| 摘要 |
Monochrome and color video mixers mix an overlay image with a default image to form a composite image to be displayed on a computer display. The monochrome video mixer includes a summing circuit for summing an overlay signal and a default signal to generate a composite signal. The overlay signal corresponds to the overlay image, the default signal corresponds to the default image, and the composite signal corresponds to the composite image. The video mixer includes a comparator that has a first input for receiving the composite signal and a second input for receiving the overlay signal. The comparator compares the signal level measured at the first input with the signal level measured at the second input. In response to the comparison, the comparator provides an enable signal in the presence of a default key color signal. In response to the enable signal, an enabling circuit in the video mixer provides the overlay signal to the summing circuit. The enabling circuit provides the overlay signal to the summing circuit when the enable signal indicates that the first input signal level is equal to the second input signal level, or, alternatively, when the first input signal level is less than the second input signal level. A color video mixer includes a number of the above-described monochrome video mixers, wherein each monochrome video mixer provides a separate output to a color gun of the display device. The overlay signal from each mixer is passed on to its respective color gun (through the mixer summing circuit) only if all the comparators (for all monochrome mixers) provide an enable signal.
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