Patent GB-969310-A (GB)
Title: Pulse modulation communication system
IPC: H04J3/00
Filed: 1963-05-06
Abstract: 969,310. Pulse modulation systems; diode gating circuits. BENDIX CORPORATION. May 6, 1963 [May 18, 1962], No. 17808/63. Headings H3P and H4L. In a communication system the transmitter comprises means for providing discrete samples of a time varying signal to be transmitted, the samples being of short duration in comparison to the interval separating successive samples; means for storing the samples; and means for transmitting the stored samples in rapid succession during an interval short in comparison to the interval separating successive samples. In the transmitter shown in Fig. 1, signals from an audio source 10 pass through a filter 11 and amplifier 12 to four bi-directional gates 17-20; these gates are opened in sequence for 1 microsecond every 100 microseconds by pulses A-D on lines 37-40, these pulses being derived from a pulse generator 22. The last pulse D of a group of four pulses also passes to a 10 microsecond delay line 58 provided with taps which indicate the address of the desired receiver. In the example shown, the taps are at 0, 3, 5 and 9 microseconds and pulses E-H are produced with this spacing. Pulses E-H open gates 61 and 72-74 respectively and allow the sampled audio amplitudes stored at the outputs of gates 17-20 to pass through gate 62 and amplifier 63 to frequency modulate an oscillator 64. The output of oscillator 64 is amplified in unit 65 and pulsemodulated by pulses E-H in unit 66 before being transmitted. Within 10 microseconds four pulses representing samples taken over an interval of 400 microseconds are transmitted, and the transmitter can vacate the channel for 390 microseconds to make room for others. At the receiver, Fig. 4, received signals pass through amplifier 82 to an amplitude modulation detector 83 and video amplifier 84, the output of which feeds a tapped delay line 85 which is the complement of tapped delay line 58 at the transmitter. The correct pulse group will produce four simultaneous outputs from line 85, and these are detected by a coincidence detector 86. The output of detector 86 is used to produce pulses A<SP>1</SP>-D<SP>1</SP> which are spaced 100 microseconds apart and correspond to pulses A-D at the transmitter. The output of amplifier 82 also passes to a frequency discriminator 108 which converts the four frequency modulated pulses E-H into amplitude modulated pulses E<SP>1</SP>-H<SP>1</SP>. Pulses E<SP>1</SP>-H<SP>1</SP> pass to a tapped delay line 109 complementary to delay line 58 at the transmitter so that after 10 microseconds each pulse is present at its appropriate tap on the delay line. At this time a pulse derived from detector 86 opens gates 111-114 and allows pulses E<SP>1</SP>-H<SP>1</SP> respectively to be stored at the outputs of these gates. The stored outputs are released in sequence at 100 microseconds intervals by pulses A<SP>1</SP>-D<SP>1</SP> which open gates 120-123, and pass through OR gate 137 to a low-pass filter 138 and the output circuits. The output circuits include a notch filter 139 which eliminates any tone of frequency 2500 c.p.s. (the recurrence rate of groups of four pulses) which might be introduced in the signal by unbalance in the storage and amplifying circuits. Diode gating circuits.-Audio frequency signals on the input line 13, Fig. 3, are sampled at 100 microsecond intervals by 1 microsecond pulses applied to terminal 37. An input pulse at the primary of transformer 43 produces outputs in the two secondary windings of such polarity as to allow diodes 51 and 52 to conduct; the audio input can then pass through one of the diodes, according to the polarity of the signal, to charge condenser 56 to the instantaneous value of the signal. When the pulse ceases, the charges retained in filters 48 and 49 give a backward bias to diodes 51 and 52.