GB784663A - Improvements in or relating to single-sideband radio transmission systems - Google Patents

Abstract

784,663. Radio transmitting and receiving systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Oct. 21, 1955 [Oct. 25, 1954], No. 30158/55. Class 40 (5). In a single-sideband communication system, for speech signals, at the transmitter the singlesideband signals before transmission are passed through a band compressor and an amplitude modulator controlled by an envelope detector fed from the single-sideband signal, while the receiver comprises a band expander. In one embodiment, the transmitter comprises a single-sideband modulator 4, 5, 6, Fig. 1, fed from a speech source 1, through a filter 2, preemphasis network 19 and amplifier 3. The single-sideband signal is passed through a limiter 14, band compressor 15 and amplitude modulator 16 to the group modulator 8 ... 11 and transmission line 12. The amplitude modulator 16 is fed by an envelope signal derived by a detector 17 from the single-sideband signal. The band compressor 15, which may divide the input frequencies by ten, for example, comprises a counter-divider or a locked-in subharmonic oscillator. At the receiver, the main demodulator 26, Fig. 2, is supplied from the line 12 through the group demodulator 20 ... 23, limiter 35, band expander 36 and amplitude modulator 37, the latter being fed with an envelope signal derived by a detector 33 from the single-sideband signal. The output from demodulator 26 is fed through a de-emphasis network 29 to the reproducer 31. The band expander 36 comprises a frequency multiplier, which may be a class-C amplifier and output filter. An equalizing network may be provided if the transit times of the envelope channel and the bandcompression or band-expansion channels are different. In another embodiment, Figs. 3, 4 (not shown), the band-expansion and bandcompression is performed by applying the singlesideband signal to a frequency discriminator whose output modulates the frequency of a local oscillator. In other arrangements, the speech signal is converted into single-sideband signals representing different formant ranges, e.g. 300 to 800 c/s., 800 to 2000 c/s. and 2000 to 3200 c/s., at least one of the lower ranges being transmitted by the single-sideband system described above. Thus, in one embodiment speech signals from the source 1 and preemphasis network 19 are applied to three bandpass filters 48, 49, 50, Fig. 5, to separate the formant ranges which are applied respectively to single-sideband modulators 51, 52, 53. The single-sideband signals are respectively applied through limiters 57, 58, 59 and band compressors 60, 61, 62 to the group modulator 9 ... 11, that from the lowest band being modulated in amplitude in the modulator 66 by an envelope signal derived from the detector 67. In the example described, the carrier oscillator 69 of the modulator 51 operates at 57 kc/s., the upper sideband being selected and the modulators 52, 53 are fed from a common carrier oscillator 70 operating at 60 kc/s., the upper sidebands being selected. This leaves a space for the sidebands produced by the envelope modulation in modulator 66. At the receiver, the group demodulator 20... 23 feeds the signals to three frequency changers 75, 76, 77 to restore the three single-sidebands to their correct relative positions. The three signals are then respectively fed to the demodulator 26 through limiters 81, 82, 83, band expanders 84, 85, 86 and amplitude modulators 90, 95, 96. The latter are controlled by the envelope signal derived by a detector 91 fed with the single-sideband signal of the lowest speech range. The demodulator 26 feeds the reproducer 31 through the de-emphasis circuit 29. In a modification, each single-sideband path at the transmitter and receiver is supplied with an envelope signal channel and amplitude modulator and at the receiver, the recovered envelope signal from the single-sideband of the lowest speech range is applied to all the amplitude modulators, in the two upper ranges through a common A.G.C. amplifier. The other two envelope signals control the gains respectively of amplifiers connected between the single-sideband channels and the common demodulator, Figs. 7, 8 (not shown). Specifications 652,717 and 663,619 are referred to.