| 摘要 |
A chroma keying device is provided for switching between graphics images and video images. An input signal and a reference are compared to provide a switching signal for performing the switching between graphics and video. A switching signal is provided when the input signal color information indicates that the input signal represents a true black image. Thus, switching is triggered upon black in the chroma keying device of a present invention. The black level is determined when each of the red, blue and green inputs of the device receives substantially zero current. A very sensitive comparison is performed between these input lines and a reference voltage in order to detect this condition. In the chroma keying device of the present invention, this sensitive comparison is accomplished using matched diodes within a common integrated circuit.
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