Patent GB-347265-A (GB)
Title: Improvements in or relating to telegraph transmission systems
IPC: H04L27/02
Filed: 1930-01-24
Abstract: 347,265. Telegraphy. STANDARD TELEPHONES & CABLES, Ltd. and STARR, A. T., Columbia House, Aldwych, London. Jan. 24, 1930, No. 2524. Drawings to Specification. [Classes 40 (iii) and 40 (v).] Impulse combinations initially of rectangular wave-form, are robbed by filters of all but a narrow range of their harmonic components, this range sufficing to enable the signals to be reconstituted at the receiver. By the use of different harmonic ranges in different channels, several transmitters having the same dot-frequency are enabled to work over the same line. As applied to carrier telegraphy in which a single side-band is transmitted, the invention enables the side-band to be spaced apart from the pure carrier, by elimination of the lower components of the side-band. The Specification first expresses a group of rectangular pulses, by means of a Fourier integral, as a continuous frequency-spectrum, the amplitude of which is a periodic function of spectral frequency; and shows that the form of the original signal could be deduced at the receiving station provided that any fraction of the continuous spectrum could be precisely reconstituted there. This proviso being unrealizable in practice, the Specification transforms the problem from one dealing with transients to one dealing with a steady state, by assuming that transient effects die away after Nt/2 dot-periods, or Nt baud-seconds, where Nt is a number determined by experiment. The impulse-group can now be expressed as a Fourier series having as its fundamental frequency 2s/Nt, where s is the dot-frequency. For the case where Nt =5 and the Baudot code is used, four of the co-efficients of this series can be determined from the amplitudes and absolute phases of any one of a series of pairs of harmonics, and from these four, together with the changes of sign recorded by a siphon or like recorder, the original signal can be reconstituted. In one arrangement (Fig. 5, not shown) two sets of d.c. transmitters use the same line ; transmitters in the first set all work at the same dot-frequency s1, but filters are so arranged that from the first transmitter only those harmonic components which lie in the band 2s1/Nt to s1 are admitted to the line (Nt being preferably equal to 5) ; from the second only the components lying between 2s1 (1 + 1/Nt) to 3s1, and so on, the bandfrequencies increasing by 2s for each fresh channel. Each of the second set of transmitters uses a dotfrequency s2, with similarly selected transmission bands ; and six channels can be worked if 27s1= 23s2. At the receiving station demodulators are required in all but the lowest channels, and networks for correction of phase- and attenuationerrors are provided. The theory applies also to a carrier-wave, modulated by interruption, from which one side-band is suppressed. Signal bands of modulation-frequency, selected as above, may be applied to a modulator (Fig. 6, not shown), or the transmitters may act directly on the carrier (Fig. 7, not shown), before filtering is effected, unmodulated carrier being simultaneously transmitted for purposes of synchronization. As the lowest modulating frequency is 2s1/Nt and not zero, each side-band can easily be separated from the pure carrier and from its twin side-band, which may be used as a separate channel (Fig. 8, not shown). According to the Provisional Specification (Fig. 7 thereof, not shown), the pure carrier may be supplied locally at the receiver, but must in that case be synchronized with the transmitter carrier by means which are suggested in the Specification.