OTHER TECH

Pros and Cons TV engineers acknowledge that satellite and cable services don't always transmit their signals at 19 Mbps, but many claim that's irrelevant. To preserve channel capacity, these services use a technique called statistical multiplexing (see below) that borrows bits from one channel on a transponder that's showing fairly slow-moving or static images and transfers them to another that needs a higher data rate, like a channel showing a basketball game.

This works because higher data rates are only needed for fast-moving action. Even on broadcast TV, "the bit rate is dancing all over the place," says John Turner, owner of Turner Engineering and a former National Association of Broadcasters Engineer of the Year. He says that an HD signal doesn't need to maintain a high bit rate all the time to look good. If the image is static, even 1 Mbps will produce a perfectly fine high-def picture.

Satellite and cable aren't the only places where you'll find lower bit rates. Although some broadcast networks demand that affiliates stick to 19 Mbps peak bandwidth when sending their HD signal to household antennas, not all stations do. A TV station that's splitting the high-def bandwidth to send out three or more sub-channels at standard-def in addition to the main, HD one just doesn't have the capacity to keep each of them at a high bit rate.

When Peter Putman, president of the broadcast-consultancy firm ROAM Consulting, checked the bit rates of WSTN in Syracuse, New York, he found that the three standard-def sub-channels were each using around 3 Mbps, while the main high-def feed was using just 8 to 13 Mbps. "When the camera panned across a crowd during a football game, small objects would turn into blocks," he said. His local Comcast cable system, on the other hand, eschews such bit-rate finagling. Each Comcast HD channel he's checked sends its signal at 17 to 18 Mbps.