The mid-gear swap
ponder as it makes plans to insert advanced (and more efficient) codecs
Advanced codecs such as MPEG-4 and Windows Media 9 promise a leaner bandwidth profile and potentially better picture quality, but introducing them onto MPEG-2-dominated networks isn't going to be easy or quick.
With a massive base of legacy boxes deployed, no one sees advanced codecs usurping MPEG-2's dominance of digital television delivery anytime soon. But many see MPEG-4 and Windows Media 9 gaining footholds in advanced services such as video-on-demand, and there is some consensus building as to how to make them cohabitate with MPEG-2 in a cable network.Transcode or simulcast?
The strategies split into two basic migration paths. One possibility is a transcoding model, where content is shipped around in a single format in the network core and then transcoded at the edge. For example, video could be stored and delivered to an edge cable modem termination system (CMTS) unit in MPEG-2, where it could be delivered to a legacy box as is or transcoded on the spot into a smaller MPEG-4 stream for delivery over coax to a newer, more advanced box.
Given the fact MPEG-4 streams are lighter, a transcoding strategy could be useful for cable operators already simulcasting analog and digital signals, according to Lou Mastrocola, RGB Networks Inc.'s vice president of business development.
"Digital simulcast in its initial form is actually a bandwidth strain on the network as opposed to relieving bandwidth," he says. "If the system goes all digital, you will recapture the analog bandwidth, but if that were to happen simultaneously with the move to an advanced codec, the ability to transcode from one format to the other would be very beneficial."
But transcoding from MPEG-2 to MPEG-4 at the edge does have its challenges, particularly if attempted in real time.
"The interesting part is they are different codecs and temporal models, so it can be awkward to take parameters that come out of one encoder destined for one type of codec and translate them into another," says Peter Monta, RGB's co-founder and chief scientist. "It seems to us most of the penalties are in rate distortion performance. You can do it, but you give up some quality or some bit rate."
So far, there hasn't been a lot of movement toward transcoding, according to David Price, vice president of business development for Harmonic Inc.'s convergent systems division.
"There are people who are saying 'we want to do transcoding,' and anyone can do transcoding by just putting a decoder next to an encoder. That's a coarse way of doing it," he notes. "As for any large, significant niche of interest in integrating that solution for transcoding that way around, not yet."
That is the situation for Comcast Corp., which sees a future for advanced codecs in on-demand services. While the proposed Next-Generation Network Architecture (NGNA) does call for a headend with the ability to transcode video, converting content there–as opposed to converting it at a video storage hub farther out in the network–doesn't offer significant benefit, according to MSO Executive Vice President and Chief Technology Officer David Fellows.
"To be honest, it's not one that we are modeling and planning on, and I've got numbers on what it will cost and what I'll save on," he says.Simulcast is best?
The more likely strategy–particularly given the likelihood advanced codecs will pop up first in on-demand delivery–is simulcasting. That is what Comcast is eyeing for its first advanced codec rollout.
"In the video-on-demand server, you would store an advanced codec form of a movie or a standard definition MPEG-2 version of the movie and probably a high-definition version as well," Fellows explains. "Then the on-demand system can tell, set-top by set-top, whether that set-top is capable of receiving an MPEG-4 signal or another advanced codec signal, and then it would serve up that stream."
Following that model will put a storage load on video servers, but Fellows says that is more than offset by the bandwidth savings.
"The $80 billion you plowed into the ground is spectrum, and the servers are going down in cost over time," he points out. "So it is worth spending extra money on server capacity in order to preserve or free up or enable new service opportunities across the spectrum on your HFC plant."
Tony Wasilewski, chief scientist for subscriber networks at Scientific-Atlanta Inc., is seeing that duplicated content attitude gaining ground among other MSOs, as well.
"We're also hearing from inside of Time Warner that they are looking forward–they are much more amenable to duplicating content than I ever would have thought in the past," Wasilewski says. Dropping server costs may well be spurring that change, and, "I think things are happening to make that more of an easy decision to make."
In a VOD setting, simulcasting also gets around the real-time transcoding issues. "You have the complete freedom to encode from scratch each time," Monta says.
But simulcasting does face difficulties if applied to standard broadcast services, because duplicating channels in analog, digital MPEG-2 and a digital advanced codec would torpedo efforts to save bandwidth.
Given the mass of legacy boxes at work in subscriber homes "you are not gaining anything by introducing MPEG-4 in a broadcast fashion," Fellows adds. "With on-demand, you actually do save."
Others see that with the analog-to-digital conversion, enough bandwidth could be freed up to start offering MPEG-2 and advanced codec simulcasts for broadcast TV.
"You'd probably expect a trend where the higher-value services–the pay channels and certainly the HD versions of those movie channels, let's say–would be the ones that would likely get bandwidth allocated for them," Wasileweski says. "They would still stay in the MPEG-2 format but they would early [on] get the advanced codecs, as well, so the newer devices could use them."
And there is a strategy proposed to get around the simulcast bandwidth problem–enter switched broadcast, a technology BigBand Networks Inc. has championed. In switched broadcast, channels are delivered only if viewers tune to them, rather than funneling all channels down to the subscriber and letting the box do the tuning. In that way, operators can significantly slash the amount of bandwidth needed in the bandwidth constrained, coax last mile.
"So any program that isn't being watched doesn't get pushed down to that service area at all, or any program that is being watched in one format and not another format only gets pushed down in the format in which it is consumed," says Seth Kenvin, BigBand's vice president of corporate development.The first mile
But even if the bandwidth problem could be solved, advanced codecs for broadcast still face a content hurdle. While it might save on transponder space for satellite operators and program distributors to use an advanced codec to beam signals to and from a satellite link, programmers may see little benefit in converting their content to an advanced codec at the origination point.
"There is a whole ecosystem of products that need to be developed around MPEG-4, but it all starts with the Catch 22–until somebody puts it into a set-top, nobody's going to serve it up in the headend," Fellows notes. "So yes, likely at some point in order to save on satellite transponders you could pitch MPEG-4 out of the uplink, but there is no real buzz about that. HBO hasn't come and said, 'Gosh, get ready to replace all of your satellite receivers with satellite receiver transcoders'."
Despite the differing strategies and significant hurdles, Comcast is still on track in its plans to debut advanced codecs in 2006, starting with on-demand services.
"I'm optimistic about the end of next year, but I would bet my job on the end of 2006," Fellows says.