All digital video is compressed, that's the only way you can practically transmit hundreds of channels around. The question is how much they are compressed, and how. ATSC 1.0 uses MPEG-2, as did cable up until Comcast and Verizon started to use MPEG-4 for some channels. MPEG-4 is about twice as efficient as MPEG-2, but either way, you have to have an adequate bitrate for the type of compression and resolution being used.
Due to the FCC auction of the 600mhz spectrum and the resulting digital repack, many OTA channels are channel sharing, leaving less bandwidth for each channel and subchannels. It could get better with ATSC 3.0, if it ever takes off, but for the forseeable future, a lot of channels are going to be heavily compressed. If Comcast moves to MPEG-4 for local channels, they will likely get direct fiber feeds of them, but then compress them too heavily, so the video quality could get slightly better or worse depending on how they are compressing and transmitting today.
EDIT: I should say all digital video *to the consumer* is compressed. Main network feeds are as well, just a lot less than cable, satellite, and OTA signals. There is uncompressed digital video within TV and movie studios.
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That is a compression issue. Local channels vary market by market depending on the broadcaster, how they are compressing, and how many channels they are cramming onto a transmitter. Usually, DirecTV, Comcast, and OTA are getting the same signals, and DirecTV compresses them again to distribute them via satellite, but it can vary by market, and in some markets for some stations, one provider or another may get a direct fiber feed and compress it separately from what the station is doing.
National channels on Comcast all look blurry and lack detail, especially during motion scenes in sports, or dark scenes in movies because Comcast over-compresses them with a bit-starved implementation of MPEG-4, and they look terrible nationwide. Local network channels, however, will vary by market and sometimes by provider as well.
It is possible that you could get a higher quality feed from DirecTV or via OTA depending on your market.
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