Patent GB-1425156-A (GB)
Title: System for transferring wide-band sound signals
IPC: G10L21/02
Filed: 1974-05-23
Abstract: 1425156 Vocoders INTERNATIONAL STANDARD ELECTRIC CORP 23 May 1974 [4 June 1973 (2)] 23050/74 Heading H4R In a bandwidth compression system of the type described in Specifications 1,424,133 and 1,425,155 in which a wide band sound signal is divided into a low frequency band, which is transmitted direct, and a number of higher frequency bands, the amplitude information of which is transferred, and in which at the receiver the amplitude information signals modulate equivalent signals lying approximately in the middle of the respective bands to be added to the lower frequency band for reproduction, the modulated equivalent signals are additionally modulated by a narrow band noise signal before addition to the low frequency band for reproduction. The arrangement provides a more natural sound on reproduction. The noise signal modulation may be of variable depth with the modulation depth controlled from the transmitter. Fig. 3 shows a preferred embodiment of a transmitter for the system in which the low frequency band is selected by filter 3 for transmission direct and the high frequency bands are selected by filters 4, 5, and 6, feeding rectifiers 7, 8, and 9, to provide signals responsive to the amplitudes of the partial bands selected by those filters. The amplitude signals are sequentially sampled by switch 11, a synchronizing signal added at 50, and the resulting signal modulates a pilot signal from oscillator 14 whose frequency is just above the if band selected by filter 3. The synchronizing signal is derived from the generator 51, which controls the step frequency of switch 11, by a divide by 2n stage 52, where n is the number of switch positions per revolution, and a tunable filter 521 which provides a control on the level of third harmonic in the synchronizing signal, which level is used to control the noise modulation at the receiver. At the receiver, Fig. 5, the pilot carrier signal is selected by filter 24 and demodulated at 25 to provide the sequential amplitude samples, to be fed to switch 27, and the synchronizing signals which is used to synchronize an oscillator 55, driving the stepping switch 27 in synchronism with switch 11 at the transmitter, while a divider 552, modulator 551, and filter 552, determine the level of the third harmonic of the synchronizing signal. The amplitude signal samples from switch 27 are stored on capacitors 28, 29, 30, and modulate the outputs of respective oscillators 34, 35, 36 to provide equivalent signals which are added in 371. The level of the third harmonic of the synchronizing signal controls the level of a noise signal from generator 372, and the noise signal output is fed via a filter 555, having a band width which is about half the frequency spacing of the equivalent signals, to modulate the equivalent signals in modulator 373 before they are added to the low frequency band in adder 564 and reproduced. In a modification Fig. 2 (not shown) each of the equivalent signals is split into quadrature related signals, corresponding phase signals added, and the two resulting quadrature related signals modulated by respective different noise signals before they are added together, and to the lf signal, for reproduction.