Vancouver:
In the old days (a few years ago) there was no such thing as high definition broadcasts over the air (or anywhere). What you picked up over the air by sticking rabbit ears on top of your TV was 480i television. What we now call standard definition televsion (SDTV). It was about a 640x480 interlaced display format that you could get for free with an antenna and the format it was delivered in fit the NTSC broadcast standard.
ATSC is a new set of standards that includes many formats. Any ATSC tuner has to receive all the new formats which include: 1080i (HDTV), 720p (HDTV), 480p (EDTV), and 480i (SDTV). An ATSC receiver is designed to pick up free broadcasts that come over the air... like a set of rabbit ears on your tv. You must put an antenna up to receive the stations with an ATSC receiver. The antenna really should be a roof mounted version as high as possible for good reception. Even then, it depends on how close you are to a broadcast station.
Do you need it?
Well, probably not.
But possibly! Where I live, Washington DC area, cable companies include NBC, ABC, CBS, FOX, ESPN, SHOWTIME, HBO, and a couple of other channels in HDTV. An ATSC tuner MIGHT pick up the WB. But, that's about the only channel I would get from an ATSC tuner that isn't already included on my cable box.
If you are using DirecTV or Cable, then you must check with your provider to find out exactly what HD channels they broadcast to your home. Then you will know whether you need an ATSC tuner.
Note: If you didn't get a TV with built in tuner, they are sold separately as stand alone boxes like your cable box or like a VCR.
NOTE: A TV marked HDTV or EDTV has nothing to do with what it can display on the set. It strictly has to do with the resolution of that device. That is, a HDTV can only be called an HDTV if it has a resolution above 720 lines. It still must scale everything to fit into the actual resolution it uses. An EDTV gets its name becuase it uses 853x480 pixels. Or 480 lines of resolution (480p). This doesn't mean it can't display HDTV. It just means that it must convert 720p or 1080i to 480p to fit onto the screen.
It also doesn't mean that at 42" people spending thousands more for the extra pixels aren't simply being ripped off. It all depends on how far you will be from the display device and how well the display device actually deals with the scaling necessary to fit an image on the screen.
There are about twice as many pixels in a HD display as there are in a ED display. This means that if you think the HD display looks absolutely awesome from 5 feet away, then the ED display will look identical from 10 feet away (all other factors remaining the same). Or, about where you might place it in a bedroom. All other factors include the quality of color, scaling, image reproduction, brightness, contrast ratio. People often buy HDTV displays because of the marketing involved with pushing HD over ED. It is fantastic, yet sad, to see how many people have bought into the idea that an HDTV is actually what is required to get the most out of your viewing experience.
