How do you pack three pounds of programming into a two-pound bag — without bruising the goods?
Comcast Media Center has developed a way to fit three high-definition TV signals into one 6-Megahertz carrier, which is ordinarily enough space to fit just two HD channels without sacrificing picture quality.
To preserve quality while boosting quantity, the cable operator’s Denver-based digital-media services unit is using a “second-pass” MPEG-2 encoding system from startup Imagine Communications, according to people familiar with the project. The technology uses statistical multiplexing to stack together three signals at variable bit rates into one 6-MHz quadrature amplitude modulation (QAM) channel.
CMC delivers six HD channels — A&E HD, HGTV HD, Food Network HD, National Geographic Channel HD, Starz HD and Universal HD — via two transponders on SES Americom’s AMC-18 satellite, as part of its HITS Quantum digital video service for smaller operators.
On the receiving end, HITS customers can take those two groups of HD channels and distribute them in two QAMs without additional modifications.
The process is not as simple as just multiplexing in a third HD, according to a research paper by CMC director of advanced engineering Ren Finley. The paper, issued in June at the Society of Cable Telecommunications Engineers’ Cable-Tec Expo 2007, said “significant engineering is required” to ensure video quality remains as good as the traditional configuration of two HD signals per QAM.
One of the key factors in achieving the “3 to 1” ratio, Finley’s paper said, is selecting the appropriate mix of channels based on overall picture complexity. The optimum configuration is to have one “difficult” channel with some high-action video, at a typical bit rate of 14 megabits per second; one “easy” channel with very little high action at 10 Mbps; and one “normal” channel in between them at 11 Mbps. A single QAM provides about 38 Mbps of bandwidth.
CMC declined to comment on whether it was working with Imagine.
Marc Tayer, Imagine’s senior vice president of marketing and business development, said the company is testing its system with three “major operators” but he wouldn’t name them.
Imagine claims its video-quality algorithms are able to either reduce bit-rate of HD video without reducing quality or increase bit-rate while boosting quality. Essentially, the system can provide more signals at the same quality or more quality at the same bandwidth, according to Tayer. “This lets you have your cake and eat it too,” he said.