Patent GB-582080-A (GB)
Title: Improvements in systems for the electric transmission of sound and other waves
IPC: H04K1/02
Filed: 1940-01-16
Abstract: 582,080. Pulse modulation. STANDARD TELEPHONES & CABLES Ltd., BEATTY, W. A., and HOUGHTON, L. W. Jan. 16, 1940, No. 952. [Class 40 (v)] A pulse train in which the duration of the pulses is made representative of the amplitudes of a sound or other wave by varying the times of occurrence of the leading (or trailing) edges of the pulses (main time variation), their other edges occurring at fixed intervals, as described in Specification 523,575, is modified in accordance with the amplitudes of a second wave by varying the times of occurrence of the fixed edges or of the pulses themselves. The modification may be in respect of a stereophonic channel or of a sub-audible wave indicating the volume compression or expansion of the first wave or it may be in respect of a sound wave scrambled so that it may be reconstituted, the main time variation being constituted by the same or another wave scrambled at random and acting as a camouflage. As described the modification comprises a " flutter " imposed on the fixed edges of the main more deeply modulated pulses as described in Specification 579,126 and the second wave may be separated from the first as described therein. The present Specification is concerned mainly with secret transmission and modified receivers. As shown in Fig. 1, a sound wave from 11 is scrambled at random at 12 and scrambled on a known plan in 13 the outputs of 12, 13 being used for the double-time-modulation of pulses at 14. The connection between 13, 14 may be closed in known manner only when there is an output from the random scrambler 12. The resultant pulses are of the form shown in Fig. 2 in which the leading edges 22 are modulated in accordance with the random wave and the trailing edge 16 fluttered in accordance with the planned scrambling. To separate the " flutter " channel from the main channel, the pulses are passed through a capacity-resistance combination whereby negative pulses 'arise from the leading edges and positive " flutter " pulses by the trailing edges the negative pulses being removed by an amplitude filter to produce the pulse train shown in Fig. 3. The pulses occur at different intervals representative of the change of amplitude of the modulating wave. Another method consists in the use of the circuit shown in Fig. 5. The variable duration pulses (positive) are applied at terminals 31, 32 to a network comprising resistance-capacity combinations 33, 34 and 44-46 of different time constants, the first having the shorter. The resultant pulses are superimposed in opposite senses in the grid circuit of valve 37 and consist of a series of short positive pulses marking the leading edges and short negative pulses (flutter) marking the trailing edge, the amplitudes of the two sets of pulses being balanced by adjusting resistance 34 to prevent breakthrough of the main channel. The grid of 37 is biassed positively to remove the positive pulses from its output which consists solely of pulses corresponding to the " fluttered " edges. This output is applied to a " detector 52 which reconstitutes the original wave, subsequently amplified at 63. Specification 528,192 also is referred to.