The Bandwidth Trifecta for Program Networks
by Leslie Ellis // November 12 2007
If your company moves high definition TV to its distributors by way of satellite, there’s yet another bandwidth saving superstar to help you get 10 more pounds of HD into that five-pound bag.
What it is: A new way of saving room on those spendy satellite transponders, which can cost a content owner upwards of $125,000/month.
How big of a bandwidth savings? About 30%, say tech people at program networks and aggregators. One content-side engineer explained it this way: “Our transponders right now can carry 47 Mbps of video, and the S2s will carry somewhere between 65 and 72 Mbps.”
That extra 18 to 25 Mbps makes room for another two or three HDTV streams per transponder.
In other words, the satellite transponder that currently carries two or three HD streams (using existing MPEG-2 compression and existing modulation/coding techniques) could now carry five or six (using advanced compression and new modulation/coding techniques).
“The “S2” in that mention, by the way, is verbal shorthand for “DVB-S2,” which is the bandwidth saving superstar of this week’s translation.
“DVB-S2″is the name of a technical standard. It breaks down like this: “DVB” stands for “Digital Video Broadcasting,” and is the major standards-setting body in Europe. “S” is for satellite. The “2” is the version number of the standard.
(Version one of the DVB-S standard, as a point of reference, blueprinted more than 100 million digital satellite receivers, worldwide. That’s since 1993.)
The underlying context of DVB-S (versions 1 and 2) is all about how to anticipate (and thus prevent) the errors that inevitably occur when blasting bits up and down from space.
From here, the DVB-S2 detail can corkscrew way into the tech-funk. (This is generally true of anything involving satellite technologies. If you find cable tech-talk daunting, spend an afternoon with a satellite engineer.)
Here’s an example from an email last week: “The LDPC codes replace the Viterbi Forward Error Correction (FEC) of DVB-S, whilst the Reed-Solomon code is replaced with a different BCH (Bose-Chaudhuri-Hocquenghem).”
How DVB-S2 Works
How it works requires a brief refresher on how satellite transmission works. For starters, the modulation has to be sturdier, because sending stuff up into space is much harsher than sending stuff over a wire. Satellites traditionally use a form of modulation called “QPSK,” for “quadrature phase shift key.”
Don’t get hung up on the language. To put it in context, the QPSK modulation used for space transmissions is also used by cable systems to transmit in the upstream (home to headend) direction. The upstream path of cable is theoretically as hostile (in different ways) as space.
When you’re sending stuff into a hostile environment, even if you’ve taken extra precautions when imprinting it onto the carrier (modulation), you need to carve out room for reconstructive surgery — just in case something gets really mutilated during the ride.
The safety mechanisms are known as “forward error correction,” or FEC. Techniques vary. Understanding them involves heavy math. Short version: Send extra bits that know where they’re supposed to go, if they get called upon on the ground to stand in for missing data.
DVB-S2 is an improvement in forward error correction, which harnesses improvements in modulation. It’s already in use by some content owners and aggregators. It’s not something that can be easily adopted by DirecTV and EchoStar, for legacy reasons: On the receive end, it requires gear that can demodulate and de-code in the new way.
Conversationally, DVB-S2 tends to move in step with MPEG-4 compression. The reasoning: You’ll need new gear to do MPEG-4, and you’ll need new gear to do DVB-S2. Might as well make both changes at once.
And so ends this unintended mini-series on HDTV and bandwidth. To sum it all up: People who pay to move big HD streams over satellite will soon get a trifecta of bandwidth savers.
One is advanced compression, like MPEG-4. Two is better modulation, to move stuff up and down from space. Three is better error correction, to reconstruct streams on the ground.
This column originally ran in the Technology section of Multichannel News.