Envelope Detection of Vestigial Sideband Signals with Application to Television Reception

Abstract

This paper contains a simple derivation of the fact that for a wide class of ideal vestigial sideband filters and with no distortion in transmission, the in-phase component of the waveform at the input to the detector is the modulating signal itself; the quadrature component is produced by a linear filtering operation on the modulating signal. When the vestigial sideband forming filter has a linear skirt, the impulsive response of this linear filter is h(f) _ sin bsr 2 where b is related to the extent of this skirt. For narrow band modulating signals, the filter described by h(f) is equivalent to a differentiator. Alternate models for the VSB format are proposed. One is chosen in which the quadrature-component-producing filter has an impulsive response which is amenable to computational use. With it, the response of an envelope detector to typical television test waveforms is computed. These results are applied to computing the phase error in the reference used in a quasi-synchronous detector whose input is taken from a conventional IF filter. In an analysis of steady state color error, by evaluating the effect of quadrature distortion using colorimetric coordinates which are subjectively significant, it is shown that the color subcarrier should be attenuated by approximately 7 dB to reduce color shifts in green to imperceptibility in a side-by-side test.