I "think" this is because the audio is mis-tagged as being DD5.1 but really isn't, thus your receiver is treating it as 5.1 and why there is no center channel. I used to get this on WABC-HD and still do on occassion, however jmbladen, I didn't notice it during the Orange Bowl.
When I do, I just change to standard audio instead of the 5.1 and it sounds fine. Annoying tho as this has been going on for a while now.
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