Patent GB-684307-A (GB)
Title: Improvements in multiplex radio communications systems
IPC: H04J1/20
Filed: 1951-07-16
Abstract: 684,307. Multiplex radio signaling; frequency modulation. MULTIPLEX DEVELOPMENT CORPORATION. July 16, 1951 [July 31, 1950], No. 16818/51. Class 40 (v). A radio station transmitting audio signals by frequency modulation is arranged to transmit facsimile or like signals also on the same carrier wave by the use of a supersonic sub-carrier which is first frequency-modulated by the facsimile signals and is then used to frequencymodulate the carrier. The invention is particularly applicable to a F.M. broadcasting network with associated relay stations at which means are provided for relaying both types of signals and for recording and monitoring the facsimile signal. Figs. 1, 3 and 11 are block diagrams of a transmitting, receiving and relaying station respectively. In Fig. 1, the audio program originating at 10 is passed through amplifier 11, pre-emphasis network 12, low-pass filter 13 suppressing 15 kc/s. and higher frequencies, to a mixer 14. The facsimile signals from a scanner 19 are amplitude-modulated on a 10 kc. wave, amplified at 20 and rectified at 21, the signals then passing through a low-pass filter 22 to frequency-modulate a multi-vibrator 23 through the range 23 to 28 kc/s. The output passes via a band-pass filter 25 giving uniform response over the pass-band to a screened line 26 to the mixer 14. Both signals are then fed to the main frequency modulator 15, power amplifier 16 and aerial 18. The level of the two types of signal is such that the facsimile signals provide a lower modulation index than the audio. The receiver station, Fig. 3, employs normal circuits for receiving and discriminating the F.M. carrier, the sub-carrier and the audio signal appearing across the load resistors 48A, 48B of the rectifier 47. The audio signal is modified by a de-emphasis network 52, 53 and is fed through an amplifier 56 to the speaker 32. The sub-carrier passes through a shielded line 33 and amplifier 34 to a high-pass filter 35, and thence through a further amplifier 36 to a frequency discriminator 37, amplifier 38 and facsimile recorder 39. At a relay station, Fig. 11, the received carrier wave is de-modulated at 310 and the audio signal, selected by a low-pass filter 314, is amplified at 315 and passed through a preemphasis network 316 to a mixer 317 and thence to a frequency modulator 318 and power amplifier 319 for re-transmission on a new carrier. The demodulated signal from 30 is also fed through an amplifier 325 and high-pass filter 326 to select the supersonic sub-carrier which is then amplified at 327, filtered by bandpass filter 328, again amplified and fed via the mixer 317 to the carrier modulator 318. A connection 332 to a sub-carrier discriminator 333 enables the facsimile to be recorded at 334. The transmitter circuit is shown in Fig. 6, in which the scanner 19 provides an amplitudemodulated sub-carrier which is fed through an attenuator 101 and transformer 102 to a rectifier bridge 105a. From this the facsimile signals pass through a low-pass filter to a freerunning multi-vibrator 23A normally operating at about 23 kc/s. but shifted in frequency up to about 28 kc/s. by the signals. The signal is fed to both grids through resistors 112, 113, one of which is variable. The F.M. output is passed through a cathode-follower valve-pair 24A, resistor pad 129, band-pass filter 25 and variable attenuator 137 to a coaxial connector 26 by which the F.M. sub-carrier is fed to the mixer 14 of Fig. 1. The lower part of Fig. 6 indicates the normal mains supply arrangements. Fig. 7 is a circuit diagram of a relay station, in which the mixed audio signals and the F.M. sub-carrier of the facsimile signals are fed through a cathode follower 34A to a highpass filter 35 which rejects the audio signals. The sub-carrier output is passed through separate potential-dividers 218, 219 to two separate paths, each comprising an amplifier 36A feeding a cathode-follower 36B. One cathode output is taken through a transformer 235 to terminals 237A, 237B which feed the F.M. sub-carrier to modulate the frequency of the relay transmitter. The other cathode output passes to a frequency discriminator network 37, the output of which, now bearing an amplitude modulation, is fed to a valve-pair 38A. The first of this pair is connected as a normal amplifier, the output of which passes to the second valve arranged as a cathode-follower. The cathode output comprising a sub-carrier amplitude-modulated by the facsimile signals is connected to terminals 264A, 264B which may be connected to a normal picture recorder at the relay station.