The Language of Video Encoding
by Leslie Ellis // October 29 2007
By now you’ve probably heard the one about the big cable operator with the big plan to boost digital capacity by 30% next year.
That’s good for the HDTV channel explosion, obviously. Happiness accelerator: No ditching of the 14 million or so digital boxes already working in people’s homes.
The big cable operator is Comcast. The capacity gain involves a compression improvement, disclosed partially at recent investor’s conference.
The compression technique launches widely in January, so details will assuredly follow soon. What’s likely to be involved is a largely overlooked member of the bandwidth preservation family: The digital video encoder.
For that reason, the subject of this week’s translation is a brush-up on the language of video encoding. Chances are high that this topic will nudge its way into your conversational life very soon, especially if you follow cable’s shelf space situation with any fervor.
Knowing going in that this batch of improvements necessarily centers partly on the existing type of video compression, known as MPEG-2. (The “MPEG” stands for “Moving Pictures Experts Group.”) It also probably involves some new video processing techniques that are heavily focused on measurable video quality.
Know also that there’s really no other way to apply newer types of compression, like MPEG-4, to a bandwidth problem, without installing set-top boxes that know what to do with an incoming MPEG-4 stream.
Three terms tend to pop up repeatedly when talking about how to squeeze — encode — a digital video signal: “Dual pass,” “open loop vs. closed loop,” and “lossy vs. lossless.”
Let’s start with dual pass. Not surprisingly, it’s a way of compressing video in two swipes. Swipe one is the encoder’s best shot at using the components of the MPEG-2 standard to squish down a video.
Swipe two is almost always the secret sauce of the encoder manufacturer. It’s a full second look at the compressed stream, to find ways to squeeze the bit rate down even more.
Right now, with HDTV mostly a volume game, bit rate reduction often leads any compression discussion. The next HDTV chapter, though, will be about picture quality. The ideal encoder accomplishes a good squish without noticeably degrading the quality of the picture.
Fact: Most professional-grade encoders use “dual pass” techniques. The real action is in what they do within that second pass.
Soon after “dual pass,” you’ll run into “closed loop” and “open loop.” The loop is the linkage (or not) between a video encoder and a statistical multiplexer.
Refresher: If the work of a video encoder is to squish one digital video stream, the work of the statmux is organizing lots of those squished streams for the ride toward homes. (Lingo translation: “Statmux” is tech-talk shorthand for “statistical multiplexer.” “Mux” is also acceptable. Both remove more than five syllables.)
A good statistical multiplexer is like my friend Diana, who can take one long look at the overhead bin on a small airplane, and at the pile of stuff needing stowage — then magically fit the hat box, guitar, duffels, crutches, rolling bags, ficus tree, parkas and backpacks into the bin.
The loop that’s being closed in a “closed loop” scenario is the one that’s created between the encoder and the statmux. If it’s closed, those two machines are working together to organize bits for the ride. If it’s open, they work independently. There are pros and cons to both.
At some point, you may hear mention of “lossy.” (If you do, you’re hanging with the advanced class.) What’s lost in “lossy” compression is an exact pointer to the original material. Lossy algorithms tend to make files smaller.
Lossless compression is a more mathematical bit tossing, so that the underlying content can always be perfectly reconstructed. It’s sort of like when you use a “zip” program to shrink an important file you need to send to Harry, because otherwise Harry’s email server keeps kicking it back to you with a note that it’s too big.
Encoder-speak is coming back into the high-tech vogue because necessity is the mother of invention: HDTV signals are huge. Bandwidth is precious. The installed base of digital boxes using MPEG-2 compression dwarfs the number of boxes that can “see” MPEG-4 signals.
Something had to be done.
This column originally appeared in the Technology section of Multichannel News.