Carrier phase recovery scheme for a low-frequency vestigial sideband video transmission system

A vestigial sideband system seems attractive for transmitting video signals over media of limited bandwidth. Linearity constraints in the unavoidable intermediate equalizers (repeaters) limit the level of the transmitted carrier to such an extent that envelope detection is precluded. In a proposed video system the transmitted signal includes a partially suppressed carrier and an out-of-band pilot tone. At the receiver a voltage of the exact carrier frequency is regenerated from this pilot tone. But synchronous detection of a video signal requires the exact phase of the incoming carrier as well. Distortion considerations limit the width of the guard bands around the partially suppressed carrier to such an extent that conventional filtering cannot be used to retrieve its phase. A special type of a phase-locked loop which adjusts the phase of the locally generated carrier to that of the incoming partially suppressed one was therefore designed and tested. Design considerations, circuit diagrams and test results for this circuit are presented here. Subjective tests have demonstrated the feasibility of the proposed system for transmitting a video signal of 1-MHz bandwidth over a repeatered twisted pair.