Patent GB-927842-A (GB)
Title: Improvements in or relating to speech signal transmission systems
IPC: H04B1/66
Filed: 1962-02-07
Abstract: 927,842. Vocoders. PHILIPS ELECTRICAL INDUSTRIES Ltd. Feb. 7, 1962 [Feb. 10, 1961], No. 4732/62. Class 40 (4). In a formant tracking vocoder the speech band is divided into frequency ranges and the lower range is transmitted unaltered, at the transmitter the upper frequency channels contain frequency and amplitude detectors which derive for transmission voltages varying with the amplitude and frequency of the energy within that range; and at the receiver the lower channel besides being used normally may also control the frequency of a sawtooth relaxation oscillator to oscillate at the fundamental frequency of the energy in the lower channel, the oscillator output being fed to amplitude modulators and frequency shifting devices in the upper frequency range channels which are controlled by the voltages referred to above. As shown, the incoming signal of frequency 0.3-3.2 kc/s. is split by the filters 8, 9 and 10 into three frequency bands 0.3-0.8 kc/s., 0.8-2.0 kc/s. and 2.0-3.2 kc/s. respectively. The lowest band is transmitted unchanged. The middle band is fed to a frequency detector 11 to produce a D.C. voltage on lead 19 as a measure of the frequency. This band also feeds an amplitude detector 13 whose D.C. output on lead 18 has a bandwidth of 70 c/s. The upper band is similarly treated in channel 7 to produce D.C. signals on leads 20, 21. At the receiver the lowest band is reproduced unchanged and is also fed to an amplitude detector 77 and 300 c.p.s. low-pass filter 48 from which is extracted the fundamental pitch of the lower band signal. This signal feeds a frequency detector 49 which in turn controls a relaxation oscillator 30 which generates a sawtooth wave at the frequency of the fundamental pitch of the speech signal. The sawtooth signal modulates in modulator 31 a 32 kc/s. signal from oscillator 32 and this signal is fed to variable gain amplifiers 34 and 35 where the amplitude is controlled in accordance with the D.C. signals on lines 18 and 20 to correspond with the amplitudes of the original speech signals in the appropriate frequency ranges. The output from amplifier 34 is then transposed to the appropriate frequency within the range 0.8-2.0 kc/s. by modulator 36 which is fed by an oscillator 40 whose frequency is varied within the range 31.2-30 kc/s. under the control of the output of the frequency detector 11 in the corresponding band compression channel of the transmitter. The upper frequency band signal is formed in a similar way in channel 29. The three frequency bands are then combined for feeding the output circuit 45-46. Transmission of the signals on leads 17 to 21 is effected as follows: the lower band, 0.3 to 0.8 kc/s., is modulated to 6.3-6.8 kc/s. The signals on leads 18 and 19 are amplitude and frequency modulated respectively on to one carrier of 6.86 kc/s. The signals on leads 20 and 21 are similarly modulated on a carrier of 6.96 kc/s. The three signals are then combined to occupy the band 6.3 to 7.0 kc/s. and this band may be further transposed for transmission over a cable or other link.