Post
by Qyot27 » Thu Mar 18, 2010 10:21 pm
More like - just as it was in MPEG-1 and MPEG-2, there is no limit on the number of different types of audio/video/etc. that can be ratified under the MPEG-4 collection of standards. MPEG-4 is simply the first I know of that has two *video* standards within it. But the standards for MPEG-1 and MPEG-2 (and yes, even MPEG-4) have several different types of *audio* contained in them.
MPEG-1 contained:
MPEG-1 Layer 1 (MP1, MPGA, etc.), MPEG-1 Layer 2 (MP2, Musicam), MPEG-1 Layer 3 (MP3)
All three of those are distinct, they are not related to one another. Musicam and MP3 in particular.
MPEG-2 contained:
MPEG-2 Layer 3 (which is more an extension to MP3), MPEG-2 Advanced Audio Coding (MPEG-2 AAC)
MPEG-4 contains:
MPEG-4 Advanced Audio Coding (MPEG-4 AAC - which differs from MPEG-2 AAC in # of profiles and bitstream formats and so on; from what I understand, the actual methodology for Low Complexity Profile, Main Profile, and RAW AAC are the same in both), and MPEG-4 Audio Lossless Coding (MPEG-4 ALS). The standard for the MP4 container also specifies that it can contain any of these formats above, or even AC3.
In terms of MPEG-4, these audio standards were not all ratified at the same time. MPEG-4 AAC was ratified at the same time as MPEG-4 itself, in 1998. ALS was ratified in 2005/2006. The same is true of the difference with Part 2 (MPEG-4 Visual) and Part 10 (H.264/MPEG-4 Advanced Video Coding). Part 2 was ratified in 1998, Part 10 in 2003. Both are MPEG-4, but they are not all that related to one another, just as AAC and ALS aren't. Unlike Part 10 (H.264/AVC), which has a mostly* feature-complete set of encoding tools available, Part 2 is not anywhere near fully implemented for consumers - because Advanced Simple Profile dominated early, and since all the focus went into it, the other profiles (such as Main, Core, Studio, and so on...there's something like 21 profiles in total) were left out to dry. And then the focus shifted to H.264 in 2004/2005, which has a lot more standard industry implementations and more of a reason not to focus on just one profile within it.
*x264 and many consumer-level H.264 encoders don't support some of the higher colorspaces the standard itself defines, but in time they'll probably be added.