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Abstract

This report describes subjective measurements of the visibility of delayed images superimposed on a 405-line television picture. The visibility of these images, which can occur as a result of a reflected signal caused by a mast or other reflecting object in the propagation path of the direct signal, depends on the amplitude and phase of the reflected signal relative to those of the direct signal. The measurements described here were made using superimposed video signals with a choice of relative polarity. Therefore these results apply to the practical case involving radio-frequency signals only when the two carriers are in exact phase coincidence or exact phase opposition; these are the two most visible conditions.

In these conditions, and with a delay between 3.0 u s and 10 u s, a reflected signal 38 dB smaller than the direct signal will not be more visible than "just perceptible" to 99% of viewers. If its relative amplitude is increased to - 32 dB, the reflected signal will be no more visible than "just perceptible" to 75% of viewers. These figures apply for pictures of extreme contrast and which have large areas of uniform background.

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