Method and apparatus for generating an auxiliary timing signal in the

Abstract

An auxiliary signal including a burst of oscillations and a blacker-than-black horizontal synchronizing signal is inserted in the horizontal blanking intervals of an HDTV video signal by first generating a digital auxiliary signal in a PROM operating at a clock frequency of 27 MHz. If the video signal from which the clock frequency and other control signals are derived is an analog video signal, the digital auxiliary signal is converted to an analog signal, filtered and clamped before it is additively combined with the analog video signal which has been similarly clamped. In that case the digital auxiliary signal stored in the PROM is predistorted so as to compensate for the damping effects of digital-to-analog conversion and low-pass filtering. When the video signal is available as a digital signal, the PROM outputs are clocked through a register enabled only in the horizontal blanking intervals and the digital video signal is clocked through a register disabled in the horizontal blanking intervals, so that the outputs of the registers can then go to a common digital-to-analog converter. In this case the PROM handles 9 bits, the most significant one of which goes to a MSB input of the digital-to-analog converter, to which the video signal has no connection, in order to produce the blacker-than-black synchronizing signal.