Patent GB-1425155-A (GB)
Title: System for transferring wide-band sound signals
IPC: G10L19/00
Filed: 1974-02-14
Abstract: 1425155 Bandwidth compression systems INTERNATIONAL STANDARD ELECTRIC CORP 14 Feb 1974 [28 Feb 1973 26 April 1973] 6753/74 Heading H4R In a bandwidth compression system a low frequency portion of the signal is transmitted direct, via lp filter 3, while the upper frequency portion of the signal is divided into a number of ranges 4, 5, and 6, and a pilot frequency, or frequencies, is positively modulated with the amplitudes of those ranges in such a way that the time average of the modulated pilot signal is lower relative the time average of the low frequency range by an amount such that the low frequency range signals audibly mask the pilot signal on reproduction on a conventional receiver while on a receiver according to the invention the pilot signal is demodulated and the amplitude signals recovered and used to modulate respective synthetic signals located in the middle of the respective upper frequency ranges to provide synthesized signals which are added to the lf range before reproduction. As described the low frequency portion extends up to a frequency in the range 4 to 7 kHz and is selected by filter 3 and fed to a combining circuit 17. The high frequency is divided into a number of partial ranges by filters 4, 5, and 6, e.g. into 12 partial ranges per octave, the amplitudes are detected by rectifiers 7, 8, and 9, and then fed successively by rotary switch 11 and adder 50 to modulate a pilot signal from generator 14 in modulator 13. The output of the modulator being fed to the combiner 17 for transmission. The clock frequency from generator 51 which controls stepping of the switch 11 is fed to a divider 52 for division in the ratio 1 : 2n, where n is the number of high frequency channels, and added, at 50, to the time multiplexed amplitude signals before they modulate the pilot frequency in modulator 13. At the receiver, Fig. 5, the incoming signal is applied direct to a combiner 564 and also to a band pass filter 24 to isolate the pilot frequency from the low range frequencies. The pilot frequency is detected at 25 and fed via switch 27 to storage capacitors 28, 29, 30, to control modulators 31 modulating synthetic signals from oscillators 34, 35, 36, located respectively at the centres of the partial ranges of the high frequency signal portion. The resulting synthesized signals are combined at 371 and fed to the second input of combiner 564 to be added to the incoming signal for reproduction. In addition the pilot signal from detector 25 which contains modulation at a frequency 1/2n x clock frequency is compared with the output of a 1/2n divider 54 connected to the clock generator 55 in phase comparator 53 to derive a phase control signal for clock generator 55 to maintain synchronism between the stepping switches at the transmitter and receiver. The clock generator 55 output is also fed to a second divider 541 which provides a signal 90 degrees out of phase with the divider 54 and the output of this divider is compared in phase comparator 531 with the received synchronizing signal and the resulting signal used to operate an automatic gain control circuit 241 on the pilot signal circuit, and also, via a threshold device 562 to maintain a switch 563 on the synthesized signal output to inhibit the synthesized signal should the pilot signal not be present or synchronism not be maintained. Modulation of the pilot signal at the transmitter is maintained so that its level is always 10 dB below the low frequency signal amplitude, which would normally occur automatically since most signal sources will have low frequency signals present if upper range signals are present. The pilot frequency is preferably at a frequency just above the upper limit of the low frequency range.