Patent GB-620715-A (GB)
Title: Improvements in or relating to electric carrier frequency modulation system
IPC: H04L27/16
Filed: 1946-09-20
Abstract: 620,715. Frequency modulation. STANDARD TELEPHONES & CABLES, Ltd., TERRY, V. J., and HARGREAVES, T. F. S. Sept. 20, 1946, No. 28298. [Class 40 (v)] [Also in Group XXXIX] In a multi-channel frequency modulation carrier telegraph system for operation over a communication medium at least part of which may produce a change in the mean carrier frequency of each telegraph channel, a pilot wave of a given frequency is transmitted with the frequencymodulated waves (over at least the said part of the medium) and means controlled by the received pilot wave either correct the change produced in the mean carrier frequency of each channel, or compensate the effect of such change. In the embodiment of Fig. 1 an 18-channel frequency modulation telegraph system is operated over one of the channels of a multi-channel amplitude modulation carrier telephone system. The telegraph channel carriers have frequencies spaced 120 c.p.s. apart from 420 to 2460 c.p.s. An unmodulated pilot wave of 300 c.p.s. is transmitted as a 19th channel. The transmitting part of each telegraph channel (of which only four are shown) comprises a frequency modu. lator 1 of conventional type followed by a band pass filter 2, while that of the pilot channel comprises a pilot oscillator 3 and filter 4. The outputs of all the telegraph channel filters 2 and the pilot filter 4 are connected to the amplitude modulator 5 of one of the channels of the multichannel telephone system, and thence via band pass filter 7. line amplifier 8, and directional filter 9 to the line. The receiving equipment comprises directional filter 11, line amplifier 12, the channel band pass filter 14, amplitude demodulator 15 and filter 17, telegraph filters 18 and a pilot filter 19. The filters 18 are connected through limiters 20 to conventional frequency demodulators 21, and the filter 19 is connected through a limiter 22 to the pilot demodulator 23. The frequency of the local oscillator 16 feeding the demodulator 15 is controlled by a reactance valve 24. The received pilot wave is applied to a frequency discriminator included in the demodulator 23 which derives in known manner a voltage dependent on the frequency shift suffered by the pilot wave. This voltage is. applied to the control grid of the reactance valve 24 and varies the frequency of oscillator 16 so as to correct any departure of the received pilot frequency from the original transmitted value. This embodiment involves a modification to the carrier telephone receiving equipment. This is avoided by the arrangement of Fig: 2, in which an additional frequency-changing device 25 is introduced between the filter 17 and the telegraph receiving equipment, the oscillator 16 (Fig. 1) being allowed to operate normally. The device 25 consists of a modulator 27 followed by a demodulator 29, provided with oscillators 30 and 31 respectively, the frequency of the former being controlled in accordance with the received pilot frequency through a reactance valve 32. The small residual frequency shift. remaining in these two embodiments may be neglected. In the embodiment of Fig. 3 (not shown), a closer correction is obtained by separating out the received pilot wave, modulating the received telegraph signals by a frequency, the sum of the received pilot wave frequency and a frequency F produced by a local oscillator, and demodulating by a frequency, the sum of F and the correct pilot frequency (produced by a local stable oscillator). A similar method which involves only one extra local oscillator is shown in Fig. 4. The pilot wave is separated by filter 34 from the telegraph channel waves which pass to modulator 37. A local oscillator 35 generates a frequency of 2580 c.p.s. (i.e. 120 c.p.s. higher than that of the highest telegraph channel carrier) which is supplied together with the received pilot wave of frequency (300+df) c.p.s. to the auxiliary modulator 40. The upper sideband (2880+df) c.p.s. is -selected by filter 42 and is applied to modulator 37. Considering the first telegraph channel, the carrier frequency received will be (420+df) c.p.s. so that the corresponding output carrier wave from 37 will be 2460 c.p.s., the same as that of the 18th channel as transmitted. The order of the channels is then inverted and the frequency shift eliminated. Re-numbering of the channel receiving equipments is therefore required. The embodiments of Figs. 5 and 6 (not shown), correct the frequency shift by applying a bias to the telegraph receivers opposite to that produced by the shift. The compensating bias is derived as a function of the shift suffered by the pilot wave by means of a frequency discriminator.