GB629861A - Improvements in electric wave transmission systems - Google Patents

Abstract

629,861. Modulating systems. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 19, 1947, No. 33670. Convention date, Oct. 15, 1943. [Class 40 (v)] A wide-band signal, e.g. an audio broadcasting signal, is converted into a carrier wave with single side-band by first producing a carrier with two side-bands and then eliminating one of the bands by successive stages. Fig. 1 shows the arrangement of the stages, and Fig. 2 the frequency spectra at the successive stages. The signal band I of Fig. 2 is applied through a filter 2 to a balanced modulator fed with a carrier from source 4, giving a weak carrier and two side-bands II. These are passed to a second balanced modulator 5 fed from source 6 with a carrier of frequency 4 kc/s. higher than the source 4. From the resulting frequencies, the spectrum III is selected and is reduced in band-width by a filter 7 so as to include one complete sideband, a 4 kc/s. carrier, and a section of the other side-band, as shown at IV. This spectrum is then subjected to a double change of frequency at 8 which brings the carrier down from 4 kc/s. to 450 cycles, as indicated at V, and is applied to a band-pass filter 9 which rejects the residual lower side-band and passes the carrier and upper side-band VI. This single side-band is now raised in frequency by modulating it at 10 in two stages on carriers which differ by 4.5 kc/s. (see curve VII), and is passed through a filter 11 which passes the new carrier of 4950 cycles and the upper side-band VIII. Further frequencychanging stages 12 precede the radio transmitter 13.