GB1502879A - Band-width reduction - Google Patents

Abstract

1502879 Secret transmission BATTELLE-INSTITUT EV 1 May 1975 [6 May 1974] 18233/75 Heading H4R In a bandwidth compression system an input signal from a source 1 is frequency shifted by a single sideband suppressed carrier modulation system 2, 3, e.g. using a carrier from oscillator 11 of about 10 MHz. The resulting sideband has its amplitude variations suppressed in a.g.c. unit 4 and is applied to a phase comparator 6 in a phase lock loop system 5 in which the other input to the comparator is derived from a voltage controlled oscillator 7 via a multiplier 8, and the output of the comparator controls the oscillator frequency, so that the oscillator frequency is locked to a sub multiple of the incoming signal. The output of oscillator 7 is also applied to a variable gain amplifier 9, controlled by the gain control voltage from the a.g.c. unit 4 so that the original variations of the signal input are impressed on the output of oscillator 7. The output of the amplifier 9 is shifted to base band by a modulator 10 and filter 10', the modulator being fed from carrier oscillator 11 via a divider having divider ratio which is the inverse of the multiplier ratio of multiplier 8. At the receiver the incoming signal is shifted to a high frequency range by a single sideband suppressed carrier modulation system 12, 13, the signal multiplied in frequency, by the same factor as the division in the transmitter, in multiplier 14, restored to base band by modulator 15, and high frequency modulation products removed by filter 17 to provide an output signal of similar form to the original signal. The arrangement can be used for bandwidth economy in transmission or recording or, with speech input, as a secret transmission system since the output from the transmitter Fig. I is unintelligible.