GB613877A - Improvements in or relating to arrangements for the transmission of electric signalsoccupying a wide frequency band - Google Patents

Abstract

613,877. Frequency scrambling. STANDARD TELEPHONES & CABLES, Ltd., and HODGSON, K. G. May 20, 1946, No. 15184. [Class 40 (iv)] [Also in Group XL (b)] An electric transmission system for signals occupying a wide frequency band in which the power level of one section is lower than the power level of another section, e.g. television or facsimile signalling systems, comprises means for dividing the band into a plurality of subbands, two of which respectively include the said sections, frequency changing means for shifting at least one of the sub-bands to cover a different range of frequencies, a plurality of communication circuits, and means for transmitting one or more of the sub-bands over each of said circuits, the low-level band being transmitted at a different relative level or over a different type of circuit from those of the other sub-bands. The invention contemplates a 1000 line colour television system and an equation is developed which shows that the bulk of the power in the band spectrum of such a system resides in the lower frequency components. In one application, Fig. 3, the output of a television transmitter 1 is divided into three sub-bands by means of filters 2, 3, 4, the frequencies in each sub-band being then shifted to other bands in frequency changers 5, 6, 7 suitable for transmission over communication circuits 10, 11, 12 (e.g. existing cable circuits &c. designed for 400 line systems). In the case of the lowest sub-band, e.g. that produced by the filter 2, the power level is such that it is suitable for direct transmission over circuit 10. The higher sub-bands produced by 3 and 4 are at lower power levels and amplifiers 8, 9 respectively are employed to allow transmission of these sub-bands at relatively higher levels than the lower bands, with a consequent improvement in the signal to noise ratio. At the receiving end the subbands are shifted back to their original positions in frequency changers 13, 14, 15 and fed to the receiver 19 after filtering in filters 16, 17, 18. Attenuators 20, 21 corresponding to the amplifiers 8, 9 are employed to restore the original power levels of the higher sub-bands. If the band spectrum of any of the sub-bands is suitable for direct transmission the frequency changing device can be omitted and replaced, if necessary, by an amplifier. In an alternative arrangement all amplifiers and attenuators are omitted at the transmitting and receiving ends and any necessary raising of the power levels effected by means of repeaters of different power handling capacities, or differently spaced, in each of the communication circuits. Four numerical examples are given to illustrate the design process of a communication system for a television band extending from 0-15 Mc/s which employs two cable circuits of given characteristics over which the minimum frequency to be transmitted is 500 Kc/s. Repeaters of similar maximum power output but having variable gains and band-widths are used on each of the cable circuits and tables are given showing the maximum permissible repeater spacing, assuming a maximum repeater section loss of 60 dB, for various dividing frequencies of the full television band. The practical arrangement of two of these examples is described with reference to Figs. 5 and 6 (not shown).