Patent GB-940507-A (GB)
Title: Improvements in or relating to pulse modulation systems
IPC: H03M1/00
Filed: 1960-04-28
Abstract: 940,507. Pulse modulation. WESTERN ELECTRIC CO. Inc. April 28, 1960 [May 13, 1959 (2)], No. 14878/60. Heading H4L. A pulse modulation system comprises means for supplying D.C. pulses of amplitude variable within a predetermined range to an output circuit during predetermined spaced time intervals, and means for converting the train of unidirectional output pulses into a train of pulses which varies about a predetermined reference potential intermediate in the amplitude range. By this means the D.C. component is removed or made substantially constant. A pulse amplitude modulator, Fig. 1, comprises five input channels which are sampled in succession by positive pulses on leads D1 to D5 from a timing pulse generator 16, which produces a negative output on leads D1 to D5 in the intervals between sampling. A lead -D is negative during each sampling period and positive in the remaining periods. When a negative potential appears on lead D1, for example, diode 15 in the first sampling gate is forward biased, thus reverse biasing diodes 12, 13 and blocking transmission from the signal source 11 to the output lead 14. During the first time slot the positive pulse on lead D1 reverse biases the diode 15 and allows the diodes 12 and 13 to conduct and allow a signal sample to pass to the common output lead 14. A negative potential supplied via a resistor 18 provides a reference potential during sampling periods. An additional gate 62, 63, 65 is controlled by the signal on lead -D, and returns the output lead 14 to a reference potential which is midway in the signal sample amplitude range, in this embodiment earth potential, in the guard spaces between sampling intervals. In a further embodiment, Fig. 3, for decoding a four-digit pulse code modulation signal which is in inverted form, the pulse code groups are received in serial form on a lead 110 and converted into parallel form by a shift register consisting of pulse regenerators 111, 113, 115 and 117 and one-digit delay lines 112, 114 and 116. The parallel output from leads A is supplied to input terminals of corresponding AND gates 118 to 121, the other input terminals of the AND gates being energized through a phase-inverting amplifier 122, from a lead -D4 which is negative during the fourth time slot of each code group and is supplied by a timing generator. The output leads of the AND gates are connected to like input terminals of respective flip-flops 123 to 126 which control the initiation of each reconstructed signal sample, the other input terminals of the flip-flops being energized through a phase-inverting amplifier 127 from the lead -D3, which is negative during the third time slot of each group, to re-set the flip-flops. Respective switches 128 to 131 are controlled by the flip-flops, each switch connecting an output lead either to earth when energized by the associated flip-flop or to a negative reference potential -E at other times. The switch output leads are connected through respective weighting resistors R, 2R, 4R, 8R, to an output lead 136. An auxiliary resistor 137 of value R, i.e. the value representing the most significant digit, is connected either to the reference potential or to earth by a switch 138 controlled by a flip-flop 139, the two inputs of the flip-flop being connected to amplifiers 122, 127, respectively. In operation, each negative pulse on lead -D3 triggers the flip-flop 139 and connects the resistor 137 to earth and each negative pulse on the lead -D4 returns the flip-flop and connects the resistor 137 to the reference potential. Thus the weighting resistors when selected under the control of the received code groups are connected to earth when the auxiliary resistor 137 is connected to the reference potential and at the termination of the resulting amplitude modulated pulse the weighting resistors are connected to the reference potential and resistor 137 to earth. The resulting output pulses, instead of extending positively towards earth potential from the negative reference potential extend in either direction from an intermediate reference potential of value 2/3  E REF . A similar seven-digit decoder using transistor circuitry is described. Specification 845,344 is referred to.