Verfahren und Vorrichtung zur Überlagerung von digital generierten graphischen Zeichen über ein analoges Videosignal

摘要

A system for overlaying digital character signals on an analog video source signal including a predetermined color subcarrier includes a subcarrier phase lock loop (14), a digital character generating device (28), a digital video encoder (18), and a switching device (24). The subcarrier phase lock loop separately generates a color subcarrier and a system clock signal which are locked to the color subcarrier of the analog video source signal. The digital character generating device detects horizontal and vertical timing of pixel information in the analog video source signal, and generates digital character signals that are to be overlaid in predetermined pixels of the analog video source signal. The digital video encoder is responsive to the color subcarrier and system clock signals for generating a separate color subcarrier which is locked to the color subcarrier of the analog video source signal. The digital video encoder also converts the digital character signals from the digital character generating means into an analog video output signal that includes the color subcarrier generated in the digital video encoder. The switching means directs the analog video output signal from the digital video encoder or the analog video source signal to an output of the system during pixels times when a digital character is to be overlaid or not overlaid, respectively, on the analog video source signal. $$