| If doing a Chroma Key on the MX50 the B bus must be the key source signal. If you are tying up the B bus that way the best you could do is "cut" between sources on the A bus.
What you are trying to do is more typically done as a downstream key. In this case you would use the EXT CAM input on the back of the switcher as the luma key source (from your computer), and your fill would have to be white or a color matte of your chosing. You would be able to cut or dissolve the backround image as well as independantly fade in or out the key that way.
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Tom D'Angelo
New York City
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