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digital cable is not mandatory. MSOs switch to digital because they can compress the signal and shove more channels into 1 frequency.
Also not all channels on the digital tier are truly digital.
MSO /Cable companies/, at least Comcast, are doing what's known as Digital Simulcast - or simulcasting the analog channel lineup in digital. i.e. Channels 2 to 79 are analog but the cable company simulcasts them on the same channel in digital for which you need a digital set top box.
The cutoff date for analog over-the-air TV is February 2009.
Your local stations usually send their digital channel via fiber to the closest cable plant or via satellite and that's re-broadcast on the cable network, usually without additional compression.
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