All providers broadcast standard defintion 4:3 as MPEG-2 for Video and AC3 for Audio both A/V industry standards even for DVD players and DVD discs that we all use. But DTV predates these standards they use a similar encoding modulation. They use QPSK instead of MPEG-2. Both encoding schemes use almost 50:1. This is an area that I know very well because I do this for a living.
Example: Let say a hockey game of about 2 hours video taped on a digital format like DVcam if they broadcast it as is it will take about 30GB of digital video broadcast DVB. Where as if you encode to MPEG-2 & AC3 it will be down to 3G byte 10:1. Also encoding frame by frame requires compression in Mega pixels per frame similar to your digital photo camera 5MP or 6MP. Now that it is encoded you need to decode it back so you need a decoder.
No Manufacturer that makes generic decoders like to bother with equipment that receives only QPSK the current encoding modulation for DTV instead of MPEG-2 and AC3 the current industry standards for SD by all providers. But this doesn't mean they are safe forever may be for SD 4:3 standard broadcast. But for MPEG-4 HD is another ball game??? All providers today use MPEG-4 for HD
