642,330. Secret transmission ; multiplex systems. STANDARD TELEPHONES & CABLES, Ltd. Oct. 6, 1947, No. 26808. [Class 40 (iv)] [Also in Group XL (c)] For transmission over a communication medium a coded wave is produced from a signal wave the amplitude of which is periodically sampled, the amplitude of the coded wave at each of a number of specified instants of lower periodicity being determined by the amplitudes of a plurality of samples of the signal wave according to a specified code. In the embodiment described in detail the coded wave is an amplitude modulated pulse train. For multiplex transmission a plurality of signal waves are sampled and the amplitude of the coded wave at the specified instants is determined by the amplitudes of all the signal wave samples at corresponding sampling instants, again according to a specified code. In one embodiment, a speech wave is sampled 6,000 times a second and 3,000 pulses per second are transmitted the amplitudes of which are capable of 900 different values, the sample amplitudes being each capable of assuming 30 different values. Thus for example, if the amplitude of one sample is 23 and the next 6, the transmitted pulse conveying both values is given a coded amplitude of 23x30+6=696. The speech wave source 1, Fig. 1, is fed to two multivibrators 2, 4 the latter connection including a delay network 3 of time constant <SP>1</SP>/ 6000 sec. The pulse generator 5, producing pulses having a recurrence frequency of 3,000 cycles/sec., is also connected to the multivibrators which produce duration modulated pulses. These are converted into amplitude modulated pulses having thirty possible discrete values by passing them through gates 6, 7 respectively which are controlled by a 3,000 kc/s. pulse generator 8, and integrating the resulting discrete number of 3,000 kc/s. pulses at the output of the gates in circuits 9, 10 respectively. The resulting. simultaneously occurring amplitude modulated pulses are added in circuit 12 after those from circuit 10 have been attenuated in circuit 11 by a factor of thirty. The output pulse train may constitute the signal transmitted over the communication medium. Alternatively a wave may be produced from the pulse train by eliminating the pulse recurrence frequency in a low-pass filter, the resulting wave having an amplitude at intervals of <SP>1</SP>/ 3000 sec. corresponding to the coded pulse amplitudes. The receiver decoder, Fig. 2, is described as arranged to deal with the coded pulse train from a transmitter as in Fig. 1, but using only ten possible sample amplitudes and hence transmitted pulses which can assume 100 different amplitudes, the tens digit conveying one sample amplitude and the units digit the other. The decoding principle used may, however, be applied to signals of the type produced in Fig. 1 for example, the corresponding possible sample and pulse amplitudes being 30 and 900 respectively. The incoming pulses from circuit 13 phase modulate a carrier from generator 15 in modulator 14, and the modulator output is frequency-multiplied to a frequency F in circuit 16, the multiplier factor being such that the phase of those pulses corresponding to input pulses of amplitudes 10, 20, 30, &c. is shifted by an integral multiple of 2#. The phases of intermediate pulses are therefore determined by the units digit only of the coded value. A standard phase is produced by the frequency multiplier 17 fed direct from generator 15 and a circuit 18 . . . 21, as described in Specification 556,208, [Group XL], produces an output at a frequency f which is linearly related to the phase of the variable output from multiplier 16. The frequency discriminator 22 produces an output voltage proportional to frequency which modulates in circuit 24 the amplitude of pulses from a generator 23 having a recurrence frequency of 3,000 cycles per sec. After adjustment of their amplitude level in circuit 25 the pulses are subtracted in combining circuit 26 from the original pulses from circuit 13 to yield another pulse train in which the amplitudes are proportional to the tens digits of the code number. The pulse train from 24 is also delayed in a network 27 by <SP>1</SP>/ 6000 sec. to produce pulses which occur at the middle of the intervals between the original pulses. The amplitude level of the pulses from circuit 26 is adjusted in circuit 28 so that the pulse corresponding to magnitude 10 has the same amplitude as that corresponding to a magnitude of unity from the output of network 27. The mixer 29 is fed from circuits 27 and 28 to produce a 6,000 cycles/sec. pulse train having amplitudes corresponding to the original speech signal which is recovered by a low-pass filter 30. The signal wave may alternatively be coded by the use of sampling pulses in sets of three or more, the transmitted pulse then denoting, by a system of numeration using three or more digits, the amplitudes of three or more sample pulses and the digits may have higher values than nine. When a continuous coded wave is derived from the coded pulses, at the receiver it is passed through a gating circuit operating at a frequency of 3,000 cycles/sec. and controlled, for example, by generator 23, Fig. 2, to derive the coded pulses again. For synchronization, a synchronizing wave may be transmitted over a special channel or as a weak 1,500 cycles/sec, tone superimposed on the communication channel. The system of Figs. 1 and 2 may be used for transmitting two separate signals applied directly to the multivibrators 2, 4 respectively, and, at the receiver, circuits 27 ... 30 are omitted, the signals being derived from circuits 24, 26 respectively. The system may also be extended to three or more channels using codes with three or more digits. Specification 535,860 also is referred to.