GB649312A - Improvements in or relating to modulators for carrier telegraph systems - Google Patents

Abstract

649,312. Code telegraphy. STANDARD TELEPHONES & CABLES, Ltd. Sept. 3, 1948, No. 23224. [Class 40 (iii)] [Also in Group XL (c)] In order to avoid distortion in a carrier frequency telegraph system when the signalling frequency approaches the carrier frequency, the leading and trailing edges of the signal pulses are rounded and are fed to a modulator which has a linear response characteristic over a limited range but is driven beyond that range by the pulses. As shown, the initial pulse signals a, Fig. 3, are fed to a shaping network 8, 9, Fig. 1, or to a low pass filter, to give the pulses the shape shown at b, Fig. 2. The pulses and the carrier from the source 5 are fed to a modulator 4 which if operated in its linear range would produce carrier pulses shaped as at c, Fig. 3. The modulator, however, is operated much beyond its linear range, thus producing the carrier pulses d, Fig. 3. The signals may be transmitted as phase reversals of the carrier. They may alternatively be transmitted as shifts of the carrier frequency ; for this purpose the frequency modulator may be of the kind described in Specification 617,139, [Group XL (c)], and the signals used to overdrive the auxiliary amplitude modulator employed in that system. The carrier frequency is described as being in the audible range, and multiplexing by the use of different audio frequencies is referred to.