A squisher of digitized video, the advanced video codec variously shows up under the following names: MPEG-4 Part 10, JVT, H.264, MPEG-AVC or simply AVC. It was jointly created by ISO-MPEG and ITU-VCEG. A competing standard VC-1 from Microsoft is currently under standardization process in SMPTE.
All are newer ways to squeeze video than is possible within the predominantly deployed version, known globally as “MPEG-2” (where “MPEG” stands for Moving Pictures Experts Group). What’s “advanced” about advanced video codecs, perhaps obviously, is their compression rate: They squeeze video further than MPEG-2. That means more digital video content can be sent, and stored (think DVRs here), than now.
A “codec” is an engineering coupling of the words “coder” and “decoder.” In the case of advanced video codecs, a piece of video encoded with MPEG-4 and its counterparts, at a rate of 1 Mbps (and dropping), looks essentially the same as a piece of video encoded with the existing MPEG-2 stuff, at 3.75 Mbps.
In essence, advanced codecs produce thinner streams that work as well as thicker streams, to do the same thing.
About All the Names
Why so many names for the new compressors? Without going into the lifecycle of a technical standard (which can outlive even healthy dogs), it goes like this: Two different standards-setting groups (MPEG and the International Telecommunications Union, or ITU) were both working on an advanced video codec. Naturally, both went by different names — MPEG-4 and H.264, respectively.
The two groups decided to combine their efforts, calling themselves “JVT,” for Joint Video Team. But old names die harder than old habits. The MPEG people started calling the work of the combined group “MPEG-4 Part 10,” because they’d had nine parts before the merged codec came along. The ITU people kept on calling it H.264. (Most people pronounce it “H dot 264.”)
That was too confusing. Ultimately, the JVT opted to call its codec “AVC,” for “Advanced Video Codec.”
What’s Advanced About It
At a structural level, AVC isn’t much different from MPEG-2, experts submit. It’s still all about removing the parts that are the same, from one frame of digitized video to the next. It turns out that lots of things are the same, one frame to the next. But we don’t notice them, because human eyes like movement better.
To compress by removing repetition requires solid reference points. In MPEG-2, there are two: I (“intra” or “initialization”) frames and P (“predictive”) frames. A third, the “B (bi-directional) frame,” can be predicted using the prior two reference frames.
In AVC, two or more frames (including B-frames) can be used as reference frames. AVC also introduced a new tool known as “intra-prediction,” which MPEG-2 video coding does not have.
Initialization frames, or I-frames, are regularly occurring reference points that initiate a compression sequence. P-frames, true to their name, predict the next frame, based on the sameness of the frame following the I-frame. B-frames look forward and backward, to anticipate and build a forthcoming frame.
That methodology remains the same in JVT/AVC. What’s different is how the squeezing is accomplished. Some methods use a technique called “entropy coding.” Others use “discrete cosine transform coefficients.” Suffice it to say that JVT/AVC brings further efficiencies to motion compensation, artifact filtering, and about a dozen other compression-related processes.
Tactically, up-shifting to an advanced video codec, which is not backward compatible with MPEG-2 video coding standard, almost certainly means new equipment in the home — like new set-top boxes, or digital TVs. Early units will likely contain both MPEG-2 and advanced video codecs.