On MPEG-4, 'All Digital' And Advanced Video Compression

One of the many moving pieces in cable's consideration of an "all-digital" network is the "advanced video codec," which usually travels with a handful of loosely synonymous terms. You've heard them: MPEG-4, MPEG-4 Part 10, JVT, H.264, AVC. (There are others, but, five is plenty muddy enough for now.)

Let's start with the "advanced video codec."

An advanced video codec is a squisher of digitized video. "Advanced" means it's better than what we use today, which is MPEG-2. ("Better" because video squishes further, which means more can be sent, and stored, than now.)

"Video," in this case, means not audio, nor transport.

Frees spectrum

A "codec" is an engineering coupling of the words "coder and "decoder." The duet within it implies the need to squeeze video down on one end, and unsqueeze it at the other end. Sort of like orange juice concentrate, but without the water.

Advanced video codecs fit into cable's "all-digital" conversations as an option (among several) for freeing up more bandwidth once a channel is digitized.

The logic goes like this: A piece of video encoded with the new stuff, at a rate of 1 Megabit per second (and dropping), looks essentially the same as a piece of video encoded with the existing MPEG-2 stuff, at 3.5 Mbps.

Thus, a thinner stream works as well as a thicker stream to do the same thing.

Why so many blasted names for the new compressor?

Without going into the life cycle of a technical standard (which can outlive even small dogs), it goes like this: Two different standards-setting groups (the Moving Picture Experts Group and the International Telecommunications Union, or "ITU") were both working on an advanced video codec.

Naturally, both codecs went by different names ("MPEG-4" and "H.264," respectively.)

Ultimately, MPEG and the ITU merged their efforts, and coined the combined group "JVT," for "Joint Video Team."

'Part 10'

So, as these things go, one group called the work of the merged effort one thing, and the other group called it something else. Specifically, the MPEG people called it "MPEG-4 Part 10," because they'd had nine new parts before the merged video codec came about, which made it the tenth. The ITU continued calling the codec "H.264." (Most people pronounce it "H dot 264.")

It doesn't take much facial contortion to recognize both terms — "MPEG-2 Part 10" and "H.264" — as wondrously geeky.

So the JVT dubbed the video codec "AVC," for "Advanced Video Codec."

On a structural level, AVC isn't much different from MPEG-2, experts submit. It's all about removing the parts that are the same, from one frame of digitized video to the next. Turns out there are lots of things that are the same, frame to frame, but we don't notice them, because our eyes like movement better.

To compress by removing repetition means you need solid, reliable reference points. In MPEG-2 and in AVC there are two — sometimes three — such points: I-frames ("initialization") and P-frames ("predictive"), always. B-frames (for bidirectional), sometimes.

Initialization frames are regularly occurring reference points that initiate a compression sequence.

P-frames, true to their name, predict what the next frame will be, based on the sameness in the frame that follows the I-frame. B-frames look forward and backwards, to anticipate and build a forthcoming frame.

Squishing it

That all remains true in JVT/AVC. What's different is the way the squishing is done. Translating the details would assuredly elicit that "look at the time!" response — it involves venturing into the world of entropy coding and discrete cosine transform coefficients.

Suffice to say JVT/AVC brings further efficiencies to motion compensation, artifact filtering and about a dozen other compression-related processes, all of which lunge past the space limitations of this column.

As reality goes, the advanced codec isn't yet ready for primetime. Licensing remains a nightmare, with nearly two dozen inventor companies vying for a cut.

Chip sets are in the works, but largely not yet encased in plastic gizmos.

Big software companies aren't waiting. Microsoft Corp. and Real Networks Inc. are brewing their own implementations. This is what people are talking about when "Windows Media 9" enters the conversation. Keep two eyes on it.

New box needed

For cable, upshifting to an advanced video codec like MPEG-4 AVC does (sigh) mean a new set-top. Probably one that decodes both MPEG-2 and MPEG-4 video, initially.

The point of the advanced codec is the gains in bandwidth, especially as HDTV channels continue to heave their big, beautiful selves through the plant to digital TVs. But know that the advanced codec is one part of a much larger scenario, which will undoubtedly unfold in different-but-similar MSO chapters over the next few seasons.

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