Patent GB-996285-A (GB)
Title: Speech transmission systems
IPC: G10L25/90
Filed: 1961-09-27
Abstract: 996,285. Vocoder systems. WESTERN ELECTRIC CO. Inc. Sept. 27, 1961 [Oct. 6, 1960], No. 34663/61. Heading H4R. [Also in Division G4] In a vocoder the pitch signals are time quantized and the quantized pitch signal coded, e.g. by a PCM coder, for transmission over a narrow band communication channel. Time quantizer and coder.-Incoming pitch pulses trigger flip-flop 22, Fig. 2, which is restored by the following clock pulse from generator 21, thus generating a pulse the trailing edge of which constitutes the quantized pitch pulse. Clock pulses are also fed to timer 25 to generate frame pulses at a frequency just above the expected maximum frequency of pitch pulses; the frame pulses trigger a flip-flop 24 which enables AND gate 27 which passes clock pulses to counter 282 of coder 28 until a quantized pitch pulse resets flip-flop 24 and disables gate 27, when counter 282 will register the number of clock pulses between the frame pulse and the quantized pitch pulse. One anode of each stage of counter 282 is connected to an AND gate 282a-282n, and frame pulses are fed through the enabled AND gates into delay element 284 in a digital representation of the counter condition at the end of a frame, i.e. when it indicates the number of clock pulses between the frame pulse and the quantized pitch pulse, delay element 284 converts the parallel digital representation into serial form by delaying the output of each AND gate 283a-283n a different amount. Should no pitch pulse occur during the frame, AND gate 27 will pass a preframe pulse from multivibrator 254 to the coder counter just before the counter condition is transmitted, as described above, this preframe pulse will move the counter on to its zero condition thus indicating the absence of pitch pulse during the frame. A guard circuit 23 may be inserted into the system on operation of switch S to suppress any pitch pulse occurring in a predetermined time interval after a previous pulse. Pitch pulses pass through gate 231 to the coder and also operate a monostable multivibrator 232 the output from which passes through OR gate 234 and inhibits gate 231 from passing any further pitch pulse whilst the circuit 232 is operated. When circuit 232 resets it operates a second monostable circuit 233 which inhibits gate 231 for a further period and so prevents pitch pulses passing during the time circuit 232 is recovering to its original condition. Decoder, Fig. 7.-The PCM pitch signal is fed to a shift register 72 which acts as a serial to parallel converter so that on the occurrence of the frame pulse the appropriate binary code appears at outputs 72a-72n, this resets the counter stages 75a-75n to the " ones complement " of this number so that the counter will count the decimal number equivalent of the transmitted binary number before all the outputs of stages 75a-75n energize AND gate 76 to produce a pulse at the output which may be used to energize the pitch excitation source via a pulse amplifier 78 and a delay 79. A preframe pulse is applied to the counter to restore the counter to the zero condition before the start of the following frame and since this condition is the one for which an output is derived from the AND gate 76 this output pulse is inhibited by gate 77. Specification 466,327 is referred to.