Patent GB-644936-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems of communication
Inventor: WINTLE MALCOLM FRANK
IPC: H03M1/00
Filed: 1948-02-20
Abstract: 644,936. Multiplex pulse code signalling. STANDARD TELEPHONES & CABLES, Ltd. Feb. 20, 1948, No. 5015. [Class 40 (v)] [Also in Group XL (a)] A pulse code modulator for a binary code of m units, comprises a cathode-ray tube having a plurality of target plates connected to m output conductors and means under the control of the instantaneous amplitude of the signal wave for causing the electron beam to strike one or more of the plates and deliver code pulses to the output conductors which represent the signal amplitude in the binary code. In one embodiment using a 5-unit code the cathode-ray tube has five rows of target plates 5 ... 9, Fig. 4, connected to load resistances 20 ... 24 and through condensers 26 ... 30 to the output terminal 25. For the simple addition binary code the plates are represented by the shaded areas in Fig. 3 which are supported on a mica disc 10 sprung into retaining notches formed on the inside of the glass envelope of the tube. They may be made of sheet or foil secured by adhesive to the disc or sputtered or sprayed thereon or they may be formed on the end of the glass envelope. For the staggered step binary code, described in Specification 644,935, the plates are represented by the shaded areas of Fig. 2. The undeflected position of the beam is at point P, Fig. 2, and the signal sample amplitude, applied to horizontally deflecting plate 16, deflects the beam between the limits RQ, say to point S. While it is still horizontally deflected, a vertical sweep voltage applied to plate 18 traverses the beam on line ST and produces a pulse output from those conductors connected to target plates in the path of the beam. In returning to point P the beam is suppressed by a blanking voltage applied to grid 14. For convenience in adjusting the beam deflections the mica sheet 10 may be covered with fluorescent material along a strip QPR or elsewhere. In the multiplex system of Fig. 4, a master pulse generator 31 produces short rectangular pulses with a repetition frequency of nF, where n is the number of channels and F is the repetition frequency of the pulse code groups in each channel. These pulses are fed to a shaping network 32 to provide the vertical deflecting pulses with leading edges of appropriate slope and, through a delay network 34, to the grid 14 for blanking as described. The pulses from generator 31 are also fed to a frequency divider 35 which delivers pulses at a frequency F to a delay network 36 from points on which gating pulses are fed to channel gating circuits 37 to which the channel signals are fed at terminal 38. The outputs of circuits 37 are mixed in circuit 39 to provide an amplitude-modulated interlaced channel pulse train which is applied to the horizontally deflecting plate 16 for coding in the tube. The synchronizing signal may comprise one of the code groups, provided by replacing one of the signal sources by a constant potential source, and reserved for synchronizing purposes. Alternatively, suitably-shaped synchronizing pulses may be derived from an additional tapping on distributer 36, and mixed with the code groups supplied to terminal 25. The rows of target plates 5 ... 9 may each be replaced by a single strip, for example of aluminium, the shaded areas being covered by a material, for example carbon, having secondary electronemitting properties different from those of the strip. The small amplitude pulses from the uncoated areas may be removed from the output by a limiting circuit. In another form of the invention, Fig. 5, described with reference to a 3-unit code, 2m target plates, in this case eight, 43 ... 50, are arranged in a single row and the output code pulses on the three leads 40, 41, 42, are produced simultaneously. The lead 40 is connected through isolating rectifiers 51 ... 54 to plates 43 ... 46, the lead 41 through rectifiers 55 ... 58 to plates 45 ... 48 and the lead 42 through rectifiers 59 ... 62 to plates 44, 45, 48, 49. The leads 40, 41, 41 are connected to different tappings on a delay line 63 so that the pulses are separated and occur successively at the output terminal 25. The plates connected up as shown produce a staggered step binary code similar to that of the arrangement of targets in Fig. 2. As before, the signal sample deflects the beam along the line Q, P, R, for example to the point S, and the vertical deflection then moves the beam across plate 44 on the path ST to produce, in this example, pulses on leads 40 and 42. Plate 50 can be omitted but a blank space should be left. With this embodiment of tube in the Fig. 4 arrangement, the vertically deflecting plates and shaping network 32 may be omitted, the grid 14 being biassed so that the beam is normally suppressed. The device 34 then should produce a short positive releasing pulse occurring during the period of each signal gating pulse. Specification 535,860 also is referred to. Reference has been directed by the Comptroller to Specifications 637,820 and 663,872.