Re: Digital multiplexer?

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You might give this product a look. It's called Digital Director. Although I wouldn't call it cheap, it sounds like it does what you want.
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I saw this unit on the web before. I'm not clear if this device auto-selects the newest active input or not. I e-mailed them to find out. If it only supports input selection from the front panel, it wouldn't be very useful. The built in sample rate converter is a nice touch and makes the price more reasonable in my mind.