Video Shoehorns

For cable operators, fitting more high-definition programming into networks already crunched for bandwidth is a front-burner priority for 2008.

Now, many are actively considering adopting a technique that perhaps a year ago they wouldn’t have: stuffing three HD channels into a single 6-Megahertz slot.

It’s sort of like sitting on a suitcase to get the latches closed. The approach takes advantage of every last bit of spectrum available in a typical quadrature amplitude modulation channel.

Cable’s convention has been to deliver just two MPEG-2 HD signals per 256-QAM, each of which provides 38.8 Megabits per second of bandwidth. That’s plenty of breathing room for high-definition video that peaks at 19 Mbps, enough even for demanding content like a fast-moving hockey game.

Squeezing a third HD signal into a QAM requires more careful engineering.

To achieve the “three-in-one” delivery, video must be re-encoded or otherwise modified to a lower bit rate. Then, the three HD channels are statistically multiplexed together, to gain efficiencies from the probability that not all three will peak in bandwidth (that is, have a sequence of video frames that are substantially changing) at the same time.

Of course, digital video compression has always involved a tradeoff between quality and quantity. Anybody can deliver three HD channels in one QAM — the problem is, they might look horrible.

“We believe it’s not at all prevalent today,” Scientific Atlanta vice president of Americas for digital media networks Brian Morris said. “I don’t see people transmitting three HDs in a QAM today, because of some quality limitations.”

The difference now is that newer MPEG-2 encoding algorithms have improved to the point where HD video can be compressed to the necessary bit rates without damaging quality.

“Last year, people thought this was impossible,” said Tandberg Television vice president of technology Matthew Goldman. “Now we’ve moved to, 'It’s possible, depending on the content.’ ”

One of the biggest tools in operators’ HD-expansion kits is switched digital video (see “High-Def Dash,” Jan. 14).

Switched video saves bandwidth by delivering television channels only when a subscriber requests them, assuming that fewer than half the channels are being watched at once.

In some cases, cable providers are even switching high-definition channels.

As a complement to switched digital video, the three-in-one technique promises to deliver 50% more broadcast HD channels in the QAMs cable system operators have already dedicated to high-definition programming.

Several cable companies are currently testing switched video, using next-generation encoders and statmuxes (statistical multiplexers), according to Morris. “We’re now in lab deployments with operators very heavily,” he said.

There’s no cookie-cutter approach to statmuxing three HD channels per QAM.

“It’s not something you can blindly throw three channels into the blender and always come out with acceptable quality,” Motorola senior marketing director Marty Stein said.

“It’s extremely content-dependent,” he continued. “You have to be very careful of the content you pick. If you have high-action or sports, you may be disappointed.”

Most HD channels, though, do not need the conventional 19 Mbps, said Keith Rothschild, Harmonic senior manager of cable solutions and strategy.

“The vast majority of the HD content coming out isn’t exactly your highest-complexity content,” he said. “We’re seeing more and more moderate- and low-complexity images.”

In other words, the majority of HD channels are suitable for compressing in the 12- to 13-Mbps range.

According to Rothschild, the largest factor that determines how successful three-in-one HD will work is the quality of the source video. “The biggest challenge we see in HD content is not based off the content itself, but how the content is prepared and delivered to the operator,” he said.

In the past year or so, the quality of HD signals delivered by programmers has greatly improved, said RGB vice president of product marketing Ramin Farassat.

“When we look at HD quality coming off the satellite, that starting point is amazingly good,” he said, adding, “You can’t improve on what’s already been encoded. It’s garbage in, garbage out.”

Still, three HD channels “is barely enough to really take advantage of statmuxing,” Motorola product marketing manager Neil Brydon said. By putting three high-definition feeds in a QAM, he said, there’s an increased risk of a momentary spike in bandwidth that would result in signal degradation.

Ultimately, MPEG-4 Advanced Video Coding (AVC) will provide a far bigger boost because out of the gate it requires roughly half the bandwidth of MPEG-2 to provide comparable quality. That also means a cable operator can put, say, six channels in a QAM and thereby get better statmuxing averages.

“MPEG-4 is the real way to get your bandwidth back,” Motorola’s Stein said. Plus, he noted, “MPEG-2 is at the end of the curve. You’re wringing out the last 2% of efficiencies. On MPEG-4 we’re right at the beginning.”

The catch, Stein noted, is that cable has millions of MPEG-2 set-tops already in use, so the transition to the newer format will happen gradually.

Others argue that MPEG-2 does, in fact, have more room for improvement.

Imagine Communications, a San Diego startup that is pushing an encoding and statmuxing system for delivering three broadcast HDs in a QAM, says it can further optimize MPEG-2 video by taking out additional bits while ensuring quality doesn’t suffer.

Imagine senior vice president of marketing and business development Marc Tayer claimed that when the company has shown cable operators’ best video engineers an HD signal processed by its system at a 25% lower bit rate than the source, those so-called “golden eyes” perceived no difference.

“Video compression by its very nature has to do with tricking the human visual system,” he said.

In some cases, programmers have anticipated the needs of cable distributors and are delivering their HD signals at levels best suited for the content.

Steve Pontillo, Rainbow Media Holdings’ senior vice president of broadcasting, information systems and technology, said the company delivers MPEG-2 HD feeds of four networks — AMC, We TV, IFC and Fuse — in a single QAM carrier.

“Our goal is to compress the video once, in a native HD format, and have the cable operators take our signal and put it right on to the QAM channel out to the subscriber,” Pontillo said.

The mix works because AMC, We and IFC generally do not have high-motion video, whereas the music-video-oriented Fuse is “a bit-demanding service,” he said: “The other three complement Fuse.”

Meanwhile, Rainbow, a subsidiary of Cablevision Systems, delivers Voom HD Networks’ 15 channels using a two-to-one ratio.

That’s because Voom wants to ensure the original feeds are higher quality, “knowing [distributors] are going to decode it and re-encode it.”

In the end, the level of compression considered OK comes down to individual preference, according to Tandberg’s Goldman.

“I’ll put two operators in a room and show them the same compressed video,” he said. “One will say, 'Yes, that’s good enough’ — and the other will say, 'No, that’s not acceptable.’ ”