I just noticed that Comcast is starting to convert some local broadcast channels to MPEG4. On my non-X1 DCX-3400, I can tell MPEG4 because the 30-second skip has a bit of a delay, and sometimes the 15-sec rewind doesn't actually go back. I noticed this on a recent recording of "The Tonight Show".
I checked my DTA, which has a nice virtual channel map, and it's true. Here in Chicago, it is showing WGBO, WLS, WMAQ, WTTW and WSNS as MPEG4 (code A in the map) and all on 147MHz. And yes, they are all downconverted to 720p. As far as I know, broadcast WMAQ (the NBC owned station) should be 1080i.
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