Patent GB-848607-A (GB)
Title: Electrical signalling system
IPC: H04B1/66
Filed: 1958-09-02
Abstract: 848,607. Telephone transmission systems. WESTERN ELECTRIC CO. Inc. Sept. 2, 1958 [Sept. 19, 1957], No. 28162/58. Class 40 (4). [Also in Group XL (c)] A signalling system for transmitting and receiving a message wave having an irregular succession of peaks, comprises means at the transmitting station for sampling the message wave at each peak and for transmitting to the receiving station information representing the amplitudes of the successive samples, and also information representing the irregular time intervals between successive samples, and means at the receiving station for providing a locally produced wave, closely approximating the message wave, from the transmitted information. Transmitter.-A speech wave from a microphone 1, Fig. 1, is fed through a branch circuit comprising differentiator 4, clipper 5, differentiator 7 (optional) and rectifier 6 which provides a pulse at each peak of the message wave, i.e. when the first derivative of the wave is zero, and this pulse opens sampling gate 2 connected to a coder 3 providing amplitude information in six digit pulse-code form. Successive groups of six digits occur at irregular intervals and may be transmitted directly over channel 10, but to save bandwidth, a time scale buffer 8 is preferably included. A 7 kc/s. generator 12 is connected to one input of subtractor 13 and also to a divider 14 which passes every seventh pulse to the second input of 13. Thus the output of 13 consists of six pulses followed by a space, the pulse reading out the amplitude information stored in buffer 8 at 1000 groups per second, corresponding to the average sampling rate of 1 kc/s., and the space providing a gap between each successive group. After each peak marking pulse, counter 20 counts the number of cycles performed by 10 kc/s. generator 23, the count appearing as a permutation of the voltages on output leads 21a to 21f. At the following peak, the marking pulse is fed along line 24 to open gates 25a-25f simultaneously, the information on leads 21a-21f then being fed to different tappings on delay line 27 the output of which is a six-digit serial permutation code representing the interval between two successive peaks. The peak marking pulse also, after traversing delay line 18, resets counter 20 which then commences to count the number of cycles of generator 23 occurring in the next inter-peak interval. The interval information may be transmitted directly over channel 30, but again since each code group appears at irregular intervals, a buffer 29 operating in the same manner as buffer 8 is preferably included. Receiver.-The interval information on channel 30 is fed to buffer 39, Fig. 2, which is identical to buffers 8, 29 but has the reverse operation i.e. it restores the irregular intervals between code groups. Each code group is fed to a decoder 50 producing an output whose amplitude depends on the time interval ti + 1 - ti between successive peaks. A voltage proportional to the reciprocal of interval ti + 1 - ti is produced by network 52-56. Capacitor 58 is charged linearly through resistor 57 so that the voltage across it is the time function (t - ti)/(ti + 1 - ti) = x. When x = 1 i.e. t = ti + 1 trigger circuit 61 applies a positive pulse to the grid of triode 59, thereby discharging capacitor 58, and at the same time the pulse is applied to a tapped delay line 66 to produce six output pulses which read out the next six interval digits stored in buffer 39; the digits stored in buffer 38 and shift register 43 representing the (i + 1)th and the ith peak respectively are also read out. The output of decoders 44, 41 are thus the amplitudes of the ith, (i + 1)th peaks respectively. These amplitudes are held by capacitors 45, 42 and together with the function 3x<SP>2</SP> - 2x<SP>3</SP> derived in network 71 are used to produce the interpolated waveform which is fed through low-pass filter 84 to a reproducer 83. If the buffers 8, 29, 38, 39 are omitted the output of either decoder 41 or 44 may be applied to a reproducer 47 through a low-pass filter 46.