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Is HDTV compression damaging picture quality?
- — 30 April, 2008 10:21
Do your HDTV programs look as good as they should? If not, you may be seeing the results of overcompression. A growing number of viewers and experts are claiming that increased use of compression — technology that downsizes huge high-definition video streams for eventual reconstitution on your screen — is responsible for a drop in quality.
Such charges — aimed mostly at the two biggest cable providers, Comcast and Time Warner — have been echoing around the blogosphere in the wake of reports about new compression algorithms. Contributing to the debate: An AVS Forum member's tests showed that at least ten HD networks were more compressed on Comcast than on Verizon's fiber-optic-cable-based FiOS service.
While service providers insist that quality remains a high priority, some experts say competition has made channel quantity, not quality, the top priority.
It's no secret that most digital TV is compressed and decompressed--in some cases several times — not just by cable or satellite services and over-the-air broadcasters but also by the video cameras that create the programs and the network satellite systems that deliver them to distributors. Compression happens, too, in trucks, control rooms, cable headends, and other waystations along the signal's path to your screen. The telltale signs of overcompression include tiling, little colored blocks, and "mosquito noise," which looks like flaring fireflies. The crispness of the picture can suffer, too.
Quantity vs. Quality
Compressing huge HDTV video streams, however, allows carriers to deliver more of them. "Everyone's really fighting the same issue — limited bandwidth — [but] their offerings are more attractive if they give more channels," says Peter Symes, director of standards and engineering for the Society of Motion Picture and Television Engineers. "There are big arguments in the broadcasting community about whether you should use virtually the whole of the 19.2 megabits you get for a single HD channel to deliver really good HD, or whether by using other standards or more compression you can get away with a high-definition [signal] and one or two standard-definition [signals] or maybe sell part of it for data services."
Currently, quality is "minor to a vast majority of the viewing public, because the vast majority...doesn't have very big TV screens," HDTV consultant Peter Putman notes.
But those viewers who do are starting to notice, including AVS Forum member Ken Fowler of Arlington, Virginia, a self-described audio-video enthusiast who posted the results of his Comcast-FiOS comparison.
Fowler had dropped Comcast for FiOS, but renewed his Comcast service (while retaining FiOS) in order to get Washington Nationals games in HD, and he began noticing quality differences between the two services. Some of the Comcast channels, he says, "didn't have the same pop, the same level of contrast, and there was a lot more blurring during movement."
So Fowler began recording the same shows on both FiOS and Comcast with his TiVo; he then downloaded the files to a computer and calculated the bitrates based on file size. The differences ran from just 0.7 percent more compression on Comcast for HBO HD to a whopping 38.5 percent for Discovery HD Theater.
Comcast acknowledges that it recently implemented additional compression of selected HD networks, but contends that its improved compression technology allows it to transmit three channels in the same bandwidth in which two were transmitted previously, without a loss of quality. Comcast's spokesperson adds that many comments about Fowler's AVS Forum post recognize that "our ongoing tweaking, if you will, and adjustments" are improving image quality.
Comcast isn't alone in looking to squeeze more HD into its cables. A Time Warner Cable spokesperson says that company is testing new increased-compression technology as well. On the satellite side, DirecTV and Dish Network are switching to a "more advanced compression algorithm," Putman says. Verizon FiOS, however, applies no additional compression to the network signals that it receives, a spokesperson says.
The HD broadcast format that a network uses can make a difference in the compression's impact. ABC and Fox are among those that use a progressive-scan format (720p), which Putman and other experts say tolerates compression slightly better than the interlaced (1080i) format used by CBS, NBC, PBS, and others.
Blu-ray Is the Benchmark
For consumers, the best HD experience will be with Blu-ray Disc content on a player hooked up to a display with an HDMI connector, which transmits uncompressed digital streams. "That's going to be the benchmark," says Symes.
If you're shopping for an HD service, Symes adds, "it's fairly a no-brainer: If you have FiOS or [AT&T's] U-Verse available, that's probably the way to go." Beyond that, so much local variability exists among competing cable and satellite services, he says, that the best idea is to ask friends in your area about their satisfaction levels.
But if you have an HD picture quality problem that you think is the result of overcompression, the best thing to do is to call your provider. Symes and Putman agree that overcompression and lowered quality will become an industry issue only when buyers who trade up to the biggest, highest-resolution screens notice and complain.