequencies in similar manner to switch the station for relaying in the opposite directions. Broadcast receiver, Fig. 8. A receiver for receiving the main programme and alternatively the subcarrier programme with or without the speech part thereof comprises the F.M. receiver 500, switch 602, audio amplifier 503 and loud-speaker 505. When switch 502 is in position 501, the main programme output is applied to the amplifier 503. When the switch is in position 515, the subcarrier programme is applied to the audio amplifier from the subcarrier demodulator 513 and low-pass filter 514 fed with subcarrier from receiver 500 through amplifiers 510, 512 and low-pass filter 511 therebetween. When switch 502 is in position 545 the subcarrier programme output from filter 514 is switched on and off by the latched relay 539 which is controlled by the pulsed " security " subcarrier transmitted with the subcarrier programme. The security subcarrier is selected by tuner 521 from the output of filter 511 and demodulated in circuit 531. A pulse of the " music " 85 c/s. control frequency, energizing tuned relay 532, operates relay coil 538 the armature 540 of which is held by latch 541, the input of audio amplifier 503 being connected through contacts 542, 543 to the subcarrier programme output from filter 514. A pulse of the " speech " 87.5 c/s. control frequency, energizing tuned relay 533, operates latch coil 568 to release the armature 540 and connect the input of audio amplifier 503 to earth.