Registered: Jul 2000
Location: Long Island, NY
GREAT NEWS! I'm one of those who can see the compression now.
Here is the full story:
Easing the Signal Squeeze for Satellite TV
By ERIC A. TAUB
Copyright 2001 The New York Times Company
ONE of satellite television's oft-heard selling points has been the quality of its digital pictures and sound. But some direct-broadcast satellite subscribers have found that "digital" cannot necessarily be equated with "good."
The country's two direct-broadcast satellite television services, DirecTV and Dish Network, can offer hundreds of channels by digitally compressing their signals, squeezing out extraneous, redundant information. In addition to nationwide channels like HBO and CNN, the satellite services are now offering local broadcast channels in more markets. But the more channels that are offered, the less bandwidth is available for each. Squeeze out too much data, and the signal loses its quality.
"My local channels on DirecTV definitely look softer than the regular networks," said Bob Pitchnick, a business owner in Croydon, Pa. "Football and basketball look the worst." Mike Kopriva, a computer engineer in San Jose, Calif., said: "On DirecTV's Sunday Ticket football games, the ball often disappears along with the fans into a mass of pixels. Soft and muddy is the best way to describe it."
DirecTV, the larger of the two companies, says that its picture quality will soon improve. "To accommodate the many local channels we now offer, we've pushed the limit on bandwidth," said the company's executive vice president, David Baylor. With the imminent launching of a new satellite designed to carry local channels, more space will become available. "You'll see an improvement in DirecTV picture quality by the beginning of 2002," Mr. Baylor said.
Both DirecTV and Dish Network are loath to discuss how they compress their signals or how much compression they apply. Still, the principles governing digital video compression are widely known.
An uncompressed studio-quality standard-definition video image requires 216 million bits per second to be transmitted. By compressing the signal, its bit rate can be reduced from 216 to approximately 6 megabits, allowing five channels to broadcast from one satellite transponder, which can handle 30 megabits of data per second. Each satellite has multiple transponders.
Several compression techniques are used. So-called "loss less" compression assumes that if one of the screen's approximately 300,000 pixels (or picture elements) is a particular color, the odds are the neighboring one will be the same color (if every pixel were a different color, only electronic snow would appear on the screen). So rather than transmitting data that fully describes the color of the neighboring pixel, only instructions designating the difference in color between the two is sent. An instruction describing the difference requires fewer bits than data describing the actual color. This technique recovers the least amount of bandwidth.
A more useful kind of compression, called lossy, eliminates certain picture information that the eye cannot see. For example, edges between black and white surfaces look sharper to the brain than those between green and purple, so the green and purple edges can be safely blurred by eliminating some of the data. Blur the edges too much, or sit too close to the screen, and the lack of detail becomes obvious.
When an object moves in space, data indicating the change in position must be transmitted. Pictures with static imagery, like news shows, require less bandwidth because the television receiver can be instructed to create much of the same image — the background behind the news reader, for example — over a succession of frames. Once the scene changes, many more bits are required to describe the new image. If those bits aren't available because the picture has been granted only a small chunk of bandwidth, the first few frames of the transition will look blotchy.
The worst thing to compress is sports and, particularly, soccer, said Andy Lippman, a senior research scientist at the Massachusetts Institute of Technology and a developer of the MPEG 2 compression standard, which is used by direct-broadcast satellite services. "Men are running quickly and suddenly across a field, and constantly changing directions," requiring more bandwidth to describe those changes.
Channels within one transponder can also "borrow" bits from another channel that does not need them. Called "statistical multiplexing," a sports program can use bandwidth not used by a static talking-heads show on that transponder.
If all the channels within a transponder have a lot of high-bandwidth action, there are no bits to borrow. "If you're running out of bits, the best thing to do is to soften the image before compression, so that fewer bits are needed," Dr. Lippman said. "That's what often happens to sports programming." This results in customer complaints — at least until the beginning of next year.
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