Comcast Media Center Serves up Digital Video

Signal to Noise

By Mike Robuck

Thanks to a service provided by the Comcast Media Center in Denver, cable operators don't have to do all of the heavy lifting when it comes to encoding and compressing digital signals.

Similar to its Headend in The Sky (HITS) service, CMC's digital video service became available to other cable operators this past summer. By using the CMC, Comcast is able make a digital overlay platform for both itself and other MSOs. The overlay platform takes analog channels off a satellite, encodes and grooms them, and then sends them - via SES Americom's AMC-4 satellite - to cable networks for digital simulcasting.

CMC downloads the analog signals at its location outside of Denver, as well as at another nearby satellite farm. CMC uses Tandberg and Harmonic for its encoding and uses closed loop encoding and statistical multiplexing to provide enough space for the local ad insertions.

"We arrange for all of the services to be delivered to our facility. Then we re-encode them into a compression format that is suitable so we can send them directly to the consumer," said CMC COO Gary Traver. "CMC's service is structured to be compatible with 256-QAM. The multiplexes distributed via the CMC service fit within one 256-QAM slice using ‘best of breed' encoders. The last thing we've done is making sure we're inserting the triggers that allow digital channels to have local advertising on them."

By using the CMC service, Traver said, larger cable operators are able to count on the digital video quality from CMC while smaller operators don't have to go through expensive headend upgrades in order to offer a digital tier.

"It eliminates a lot of complexities at the headend, where there is already a lot going on to support other applications," Traver said. "Full digital service represents a plant upgrade for the system, which is very important for independent cable operators who are looking to remain competitive in their markets by having sufficient bandwidth for launching other products and services."

Digital crystal ball

Traver said CMC is looking at a variety of approaches, such as advanced codecs, for future digital video delivery. He also thinks there are still some improvements to be made in compression technologies.

"The state of compression today is still too much of an art form and not enough of a science," he said. "It can be very easy to underestimate how much work is required. There isn't a plug and play solution for getting it right."

Re-compressing the signal is better than rate shaping, according to Traver. He said there could be a compounding effect when rate shaping is used on an encoded signal.

"We can achieve the same objective of reducing bandwidth without that risk through recompression," he said. "A signal at lower bandwidth looks better than one rate shaped at a higher rate."

MPEG-4 and switched digital

While the benefits of MPEG-4 for digital video are still somewhere on the cable horizon, Cox's Steve Watkins, director of digital video technology, said there is an indirect effect from MPEG-4 today.

"MPEG-4, which has a lot of interest in the field right now, is driving a lot of improvements in the MPEG-2 arena," Watkins said. "As the equipment manufacturers look toward the future of MPEG-4 over IP, and they're basically working with MPEG-4 encoding, they're able to take some of those techniques and apply them back to MPEG-2. We believe that will get us a little bit of additional savings in the MPEG-2 stream."

Watkins said Cox is also interested in switched digital for future digital deployments, especially after doing a field trial in one of its systems in 2004.

"We ran the trial and had very good results," Watkins said. "It (switched digital) has a lot of potential, but we have to do a very careful job of designing the network, choosing the appropriate channels and deploying those channels. We need to pick the right channels to be switched while allowing enough overhead for the customers to get all of the channels they want to watch. We also need the system to be robust enough, from an operational and technical standpoint, that it can respond to those requests in a timely manner."