Reduction of television bandwidth by frequency-interlace

Abstract

A method analogous to the N.T.S.C. colour television system is used to obtain a bandwidth reduction of a black-and-white video signal by a factor of approximately 2:1. The normal signal is split into two frequency bands, nominally zero to 1.5 Mc/s and 1.5 to 3.0 Mc/s. The latter is used to amplitude-modulate a sub-carrier, whose frequency is an odd multiple of half the line scanning rate. The lower sideband of the modulator output is selected and combined with the original zero-to-1.5 Mc/s band, so that the spectra of the two signals interleave. The combined signal may now be sent over a channel of 1.5 Mc/s nominal bandwidth. At the receiving end of the channel the composite signal is applied to a synchronous demodulator, fed also with sub-carrier of the same frequency as at the transmitter. The lower sideband of this demodulator is taken and combined with the received signal, to yield a “normal” video signal extending from zero to approximately 3 Mc/s, together with an “interleaved” signal. The interleaved signal is such as to give an interference pattern on the display which in a stationary picture should optically cancel after four successive frame scans. However, the pattern is built up in such a way as to give rise to a “crawling” motion which is very noticeable at close viewing distances. Photographs of typical pictures obtained with an experimental apparatus are given, showing various interference effects produced.