Method and a circuit for detecting the presence of a television or other device on the output of a video digital to analog converter

摘要

A detector circuit (1) for detecting the presence or absence of a television (2) on an output (3) of a video DAC (4) comprises a comparator (11) for comparing a voltage developed by the video signal on a control resistor R2 with a reference voltage of 0.5 volts. The resistor R2 is of 75 ohms and matches the internal impedance R1 of 75 ohms of the television (2). A latch (12) latches the output from the comparator (11) onto an output pin Q when the voltage developed across the control resistor R2 is developed by an equalisation pulse of the vertical blanking interval of the video signal. In the presence of a television (2) the voltage developed across the control resistor R2 is 0.35 volts, which pulls the output of the comparator (11) low, while in the absence of a television (2) the voltage developed across the control resistor R2 is 0.7 volts which pulls the output of the comparator (11) high. A control logic control circuit (21) reads the output pin Q of the latch (12) for powering up or powering down the video DAC (4) as appropriate.