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"  When you turn on a high-definition broadcast, you assume that your TV will come to life with the crispest, sharpest picture imaginable. But the fact is, hi-def doesn&amp;#039;t always mean high quality. The standards for what qualifies as HD\u2014the ones that go into effect with the big digital TV changeover in February\u2014were set by the Advanced Television Systems Committee (ATSC) back in the 1990s, and really only involved one major qualification: having a whole lot of pixels. In fact, there&amp;#039;s no real regulation over high-definition picture quality at all\u2014&amp;quot;none whatsoever,&amp;quot; one industry consultant told me. And that&amp;#039;s part of the reason why different HD stations often have wildly varying levels of picture quality that change from one moment to the next. Behind the scenes, content producers, broadcasters and cable and satellite providers are engaged in a constant tug-of-war over bandwidth and video quality, with no hard metrics to even define what looks acceptable. Even officials at HBO, where Generation Kill looks pretty fantastic on my TV, bemoaned the lack of a silver bullet ... for now. In order to qualify as hi-def, a signal must have either 720 horizontal lines of progressively scanned pixels (720p), 1080 lines of interlaced pixels (1080i) or 1080 lines of progressively scanned pixels (1080p, which nobody even broadcasts yet.) But there&amp;#039;s a whole lot more to the quality of digital television than the number of pixels present. After all, 1080 lines of poor-quality pixels may technically be &amp;quot;high-definition,&amp;quot; but that doesn&amp;#039;t mean it looks very good. One of the most important factors in determining picture quality is bit rate, or how much video and audio data is being sent down the pipe for each program. The technology behind digital television relies heavily on digital compression, and the ATSC specifies that digital TV use the MPEG-2 compression standard, which is also utilized by DVDs, although some satellite broadcasters use the more efficient MPEG-4 advanced video coding (AVC) standard. These compression technologies are necessary in order to deliver a large number of channels to consumers. Without these codecs, an uncompressed HD video stream could require as much as 1 gigabit per second of data capacity\u2014that&amp;#039;s 52 times the capacity of the average broadcast channel. With compression, the same stream can be shrunk almost infinitely. But compression is often used overzealously, and picture quality suffers as a result.  Many people are already familiar with this data-size/fidelity tradeoff from their experiences with digital music: MP3 files with high levels of compression may take up less hard drive space, but they sound muffled and unsatisfying. The same is true for video. When an HD signal is over-compressed, it may have the same number of total pixels, ensuring it&amp;#039;s still technically HD, but the picture is often tainted with blocky, pixelated noise and image artifacts. Surprisingly, there is little regulatory control over compression. According to ATSC president Mark Richer, the ATSC standard does not require minimum bit rates for over-the-air broadcasters, and the FCC requires only that broadcasters provide at least one service that is equivalent to analog television. Each over-the-air channel has a bandwidth of 6 megahertz, which can handle a maximum bit-per-second throughput (also known as &amp;quot;payload&amp;quot;) of 19.3 Mbps. But since each broadcaster only needs to provide a 480i digital signal, which typically demands 4 to 6 Mbps, that leaves plenty of space for other subchannels on the same channel&amp;#039;s bandwidth. Even high-definition channels (which take up anywhere from 12 Mbps for a good-quality 720p broadcast to at least 15 Mbps for 1080i) should have enough leftover data space to broadcast a quality standard-definiction subchannel. The math seems to work, but digital TV isn&amp;#039;t generally broadcast at set bit rates. Instead, most broadcasters use a technology known as &amp;quot;statistical multiplexing&amp;quot; to dynamically allocate bandwidth to various programs based on the needs of each program, ensuring the most efficient use of the available bandwidth. For example, an HD broadcast on NBC next month of a track-and-field event during the Olympics would have fast-paced movement requiring a lot of extra bits\u2014in general, the more movement there is on screen, the more pixels need to update in each frame, so the less efficient the compression of the video stream becomes. But a newscast on an NBC sub-channel, with largely static talking heads, requires a lower bit rate for the same perceived quality. So the statistical multiplexer will shift a greater portion of the bits running down the broadcast pipe to a sporting event like the Olympics. &amp;quot;The bit rate for video is incredibly flexible,&amp;quot; says television industry consultant Mark Schubin. &amp;quot;The range is enormous\u2014there are broadcasters that have done HD in as little as 6 Mbps and as much as 18 Mbps.&amp;quot; And that&amp;#039;s just in the broadcast realm, where only a few programs are potentially running on each channel. Cable and satellite services transmit hundreds of channels at a time, and there is literally no regulatory oversight at all when it comes to compression of HD content on those services. The only applicable language in FCC regulations is the &amp;quot;must carry&amp;quot; rule, which specifies that a cable company cannot down-convert a locally broadcast HD signal to an SD signal. For example, if ABC affiliate KTKA in Topeka, Kan., is sending out Desperate Housewives in 720p, Cox cable service cannot change it to 480i to save bandwidth. In order to keep cable and satellite companies from overcompressing their programs, many cable channels and major networks broker individual minimum-quality-of-service contracts with each provider. But even those are somewhat nebulous.  I asked Robert Zitter, HBO&amp;#039;s chief technical officer, what his company&amp;#039;s requirements were. &amp;quot;We do a contractual relationship with all of our distributors, and one of the items that&amp;#039;s addressed in there is what they can and cannot do with our signal,&amp;quot; he says. &amp;quot;But much of it cannot be quantified. There&amp;#039;s not a device that&amp;#039;s made that can look at a picture and say pass or fail. We all wish we had it\u2014it would make the negotiations a lot easier. Ultimately, it winds up being more subjective than quantifiable.&amp;quot; HBO delivers its programming precompressed, explains Zitter, and does make demands about what other programming the company&amp;#039;s content is multiplexed with onto the same channel (no sports channels, animated content or other high-bit-rate-demand content). But HBO is a content provider with plenty of negotiating clout that places a high priority on video quality. For many second-tier cable channels, their programs&amp;#039; compression is often left to the whims of the cable or satellite companies. And as more and more HD channels have ever-higher bit-rate demands, and as increased Internet usage puts more strain on overall cable bandwidth, the horse-trading between broadcasters over bits per second could easily get ugly. In fact, according to Hugh Brydges, of the Society of Cable Telecommunications Engineers, there is a behind-the-scenes quasi-democracy of bandwidth going on with many cable networks, where major channels get assurances of quality, while certain low-viewership channels aren&amp;#039;t really channels in the classic sense at all. Using a technology called &amp;quot;switched digital video,&amp;quot; these channels are served up to viewers upon request like a video-on-demand movie. When nobody is watching, the channels take up no space at all. For the future, the cable and satellite industries have a few tricks up their sleeves to free up some extra bandwidth for more HD programming. Satellite companies are already moving to the more efficient MPEG-4 AVC standard, which allows for more HD channels. The cable industry is moving more slowly to new compression standards and also hopes to phase out a lot of the analog channels that are currently clogging up much of its bandwidth. Interestingly, that puts them in a similar situation to the one many broadcasters are now dealing with because of the digital television conversion: Many customers with older, non-HDTVs will require new set-top boxes to view content. Alas, the only other option is to keep on squeezing that signal. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/culture/tv/a3459/4275063/>