why not the LS2/LS7? @ Apr 1st 2008 3:50AM

I said "a QAM" isn't a technical term, not "QAM" isn't a technical term.

No, OTA and cable aren't different here. Both use 6MHz slots and both squeeze the same amount of data in a 6MHz slot. It's just that OTA uses 8-VSB (in the US) and cable uses 64-QAM or 256-QAM. The reasons they use different modulation is that 8-VSB has advantages for being carried through the air (where signals have to compete with weaker signals from farther away on the same frequency bands) and 64-QAM and 256-QAM have advantages for being carried on a cable.

Both use ATSC encoding (by FCC regulation). CBS has a policy of broadcasting only a single virtual channel in a 6MHz slot, while other channels like KQED (PBS here in Northern California) broadcast 5 SD virtual channels (during the day) or 1 HD virtual channel and 1 SD virtual channel (in the evening) in a single 6MHz slot. In both of these cases, all the data is carried in single MPEG-2 transport stream (TS), and the receiver must decide which packets in the TS pertain to the virtual channel they are displaying/recording and which are for another virtual channel.

KQED's video quality suffers horribly and for the same reason as Comcast's transmissions, that is, they're just trying to squeeze too much into too little bandwidth. In KQED's case though they are doing it at origination, while Comcast is taking an already compressed signal and removing further data (sometimes called combing) from an already compressed signal.

DirecTV (and Dish) are not required by law to use ATSC (which uses MPEG-2), so DirecTV uses MPEG-4 AVC (H.264) for most of their HD, Dish does similarly. DirectTV boxes output with ATSC-type timings (like 1920x1080/60i or 1280x720/60p) but they don't use ATSC for their video signaling.

If people want to talk about what Comcast is doing, they should say a "single 6MHz slot", or (less specifically), "a single transport stream".