Ten HDTV Myths

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"All true HDTV programming looks equally great."

"I'll have to toss all current analog sets when the digital conversion kicks in."
Photograph: Chip Simons
This claim gets us to a dirty little secret of HD broadcasting: All HDTV programs are compressed--some to a greater extent than others.

The FCC allots each TV station sufficient airwave spectrum to broadcast a little over 19 megabits per second of data, but stations aren't required to devote their share to a single high-def program. They may compress an HD show enough to leave room for one or two standard-def broadcasts as well--a practice known as multicasting.

The ATSC standard includes support for MPEG2 video encoding, but it says nothing about compression levels. Broadcasting an uncompressed MPEG2 video would require 885 mbps (for 720p content) or 995 mbps (for 1080i content). A station that broadcasts a single HD program can devote only 18 mbps to it, HDTV consultant Peter Putman says; and to get that, broadcasters have to use a compression ratio of 49:1 for 720p and 55:1 for 1080i.

If a station uses its bandwidth to broadcast both an HD show and a standard-def show, the HD program has to fit into 13 or 14 mbps. And a station sending out two standard-definition channels along with an HD channel must compress the HD signal to roughly 13.5 mbps, which entails compression ratios in the vicinity of 66:1. Such high compression produces artifacts that might not be noticeable on a small CRT, but can be quite obvious on a big fixed-pixel display. These include mosquito noise, an effect in which small dots seem to surround a person's head; and macroblock errors, similar to what a fast-moving video game looks like on a PC with too little graphics power.

You can get a hint of how much a station compresses its video by learning whether it multicasts. But generally speaking, satellite and cable carriers compress HD programs more than over-the-air broadcasters do. Though they have a lot more bandwidth at their disposal than terrestrial stations, these pay-TV carriers need it for sending out the dozens of channels their subscribers expect (not to mention extras like Internet access). Dish Network has said that, because of bandwidth constraints, it will gradually move all of its customers to equipment that supports MPEG4 encoding, which is more efficient than MPEG2. But sometimes it's out of the carriers' hands, too. Pay-TV content providers such as Discovery, ESPN, and HBO also compress their programs before beaming them to the cable and satellite services.

"Standard-definition TV is unwatchable on HDTV."

Well...this is a case of hyperbole, not of outright fabrication. True, standard-def programming will never look as good as HD programming on an HDTV because of the scaling issues mentioned previously. But vendors are toiling to better the SD experience on their HD sets, and the success of these efforts varies between vendors and sets. So if you're expecting to watch standard-definition TV on an HD set, make sure that you do your own taste tests.

"I'll have to toss all my current analog sets when the digital conversion kicks in."

Though this is not strictly an HDTV issue, it is a common misconception about the digital transition, which Congress seems bent on completing by 2008. At that point your old sets won't be able to snag over-the-air broadcasts without help, but you should still be able to use them by buying inexpensive digital-to-analog converters. And cable or satellite boxes will still work because the service provider will take care of the conversion. Of course, you won't be able to experience HDTV on an analog set.

These may not be the only myths you'll encounter in your quest for the perfect HDTV--and you can't trust everything you hear (or see) in a showroom. So careful research is essential before you pay for what's likely to be the most expensive TV set you've ever bought. And that is the gospel truth.

High-Def Resources Online

High definition continues to evolve, so it's wise to keep up-to-date. The Consumer Electronics Association offers a useful primer on HDTV. For more insights from consultant Peter Putman, check out his site, HDTVExpert.com. You can discuss high-def matters from technologies to local reception with the denizens of High Def Forum.

This story, "Ten HDTV Myths" was originally published by PCWorld.

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