I keep basic analog cable just for the digital locals (ABC, CBS, NBC, FOX, WB, PBS)...and because it's cheap. I also receive digital locals via OTA, but WB and PBS are on temporary, low-power, antennas and don't come in all too well - they should come in 24/7 after going full-power.
1. Voom - satellite service is carried via 8PSK (VOOM STB)
2. Digital OTA - is broadcast using 8-VSB (ATSC tuner/Voom 8-VSB module)
3. Analog OTA - is broadcast using AM/FM (NTSC tuner)
4. Cable - analog and digital is carried using QAM (QAM tuner/Cable Ready)
A. VOOM STB will decode 1 & 2
B. A Cable Ready TV (been in existance for 20'ish years) will decode 3 & 4
C. OTA STBs will decode 2 and perhaps 3. However, newer models may also decode 4.
D. Cable HD STBs will generally decode 4.
My 2 year old Mits, with Integrated 8-VSB/QAM) digital tuner will decode 2 & 3. Or course, this assumes that cable is being broadcast in-the-clear (unencrypted). My 2 year old Mits also has an analog tuner "Cable Ready" for 3 & 4.
As of this summer, many of the larger HDTVs are supposed to come with an integrated 8-VSB/QAM tuner. The percentage of HDTVs having this tuner is gradually supposed to increase over the next few years until they are all "Digital Cable Ready".
Cable systems can send their HD broadcast locals (ABC, CBS, NBC, FOX, WB, PBS) using one of two methods: 8-VSB or QAM. Almost all cable system use QAM since it is my understanding 8-VSB take up too much bandwidth and requires additional equipment at the cable headend.
My cable company is passing broadcast locals through to its customers using unencrypted QAM. I believe the FCC has mandated that all cable companies must do this with their basic tier programming...but I am not certain. Also, my cable company is passing premium channels (HDNET, INHD, HBO-HD, etc.) through to it's customers using encrypted QAM. In this case a Cable HD STB is required to decode them. People think they need to rent an STB from the cable company in order to receive their HD. This in true ONLY if you wish to receive premium channels. All the digitals broadcast locals should be passed through "free of charge" as long as you maintain a basic/lifeline subscription.
Anyway, that's my understanding...please feel free to correct me