Patent GB-842421-A (GB)
Title: Improvements in or relating to means for selectively receiving a plurality of simultaneously-transmitted telegraph messages
IPC: H04L5/08
Filed: 1956-12-21
Abstract: 842,421. Multiplex radio signalling. SIEMENS & HALSKE A. G. Dec. 21, 1956 [Dec. 23, 1955], No. 39024/56. Class 40(5). Relates to duplex telegraph systems of the type in which a carrier frequency assumes four values f 1 , f 2 , f 3 , f 4 , for the four possible combinations of marking and spacing conditions in the two signal channels A, B, viz. space-space f 1 , space-mark f 2 , mark-spaced f 3  and mark-mark f 4 , and wherein the signals are separated at the receiver by circuits which produce phase shifts dependent on frequency. In the embodiments described the difference between adjacent frequencies is arranged to be 400 c.p.s. and the mean frequency is f 0 , as that f 1  = f 0   - 600, f 2  = f 0  - 200, f 3  = f 0  + 200, and f 4  = f 0  + 600 c.p.s. In one arrange ment, Figs. 1 and 2, three demodulators I, II, III, are tuned to the frequencies f 0 , f 0  - 400, and f 0  + 400 c.p.s. respectively each demodulator including a discriminator consisting of a condenser C and inductor L, and the incoming signals at Kl are amplified and applied to the demodulators in parallel. The voltage across each series circuit LC is fed via an amplifier VII to one side of a phase-sensitive detector GM, the other side being fed via an amplifier V12 with a component of the voltage tapped from the corresponding inductor L the resultant outputs of demodulators I, II, and III being as shown in Fig. 2. Since message A will carry spacing current on the occurrence of frequencies f 1  and f 2 , and marking current on the occurrence of frequencies f 3  and f 4 , the demodulated message A may be obtained from the output of the demodulator I. To obtain the message B, the outputs of the demodulators II and III are added and the output voltage of the demodulator I subtracted from this sum, the resultant output voltage being as shown in Fig. 2(IV), and satisfying the requirement for message B that frequencies f 1  and f 3  correspond to a spacing current, and f 2  and f 4  to a marking current. A further arrangement, Fig. 3 may employ two demodulators, the demodulator having the curve I, Fig. 3(a) having a characteristic similar to those of Fig. 1 and serving to demodulate the A signal. The characteristic II of the second demodulator is arranged to be half as steep as the central portion of the characteristic I of the first demodulator whereby the curve(b) is obtained by subtraction, which provides that the output voltage set up for frequences f 1  and f 3  is of different value and preferably of opposite polarity to that set up for f 2  and f 4 , the current conditions for receiving the B message. The two characteristics may be obtained Fig. 4 (not shown) by taking the output from a discriminator, which varies according to curve II, and applying it to a voltage divider, a tap on the voltage divider providing one output. The full voltage from the discriminator is also applied to a limiter so biased that the output is limited at frequencies f 2  and f 3  as in curve I. The two outputs are differentially combined and produce a characteristic of the form shown in Fig. 3. In an alternative arrangement, Fig. 6 phase-shifting networks consisting of series resonance circuits S 1 , S 2 , S 3 , are fed with constant current through a high resistance and isolated from one another by valves. The circuit S 1  is tuned to the frequency f 0 , and depending upon whether the incoming frequency is higher or lower than f 0  a phase shift of + 90 degrees or - 90 degrees is obtained by tapping from the circuit S 1 . The circuit S 2  is tuned to the frequency f 0  - 400 and the circuit S 3  to the frequency f 0  + 400 cycles per second. If the frequency f 1  is received the arrangement is such that a phase shift of - 270 degrees exists between the input voltage U 01  and the output voltage U s3  and if the frequency f 2  is received the phase shift is - 90 degrees on reception of f 3  it is + 90 degrees and on reception of f 4  it is + 270 degrees. A zero phase shift exists between the voltages U L1  and U s1  at the frequencies f 1  and f 2 , and at the frequencies f 3  and f 4  the phase shift is 180 degrees. In this way the message A is recovered at the terminal A. Similarly, a phase shift of  - 180 degrees exists between the voltages U L1  and U s3  at the frequency f 1 , a zero phase shift at frequency f 2 , a phase shift of 180 degrees at frequency f 3 , and a phase shift of 360 degrees at frequency f 4 , thus providing the correct conditions for the reception of message B at the terminal B.