GB522429A - Improvements in and relating to carrier wave signalling systems - Google Patents

Abstract

522,429. Distributing music. WIRED RADIO, Inc. Dec. 8, 1938, No. 35801. Convention date, Jan. 14. [Class 40 (iv)] [Also in Group XL] In a system in which different programmes are transmitted over, e.g., a power line 25, in the form of side-bands of suppressed carriers and in which a control frequency from which replacement carriers are derived is transmitted with the side-bands, any modulation of the control frequency arising during transmission is removed by combining it in a linear detector 34 with local oscillations 35 of different frequency, and of relatively-low energy level, selecting the sum or difference frequency, recombining this with the local frequency in a second linear detector 37 and selecting the original frequency by a filter 38. Since the local oscillations and the modulation frequencies are both weak relatively to the control frequency, the output of detector 34 will contain only combinations of the control frequency and the weaker frequencies and not combinations of the weaker frequencies with one another, and the sum or difference frequency selected by filter 36 will not contain the modulation. In the system shown, the carriers employed are all harmonic (30, 50, 60, 80 kc) of a crystal-controlled basic frequency of 10 kc and the control frequency is a higher harmonic (150 kc) of the same basic frequency and after the removal of modulation is passed to a submultiple generator 39 to produce replacement carriers corresponding to the suppressed carriers. Specifications 520,074 and 521,720 are referred to.