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H.262: MPEG-2, parts 2, 3, and 7

If you have ever played a DVD in your home (and the talking-head oracle on my chamber desk assures me that many of you have), then you’re familiar with MPEG-2, even if you’re not aware of it yet. That’s the codec’s claim to fame, really: it’s the video and audio codec that brought the home theater ethic to the population on a massive scale. LaserDisc, DVD’s clear predecessor from an aficionado standpoint, had the technology as far as multitrack audio, widescreen, and special features (not to mention nonlinear workflow as far as navigating the disc), but DVD contained the digital video advancement (LaserDisc video was still analog) necessary to shrink the physical size of the thing and make it palatable to the casual viewer.

H.262 was developed as MPEG-2 Part 2, to occupy a stronger and more versatile spot in the industry. It contained the ability to display video in interlaced and progressive scan, (LaserDisc couldn’t do progressive, and MPEG-1 couldn’t display interlaced) and it included a new type of frame. To this point, digital video encoding had made use of two types of frames. The first is the I-frame (intra), also called the reference frame, which consists of an entire, fully-rendered picture. A key frame in animation or a transition is something similar. It is the frame type that provides the most data, and consequentially is the most space-intensive. The second type of frame is the P-frame (predictive), also called the delta frame, which slots into the next frame slot and increases efficiency by containing only the elements of the previous I-frame that change. H.262 introduced the B-frame, or bi-directionally-predictive frame. This frame increased efficiency further, by containing only the changed elements from the frames preceding and following it. The more P- and B-frames a piece of digital video contained, the more it could pack into a given amount of space - and the more immutable the video was, the less able the user was to manipulate or edit the video in any way.

Part 2 also incorporated some HD-related elements at an embryonic level. Of the four “profiles” available for MPEG-2 encoding, two of them allowed for scalability; we’ll talk more about scalability when we theoretically arrive at the point of this column. The other HD-related element involved two coding “levels” within each profile, which provided HDTV-compliant resolution to the encode. Neither of these levels was used in practice, but the capability was there.

MPEG-2 Part 3 encompasses audio coding definitions; it allows for multichannel audio up to a 5.1 configuration, and it's the spec behind the limited advancement of audio DVDs primarily due to this asset. It also defines additional parameters and bitrates for MPEG-1 Audio Layer III, more commonly known as MP3. It's a substantive part of the MPEG-2 “build,” but a relatively obscure one. Less so is Part 7, also known as the Advanced Audio Codec, or AAC. Part 7 was designed as a replacement for MP3, being more efficient as far as compression goes while also allowing for many more discrete channels of audio and data.

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