Patent GB-1424133-A (GB)
Title: Transmission of wide-band sound signals
IPC: G10L21/038
Filed: 1973-02-22
Abstract: 1424133 Bandwidth reduction system INTERNATIONAL STANDARD ELECTRIC CORP 22 Feb 1973 [24 Feb 1972 14 April 1972 12 Oct 1972] 5780/73 Heading H4R In a transmission system for sound signals of about 20 to 15,000 Hz a lower frequency range of the signals is transmitted directly together with amplitude information in respect of a number of partial frequency ranges in the remainder of the band to be transmitted, the amplitude information being transmitted as sequential modulation of a subcarrier of frequency above the upper limit of the lower frequency range. At the receiver signals in the higher frequency range are synthesized by modulating equivalent signals, lying approximately at the centres of the partial frequency ranges, with the respective amplitude information signals recovered by demodulation of the subcarrier. As described, Fig. la, the incoming signal at 1 can be passed through an amplitude compressor 2 and split into a low frequency range, by low pass filter 3, and respective partial frequency ranges, by band pass filters 4, 5 and 6. The low frequency range is transmitted direct while the partial ranges, at the outputs of filters 4, 5 and 6, are fed to rectifiers 7, 8 and 9, to derive signals representing the amplitude of the signals in the respective ranges. The amplitude signals are transmitted by a time division sampling technique, using a switch 11. The resulting amplitude modulated pulses are fed via a pre-emphasis circuit 12 to a modulator 13 and filter 18. The modulator can provide either frequency or amplitude modulation of a carrier 14 which is in the frequency range above that transmitted by the filter 3. One pulse position in the time division multiplex cycle is utilized for a synchronizing signal provided by a negative potential source 15, the amplitude of which is also modulated to convey information as to the degree of compression provided by compressor 2. The resulting modulated signal from modulator 13 may then comprise amplitude information for the partial frequency ranges as modulation from 0 to 70%, with synchronizing pulses as modulation above 80% and compression information in the modulation range 80 to 100%. At the receiver, Fig. lb, the low frequency range is selected by filter 23, and the amplitude information carrier by filter 24. The amplitude information carrier is demodulated by demodulator 25, and passed through logarithmic de-emphasis network 26 to switch 27. The synchronizing pulses are detected by an amplitude discriminator 40 to control a clock generator 41 driving the switch 27 so that the amplitude control signals appear on the storage capacitors 28, 29, 30, and the expander control signal on capacitor 39. The amplitude control signals are used to modulate the amplitudes of the frequencies generated by oscillators 34, 35, 36, whose frequencies are set in the middle of the appropriate partial frequency ranges. The resulting signals are added to the low band signal from filter 23 in adder 37, the sum passed through expander 38, controlled by. the signal on capacitor 39, for reproduction at 42. In an alternative system, Figs. 4a and 4b (not shown), a separate sound source is split into only two bands, a low frequency band which is added to the low frequency band of a system as described above and a high frequency band convering the rest of the transmission band. The high frequency band is rectified, sampled, and the samples transmitted in time division multiplex with the partial frequency band amplitude signals. At the receiver the amplitude signal of the single high frequency range is used to modulate the output of a random noise signal, limited to the frequency range of the high frequency band, and reproduced with the low frequency and synthesized partial frequency range signals.