Notes on the effects of irregularities in co-axial cables on television transmission

Brillouin [see Abstract 1938B02269] has shown that, on account of double reflexions from two irregularities in a wide-band cable, secondary waves are superimposed on the direct wave and produce prolongation of the transmitted output signal (signal "tail"). In the case of a 400-line 25-frame television transmission over a typical co-axial cable the tail of a 4 Mc./sec. signal may be of the order of one 10th line duration; the result of an effect of this magnitude on the picture is to produce a grey background "veiling" of the image. Experiments are made with a cinematograph projector and suitably modified shutter to produce a veiling effect on a film image similar to television signal tail effect and to determine the level below which image veiling ceases to trouble the observer; this level, expressed as a relative initial mean amplitude of the secondary signals, is given as 1/30. The effect of signal tail on a rectangular image is discussed particularly in relation to the possibility of correcting the transmission by equalisers and phase compensators; such correction is possible in principle but would present great difficulty in practice.