Video overlay circuit for synchronizing and combining analog and digital

Abstract

A video/graphics overlay circuit receives an analog input composite video signal and a digital input composite video signal and combines them into a linear combination output composite video signal depending on the state of one or more mixer control signals. The two composite video input signals are each capacitively coupled to buffer and clamp circuits through which the blank or DC level of each signal is clamped to two volts. The outputs of the buffer and clamp circuits are then fed to a mixer circuit and burst separator circuits. The mixer circuit generates the output composite video signal which is a linear combination of the input composite video signals as controlled by the one or more mixer control signals. The burst separator circuits separate the burst signal from the input composite video signals. The extracted burst signals are then provided to a burst signal phase-locked loop for locking the burst signals of the input composite video signals in phase. The feedback loop of the burst signal phase-locked loop is closed through a digital encoder circuit and changes the phase and frequency of the burst signal within the digital input composite video signal until it is locked in phase with the analog input composite video signal. A burst lock signal is generated when the burst signals of the two input signals are locked in phase. A horizontal lock signal is generated when the horizontal sync signals of the two input signals are locked in phase. A black and white detector circuit generates a signal which indicates whether the analog input composite video signal is a black and white signal or a color signal.