Patent GB-252348-A (GB)
Title: Systems of radio broadcast distribution
IPC: H04H20/72
Filed: 1926-04-28
Abstract: 252,348. Colladay, E. F., (Assignee of Clement, E. E.). May 21, 1925, [Convention date]. Multiplex signalling; relaying systems; secret and selective signalling systems; calling systems; thermionic amplifiers and detectors. - A wireless broadcasting system comprises a central master station A, district stations B, local distributing stations C, and subscribers' stations D, as shown in Fig. 1. The programmes, which may originate at the microphones M at the master station or at other stations, are sent out as modulations of an intermediate frequency wave, which itself modulates a carrier wave. At the relay stations B, C the received carrier is rectified and the resulting modulated intermediate frequency wave is retransmitted on a different carrier. At the subscribers' stations D, the programmes are received by double detection, with a selection of programme on the intermediate frequency. The tuning of the aerial 13 and secondary circuit 15 of the subscribers' set is fixed, the selection being afforded by operating switches 26 - - 29 which put in circuit a desired tuned coupling 18 - - 21 between the two detectors 10, 30. When one of the switches 26 - - 29 is operated, it closes the filament circuits of the first detector 10, and the second detector 30, and connects the primary and secondary of the selected coupling 18 - - 21 in their respective plate and grid circuits. The tuning of the couplings 18 - - 20 is permanently set for the reception of normal programmes, whereas that of the coupling 21 is adjustable to enable special programmes to be received. A telephone or a loudspeaker and amplifier is plugged into the jack 42. Separate second detectors such as 30 can be associated with each coupler 18 - - 21; and a rotary switch may be employed instead of the keys 26 - - 29. At relaying stations the one or more intermediate frequency side-bands may be detected, amplified, and used to modulate a new carrier. To enable calling and two-way signalling to be effected selectively and secretly, especially between relay stations, certain carrier frequencies are allotted as receiving frequencies to each station or class of station. The station calling is identified by the intermediate frequency employed. Alternatively, a station may be called by using the intermediate frequency allotted to that station; or by utilizing a third frequency such as a 60-cycle hum to operate a tuned relay or signal at the desired station. The Specification as open to inspection under Sect. 91 (3) (a) comprises also the following subject-matter:- Secret signalling systems. To afford secrecy in the distribution from local stations C to subscribers' stations D, the programme transmitted by the microphone 47 may be masked by a hum from the A.C. mains 56, Fig. 6 (Cancelled). The two modulations are combined in the transformer 51, which is connected to a modulator 52, intermediate frequency generator 58, second modulator 59 and carrier wave generator 60, by which the aerial 62 is energized. The receiving aerial 63 at the authorized stations D is coupled to the first detector 65 which has its output circuit tuned to the intermediate frequency and coupled to the second detector 67, with the phones 69 in its plate circuit. The filaments of these valves are energized through a transformer 71 by A.C. from the same power mains 56, and the phase is so adjusted that the hum is neutralized. The plate circuits of the valves 65, 67 may also be supplied from the mains through rectifiers 73, 74, which may also assist in neutralizing the transmitted hum. Fig. 7 (Cancelled) shows another form of secret transmitter, not requiring a wire connection between the sending and receiving stations. The microphone 75 is connected to two transformers 77, 78 which modulate two separate intermediate frequency generators 85, 86 the combined output of which modulates the carrier wave generator 89 and energizes the aerial 91. A hum from the source 84 is superposed on the modulation by means of additional coils 81, 82 on the transformers 77, 78. Reversing switches are provided for reversing the phase of the respective modulations. At a relaying station such as B or C, Fig. 1, the two intermediate frequencies are separated by selective circuits after a first detection, and are then used jointly to modulate a new carrier wave. At an authorized receiving station D the two intermediate frequencies are selected by tuned circuits 107, 108, Fig. 9 (Cancelled), after a first detection at 105; and after passing the separate second detectors 109, 110 the resulting telephonic currents are combined bv the transformer 113 and telephones 114. In one position of the reversing switch 115 the hum is neutralized, and the programme alone is heard. If a second programme is transmitted by a microphone 75<a>, Fig. 7 (Cancelled), connected in place of the alternator 84, one programme will be heard in one position of the switch 115, Fig. 9 (Cancelled), and the other programme in the other position, whilst an unauthorized receiver would receive an unintelligible combination of the two. Another system of secret transmission is shown in Fig. 10 (Cancelled), involving the suppression of the unmodulated component of the intermediate frequency. The intermediate generator 119 is modulated by the microphone circuit 116, and one side-band is selected by the filter 121 and is used to modulate the carrier wave supplied by the source 127. The intermediate frequency is sent out through an amplifier 137 to a telephone or power line 140. At an authorized receiving station, the carrier wave is received on the aerial 141 and rectified by the detector 143. The inter mediate frequency side-band passes through a filter 145 and is combined in the circuit 147 with the unmodulated intermediate frequency received on the line 140. The combination passes to the second detector 151 and the resultant signal is heard in the phones 153. The unmodulated component of the intermediate frequency may be suppressed at the transmitter by the use of push-ball modulators. The carrier wave mav be derived as an harmonic of the intermediate frequency. This subject-matter does not appear in the Specification as accepted.