Patent GB-1388903-A (GB)
Title: Frequency compressors
IPC: H04N1/00
Filed: 1972-02-16
Abstract: 1388903 Bandwidth compression GRAPHIC SCIENCES Inc 16 Feb 1972 [16 Feb 1971] 47681/74 Divided out of 1388901 Heading H4R Bandwidth compression of an analogue signal is effected by detecting intersection points at which the signal A, Fig. 2a, is crossed by a levelshifted, time-delayed replica of itself B, and amplifying the signal with alternate first and second polarities as the signal passes through alternate ones of said detected intersection points. The time delay and the magnitude and direction of the level shift between the signal and its replica being such that alternate intersection points occur substantially at local minima (or maxima) of the signal. In addition higher-order frequency components generated by the inversion are filtered out so that an analogue frequency compressed signal of half the frequency content of the original analogue signal is produced. In a facsimile transmitter, Fig. 1, the facsimile signal derived by photoelectrically scanning the document 14 on drum 10 is applied via gain control circuit 3, analogue frequency compressor 5 and modulator 7 to a telephone line transmitter 9. Line frequency sync. pulses from sync. unit 31, derived in response to driving pulses from photodetector 30 receiving light reflected from a mirror 34 on the rotating drum, periodically closes switch 28 to charge capacitor 20 and provide a zero reference D.C. level for the facsimile signal. Amplifier 24 having feedback resistor 40 and resistors 42 in series with Zener diodes 44 having different breakdown voltages compensates for non-linear output of the sensor head 18. In addition a capacitor 52 is charged during each line (discharged by switch 58) from a level shifter and amplifier 54 driven by a comparator 56 supplied with the output of amplifier 24 and a reference voltage. Capacitor 52 controls FET 50 until the maximum output of amplifier 24 is just equal to the reference voltage, this maximum output being derived from the leading edge of the document for each line scan. The gain controlled output from amplifier 24 is fed via a switch 72 to a differential amplifier 78. Switch 72 is driven from a monostable multivibrator 86 through coil 88. The multivibrator also sets a flip-flop 90 which is reversed on receipt of a signal from a comparator 92 having the output of amplifier 24 applied as a first input and a level-shifted delayed replica thereof as a second input. Flipflop 90 drives a switch 82 through an OR gate 96. The frequency compressor 5 operates by detecting local minima and reversing the polarity of the amplifier 78 to thereby reflect alternate signal signals about the reference axis to form a  modified signal whose components are half the frequency of the input signal. The output of comparator 92 is shown in Fig. 2b, the trailing edge of each pulse switching the flip-flop 90 between its stable states which opens and closes switch 82 so that the output of amplifier 78 is as shown in Fig. 2c, the sharp transitions being removed by a low pass filter 120. Positive and negative reference pulses of fixed amplitude are added to the signal at the beginning of each scanning line (as shown in the waveform at the output of amplifier 78 in Fig. 1) in response to the actuation of switch 82 by multivibrator 100 and switch 72 by multivibrator 86. The pulse output from multivibrator 86 has a duration twice that of the pulse output from multivibrator 100. In modulator 7 the amplitude excursions of the output of amplifier 78 are converted from being bipolar to unipolar, switch 116 being closed, low pass filter 120, and modulated on a carrier, e.g. of 2050 Hz, from oscillator 124. A control tone of 1500 Hz from oscillator 134 is added 130 (switch 136 being closed) and the composite signal fed via a vestigial sideband filter 132 to a line acoustic coupler 138. If the mode control unit 98 is actuated to transmit data at a low speed i.e. bandwidth compression is not required, then switches 82 and 84 are closed and switches 116 and 136 are opened. Thus mode control 98, selects the modulation index for the signal to be transmitted (switch 116), determines whether or not the signal is bandwidth compressed, sets the reference level and polarity of the signal and selects the appropriate drum speed.