A normal video signal and an overlay video signal are combined by supplying the normal video signal to the base of a first emitter follower transistor while the overlay video signal is supplied to the base of a second emitter follower transistor. The two transistors are biased such that the first transistor...http://www.google.fr/patents/US4599610?utm_source=gb-gplus-shareBrevet US4599610 - Overlaying information on a video display
