In-service measurement of composite triple beats in a cable television system

摘要

An in-service composite triple beats (CTB) measurement system for a cable television (CATV) system isolates and measures weak composite triple beat components in the presence of a visual carrier modulated with program material for selected channels of the CATV system. Each channel is demodulated and digitized in a frequency bandwidth about the visual carrier for the channel (the CTB frequency range). The digitized video is sampled during the vertical interval, and corresponding samples from consecutive frames are differenced, squared and accumulated to produce a CTB value, which may be scaled and corrected for noise. Noise correction is achieved by performing the same measurement, but digitizing in a frequency bandwidth offset from the CTB frequency range. The difference between the two measurements provides a corrected CTB value. For further accuracy the automatic gain control of the demodulator may be enabled only during the vertical interval, and the frequency control of a phase locked loop that produces the clock for the digitization of the video signal may be turned off during the vertical interval. Finally the amplitude of the sync tips of the demodulated video signal may be minimized prior to digitization.