Patent GB-1399365-A (GB)
Title: Apparatus for coding audio signals
IPC: H04W88/02
Filed: 1972-08-21
Abstract: 1399365 Speech secrecy system BOEING CO 21 Aug 1972 [7 Sept 1971] 38824/72 Heading H4R In a speech secrecy system the speech is coded by modulation with a frequency which is varied in a pseudo-random fashion with the pseudo-random frequency generators synchronized at the transmitter and at the receiver by generating two frequencies at the transmitter, providing a characteristic modulation of one or both frequencies at the transmitter, and detecting the modulation to provide the synchronizing signal for the pseudo-random generators at both the transmitter and receiver. As described with the selector switch 14 in its "coded" transmission position, operation of the press-to-talk switch on the transmitter causes operation of relay K1 which, at contacts K1A, K1B, disconnects the microphone from the straight through path to the transmitter, and connects it instead to the audio coder 42, 44, 46 &c. Simultaneously contacts K1C are operated to enable one shot 96 which, after a delay of 0À8 sec., to allow time for transmission of a calling or identification signal, resets to provide an enable signal, through NOR gate 98 to counter 68. Counter 68 provides two output signals, of 976 Hz and 1953 Hz, which are added to the microphone signals in adder 42 and transmitted, by-passing the modulator 46, through contacts K2C of relay K2. A predetermined time after the starting of counter 68 a signal is produced to activate phase inverter 62 causing phase inversion of the T1 output frequency signal of the counter 68. The inversion of the T1 signal is detected by the synchronizing detector 110, which is fed with filtered and unfiltered transmitted signals via differential amplifiers and zero crossing detectors 102, 104, 106, 108, and causes operation of relay K2 which connects the mixer 46 into circuit to scramble the transmitted signals, and operates one shot 112 which initially sets shift register 82 and counters 76, 78 to preset conditions, then enables them to cause generation of the pseudo-randomly varying carrier for the mixer 46. The generator comprises a variable modulus counter 76 whose output feeds both the mixer 46 and a fixed modulus counter 78 whose out-put feeds shift pulses to shift register 82 provided with variable interconnections to form a pseudo-random sequence generator. The first five stages of the shift register 82 are coupled to the counter 76 to vary the division ratio of the counter, and hence the frequency of its output, as the stages change state. Since the counter 76 also feeds the counter 78 the rate at which the shift register steps is also varied. The audio signals, together with the two signals T1 and T2, after modulation by the pseudo-randomly varying frequency are fed through a low pass filter, which passes the lower sideband of the modulated signal, to the transmitter via contacts K1D and K1E. At the receiving station the received signals are applied via contacts K1B, which are not operated since the K1 relay is not operated by the "press to talk" switch, through the adder 42, high and low pass filters 44, 48, and the notch filter circuits 52, 54, which remove the signals T1 and T2 from the received signal, to a blanking circuit 56, which only allows signals to pass through to the audio reproducer while the signal T2 is present in the received signal. Absence of a carrier on the incoming signal causes noise which is fed from the line 89 through high pass filter 90, of cut off about 6kHz to inhibit operationof one shot 96. Presence of carrier, reduces noise above 6kHz and the one shot 96 is triggered to apply a "window" to sync detector 110. If during the "window" signal the frequencies T1 and T2 are present in the received signal they appear on lines 49 and 53 and the inversion of T1 is detected to cause operation of one shot 112 and relay K2 and the start of generation of the pseudo randomly varying carrier supply to the mixer 46, in an identical manner to that described above for the transmitter, effecting restoration of the audio signals to their original frequency, for reproduction at the receiver.