Television Hum-Buckers

Abstract

One of the worst problems for TV broadcasters is the unexpected introduction of 60 Hz hum into the video signal by ground potential differences at locations separated a few hundred feet or more. These potential differences are usually caused by unbalances and various grounding practices in ac power systems which are not under control of the broadcasters. Frequently the hum differs by several volts and when a cable carries such a spurious signal the video signal frequently cannot be easily “cleaned up” satisfactorily. A system has been developed which obviates the need for balanced lines, or clamps, or differential amplifiers. It uses a so-called “Hum-Bucker Coil” in the video cable, which is actually a 60-Hz transformer wherein the unwanted ground potential difference is applied across the primary. The resulting secondary voltage is of a phase and amplitude for exact cancellation of the pre-existing hum voltage on the cable. This action is achieved with negligible picture signal degradation or attenuation. Measurements on a small hum-bucker show that more than 50-dB reduction of hum can be achieved. At the same time, quality degradation is only equivalent to that caused by video transmission through a few feet of small-size coaxial cable.