GB1260735A - Vocoder speech transmission system - Google Patents

Abstract

1,260,735. Vocoders. G. FERRIEU, and J. M. PERSON. 15 Dec., 1969 [16 Dec., 1968], No. 61046/69. Heading H4R. [Also in Division G4] In a channel vocoder a pitch detector produces pulses of a repetition period equal to the fundamental pitch period of the input speech and a pulse sorter eliminates spurious pitch pulses whose repetition period differs from the previously established pitch period by more than a predetermined percentage. The resulting pitch period values are transmitted to a synthesizer by p.c.m. signals. At the receiver the p.c.m. signals are converted to analogue form and control the stepping of shift registers connected in loops to generate excitation signals having a preferred frequency-power distribution. Two shift registers are used, one operating during voiced sounds and the other during unvoiced sounds, the one operative during unvoiced sounds having more stages than the other. Pitch detector, Fig. 4, includes two identical sections one operating on the erect speech and one on the speech after phase reversal in inverter 100. The speech signal is applied via an amplifier 103 and capacitor 104 to a resistor 105. Speech is also fed to an amplifier 109 which phase shifts the speech by 90 degrees causing monostable 110 to produce a pulse coincident with the positive peaks of the speech fed to amplifier 103. The pulse from monostable 110 enables amplifier 107 which, if a positive voltage exists at its input, triggers monostable 108, to reset counter 121, and to make transistor 106 clamp the potential across resistor 105 to zero, following which monostable 108 will only be operated by succeeding positive pulses, in the speech waveform, having as great an amplitude as that of the previous pulse, as modified by the discharge of capacitor 104 through resistor 105. Therefore output pulses are fed to the sorter 102 at the instant of the maximum positive peaks in the speech input. Pitch pulse sorter 102, Fig. 5, receives pitch pulses from detector 101 which reset counter 121 so that counts indicating the period between pitch pulses are obtained from the counter 121. The counts obtained during successive pitch periods are compared in comparator 126, and the value transmitted to the multiplexer via gates 141, if the count is within a predetermined range of the previous count, the range varying with the count length and being determined by the non-linear decoder 122 through which the values are fed to the comparator. P.c.m. coder and decoder, Figs. 2 and 3 (not shown), are of conventional type providing compression of the signal at the coder and complementary expansion at the receiver. Excitation signal generator, Fig. 5.-Eight digit codes specifying the pitch period are entered into register 133 and used to set a counter 135. The counter counts down to zero, which state is detected by "and" gate 137 and produces a pulse to reset the counter to the number stored in register 133 and to step on each of the shift registers 130 and 131 by one step, therefore controlling the rate of stepping of the register and the fundamental frequency of the excitation signal. During unvoiced sounds a zero count is transmitted and detected by "and gate" 134 which inhibits registration of such a count in register 133 and causes switching of the excitation source from shift register 130 to shift register 131. Shift register 130 has seven stages with looping back from the last two stages via "exclusive or" gate 1300 to the input stage to provide a binary sequence of 2<SP>7</SP>-1=127 pulses, and shift register 131 eleven stages providing a cycle of 2<SP>11</SP>-1 = 2047 pulses, each forming a maximal length linear binary sequence generator. The counting rate of counter 135 is 2MHz and the arrangement is such that the period of the output from shift register 130 is equal to the pitch period while that from shifter 131 is sixteen times that period. The excitation signal is applied in a conventional manner to a standard vocoder synthesizer, Fig. 1 (not shown).