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8VSB vs. COFDM
When the digital broadcasting initiative was first passed, there were two ways to broadcast the signal over the air. One is called 8VSB for 8-bit Vestigial Sideband modulation and the other is COFDM or Coded Orthogonal Frequency Division Multiplexing. It's beyond the scope of this column to describe these broadcast encoding methodologies in detail, but suffice it to say that they are fundamentally different approaches to the problem of broadcasting a digital signal and they have different strengths and weaknesses.

Everything is a trade-off in technology, and which of these two broadcast methodologies is best is largely a matter of what you consider important. In fact, what you might think imperative for a good HDTV system, a broadcaster might not find important at all. COFDM, for instance, is much more tolerant of multipath interference and in-band interference than is 8VSB and is easier to receive, while 8VSB has the advantage of utilizing the available spectrum more efficiently. If you were a broadcaster, you'd likely find efficient use of allocated resources much more important than being able to get a decent picture with a cheaper, less sophisticated antenna. And this brings us to the topic at hand.

8VSB Is Chosen
In early 2000, the Senate Committee on Digital TV (ATSC or Advanced Television Standards Committee) made the decision (some think arbitrarily) to adopt 8VSB as the broadcast standard for US Digital TV. As most of us know, the deadline was set for 2006 for the full migration of US broadcasters from analog broadcasting over VHF to full digital broadcasting over UHF. Obviously, this deadline is not going to be met, but just as obviously, it will happen at some point. The fact is, the government wants the VHF TV spectrum back. The result is that we are stuck with a system that, while very efficient in its use of broadcast spectrum, is relatively difficult to receive - especially if you live more than a few miles from the transmitter. This makes antennas particularly important.

Although 8VSB was sold on its ability to reject multipath interference, the reality is that this aspect of 8VSB simply doesn't work very well. Unless you have a very strong, very directional signal, the results you get over the air will vary between different stations, according to the time of day, outside temperature, moving objects like aircraft and sometimes even cars and trucks. In analog, the signal might fade a bit or get multiple images from some of these obstacles, but little more. With DTV, the signal breaks-up into a mosaic tile pattern or "pixelizes" and a condition lasting more than a few video frames will cause total picture and sound loss often for many seconds. It is imperative, therefore, for those contemplating using a new HDTV set in conjunction with a home antenna (as opposed to satellite or cable) to buy the correct antenna and to install it correctly.

Outside is Best, but Directional is Crucial
Naturally, as in any discussion of reception antennas, the first, best piece of advice is to fit an outdoor, rooftop antenna. This is especially important if you live more than 10-15 miles from the broadcast stations. It's even more important if these stations aren't line-of -sight. But, in some areas, adequate reception can be had with an attic-mounted antenna, and if you have to do that, then there are other ways of dealing with the attenuated signal. If you can see the broadcast tower from your location, an indoor antenna is probably more than adequate. This does not mean, however, that a simple UHF "bow-tie" antenna will suffice. In most urban or hilly areas it simply won't do at all because the simple bow tie has no gain. That is to say this type of antenna is pretty omni directional and will pick up signals from the side and back with almost the same strength as it will pick up signals from its front. The poor multipath performance of the US 8VSB signal makes it an absolute necessity that you use a fairly directional UHF antenna for DTV; even in strong signal areas.

Multi- element UHF antennas are easy to find. Any TV shop or even Radio Shack should be able to supply one with everything you need. If you live close to the broadcasting facility, you can mount the antenna in the attic, but more than say, 15 miles from the broadcast tower is going to require an outdoor antenna. Either way, one thing will be common to both. Highly directional antennas need to be adjustable. In the "old days" rotors were only needed if you lived in an area where the different TV channels were coming from different directions. DTV needs a rotor because the multipath problems of 8VSB may require you to tweak the antenna's position occasionally to maintain decent reception. Rotors can be mounted either outdoors or in the attic. Attic mounts are actually very easy. A pair of 'U' bolts bolted around a length of standard antenna mast and anchored in the roof peak stringer will generally work fine. If you are mounting on the roof, mount the antenna at as high a point as you can find. Use coaxial cable for lead-in. DTV is quite susceptible to in-band interference, and we don't want the lead-in cable to act as an antenna for this interference.