Patent GB-1459440-A (GB)
Title: Method and apparatus for transmitting and receiving electrical speech signals transmitted in ciphered or coded form
IPC: G10L19/00
Filed: 1973-12-21
Abstract: 1459440 Speech secrecy system GRETAG AG 21 Dec 1973 [21 Dec 1972] 59584/73 Heading H4R In a secret transmission system a speech signal is analysed to provide frequency spectrum, pitch, and voiced unvoiced information, the information is ciphered, a transmission signal is synthesized by modulating the harmonics of a common fundamental frequency with the ciphered speech information and the synthesized signal transmitted. At the receiver the synthesized signal is analysed, the information signals deciphered and used to synthesize a speech signal similar to the original speech signal. As described with respect to Fig. 3 the incomng speech is converted, at 28, to digital form, each sample having 10 to 12 bits and with a clock frequency of 104 Hz, and subjected to compression by converting the samples to 6 to 7 bits, at 30, with a compression coefficient digitized as a two bit signal on line 100. Twenty five to thirty millisecond sections of speech are fed to store 31 and to a pitch and voiced unvoiced detector 68. By an auto correlation or a cross correlation technique the pitch of the incoming signal is determined and, if the signal is voiced, controls the period of a frequency generator 38 which produces digital samples of quadrature related sinusoidal waveforms of the fundamental pitch period and its harmonics over the audio range. The fourier analyser 34 then correlates each of the frequencies generated by the generator 38 with the speech section in a store 31 to produce, by combination of the correlations for the quadrature related waveforms of each of the frequencies, a set of fourier coefficients for that speech section. For unvoiced speech the generator 38 operates with a fixed frequency fundamental, e.g. of 60 Hz, and its harmonics. The fourier coefficients can be delivered unaltered, or after averaging adjacent coefficients to reduce their number, by computer 67 to a cipher device 22 where the coefficients, together with pitch frequency, and voiced unvoiced information, are modulo added to a cipher signals and the ciphered information signals fed to a smoothing computer 123 which interpolates on the incoming values, which occur once per speech section of 25 to 30 msecs., and produces a greater sampling rate with smoother variation from one received value to the next. The resulting smoothed signals are fed to the synthesizer 96 where each information signal modulates a respective harmonic of a fixed fundamental frequency, e.g. of 60 Hz, which is produced by the generator 134. The resulting digital signals are converted to analogue form in 132 and fed to the line as a synthetic coded speech signal. The analysis and synthesis can be done in parallel, using separate correlators and multipliers for each frequency generated or the process can be speeded up and the correlation and multiplication done serially using a single channel working at high speed, alternatively a reduced number of channels can operate serially at an intermediate speed. At the receiver the incoming synthesized signal is converted to analogue form in converter 30 and analysed in a similar manner to that used at the transmitter, except for using a fixed frequency for the generator 38, the same as the frequency used in the synthesizer at the transmitter. The resulting information derived from the analysis is deciphered, by subtracting the cipher signal in a modulo subtractor, and the information smoothed by interpolation in 123 before use to synthesize the original speech in synthesizer 96. When operating in the receive mode the frequency generator 134 is controlled by the received pitch information, or if the received signal indicates an unvoiced signal the generator can be used to generate a random noise signal, e.g. by switching at random intervals between two fixed frequencies. The output from the synthesizer is expanded at 79 under the control of the received compression information signal before conversion back to analogue form at 132 for reproduction. Should the transmission between the transmitter and receiver be such as to introduce a frequency shift, a test signal can transmitted periodically, e.g. during gaps in the speech, its value detected at the receiver and used to frequency shift the signals generated by the generator 38 a corresponding amount to compensate the shift in the received signal. Similarly a test signal can be analysed at the receiver and the received values stored to modify the subsequently received signals to provide equalization of the amplitude frequency characteristic of the transmission system. Volume compressor operates by determining the most significant digit present in the samples of a predetermined signal period which have been coded into 10 to 12 bits, and dropping digit positions of most significance from the digitized samples until the most significant digit is in the most significant digit position, while still leaving for transmission a minimum of say 6 bits. The number of digits dropped is signalled digitally to the receiver for appropriate adjustment of the expander. Pitch detection operates either by autocorrelation of the input signal at a range of delays, and selecting the maximum autocorrelation function as indicating a delay corresponding to the pitch period, or an integral multiple thereof, or the pitch can be determined in the analyser 34 by varying frequency of generator 38 and selecting the frequency which provides a maximum summation for the fourier coefficients of the five lowest frequencies of generator 38. Voiced unvoiced detection can be determined from absence of a definite peak in the autocorrelation function or by comparison of the sum of the five lowest frequency fourier coefficients with the sum of the five highest frequency fourier coefficients. In an alternative arrangement the section length of the speech being analysed is varied according to the speech signal being analysed, synthesis at the transmitter is performed with a uniform section length, and information is transmitted on the analysis section length for use in synthesis at the receiver.