Tuesday, August 08, 2006
MPEG4 Overview
MPEG4 is a standard that includes many "parts". The important ones in our case are Parts 2, 3, 10, and 14.
MPEG4 Part 2 is the original MPEG4 video encoding specification that specifies the resultant bitstream itself.
MPEG4 Part 3 is also known as AAC and is an audio successor to MP3.
MPEG4 Part 10 is a newer MPEG4 video encoding specification also known as "AVC" or "H.264".
MPEG4 Part 14 is the file format specification also called, generically, a "container" format.
In general, the confusion between audio encoders, video encoders, and file formats is not readily resolved by the companies that create and promote this software. I've had to learn about all of this through bits and pieces and lots of reading.
The different encoders such as DivX, XviD, 3ivx all can encode into MPEG4 Part 2-compatible bitstreams, yet they encode it using different algorithms thus resulting in various subtle differences in quality. XviD was started by DivX releasing an Open Source version of DivX, called OpenDivX, and now is developed separately. Part of the confusion is that the terms "DivX", "XviD", "3ivx" and others can refer to a company, a program, an encoder, a decoder, an encoding format or a container format. And it is frequently not clear which one is being referred to. This is particularly confusing when referring to one or both of the encoding format and the container format.
In encoders that differentiate between "MPEG4" and "H264" or "MPEG4" and "AVC" as two different ways to encode video, then MPEG4 in this case likely means MPEG4 Part 2 whereas "H264", "H.264", or "AVC" refers to MPEG4 Part 10.
So the reason for the statement that all DivX-encoded video is MP4 is that DivX-encoded video conforms to MPEG4 Part 2. But all MPEG4 video is not DivX: for example, MPEG4 Part 10-encoded video. Incidently, I don't think the first part of this statement is actually true. The latest incarnation of the DivX encoder can, I understand, create ".divx" files which are essentially AVI format enclosing among other things an MPEG4 Part 2 video. And the AVI files would not be MPEG4 Part 14. So calling the result .divx file an "MPEG4" file is misleading at best.
Incidently, Part 10 video is said to be of higher quality than Part 2 for any given file size, but Part 10 is more computationaly expensive. Leading me to surmise that the quality per battery life for Part 10 vs. Part 2 is similar if not slightly favoring Part 2.
MPEG4 Part 2 is the original MPEG4 video encoding specification that specifies the resultant bitstream itself.
MPEG4 Part 3 is also known as AAC and is an audio successor to MP3.
MPEG4 Part 10 is a newer MPEG4 video encoding specification also known as "AVC" or "H.264".
MPEG4 Part 14 is the file format specification also called, generically, a "container" format.
In general, the confusion between audio encoders, video encoders, and file formats is not readily resolved by the companies that create and promote this software. I've had to learn about all of this through bits and pieces and lots of reading.
The different encoders such as DivX, XviD, 3ivx all can encode into MPEG4 Part 2-compatible bitstreams, yet they encode it using different algorithms thus resulting in various subtle differences in quality. XviD was started by DivX releasing an Open Source version of DivX, called OpenDivX, and now is developed separately. Part of the confusion is that the terms "DivX", "XviD", "3ivx" and others can refer to a company, a program, an encoder, a decoder, an encoding format or a container format. And it is frequently not clear which one is being referred to. This is particularly confusing when referring to one or both of the encoding format and the container format.
In encoders that differentiate between "MPEG4" and "H264" or "MPEG4" and "AVC" as two different ways to encode video, then MPEG4 in this case likely means MPEG4 Part 2 whereas "H264", "H.264", or "AVC" refers to MPEG4 Part 10.
So the reason for the statement that all DivX-encoded video is MP4 is that DivX-encoded video conforms to MPEG4 Part 2. But all MPEG4 video is not DivX: for example, MPEG4 Part 10-encoded video. Incidently, I don't think the first part of this statement is actually true. The latest incarnation of the DivX encoder can, I understand, create ".divx" files which are essentially AVI format enclosing among other things an MPEG4 Part 2 video. And the AVI files would not be MPEG4 Part 14. So calling the result .divx file an "MPEG4" file is misleading at best.
Incidently, Part 10 video is said to be of higher quality than Part 2 for any given file size, but Part 10 is more computationaly expensive. Leading me to surmise that the quality per battery life for Part 10 vs. Part 2 is similar if not slightly favoring Part 2.
# posted by Greg Smith @ 6:11 AM 
