MP4 (and M4A) are
NOT encoding technologies. MPEG-4 Part 12 (which is what the M4A file type is actually called technically) is a specification for a file wrapper. If you don't know what the heck a file wrapper is, then read this:
http://wiki.jrmediacenter.com/index.php/DirectShow_Playback_Guide#Filter_Types (particularly the second paragraph).
The term MPEG is actually just a group of engineers who define standards for digital video (
Motion Picture Experts Group), who work as part of the International Organization for Standardization (ISO/IEC). Over the years, MPEG has defined a number of standards for both audio and video encoding and for file formats. The MPEG standards are all split into a number of different "parts" which define different parts of the puzzle. For example: MPEG-2 is often referred to as the "encoding format" of the video on a DVD. However, this isn't technically correct, as the AAC audio format is
also part of the MPEG-2 standard (which is an "advanced" version of MP3, which is just MPEG-1 Audio Layer 3). MPEG-2 Part 1 defines the two different container formats for MPEG video (transport and program stream), MPEG-2 Part 2 defines the video encoding and compression technique we all know from DVDs, and MPEG-2 Part 3 is the audio section of the standard (which allows for multi-channel encoding and backwards compatibility with MPEG-1 standards). In fact, AAC is actually also known as MPEG-2 Part 7!
The MP4 file type is a container format defined by the MPEG-4 Part 14 standard. It defines a way to represent audio and video data on disk, and a way to include metadata tags about that data. It does
NOT define the way in which that audio or video data is compressed or encoded (though it does specify allowable types). A M4A file is a MP4 file. They are one and the same. You can have lots of different types of M4A/MP4 files, just like you can have lots of different types of AVI files (or like there can be lots of different things inside a ZIP file).
There are other parts to the MPEG-4 standard, including:
- MPEG-4 Part 2 which specifies the Advanced Simple Profile video encoding technique you probably know as DivX or XviD (or Quicktime's MPEG-4 file format)
- MPEG-4 Part 10 which specifies a "better" video encoding technique you might have heard called H264 or AVC
- MPEG-4 Part 3 which specifies extensions to the AAC audio encoding techniques originally specified in the MPEG-2 standard (it provides the High Efficiency and Scalable Sample Rate extensions among others)
- MPEG-4 Part 8 which specifies a method to deliver MPEG-4 video content across IP networks
- MPEG-4 Part 17 which specifies a format for timed subtitles
and on and on.
So, saying supporting M4A "encoding format" doesn't really make any sense. M4A is a container format. You can have a M4A file with MP3 formatted data inside, or with AAC formatted media inside (most are), or with Apple Lossless audio, or with any number of different encoding types. In fact, since the file extension M4A isn't really part of the standard, you could just as easily have a M4A file with video data inside of it in XviD format!
Author
Topic: mpeg 4 audio / m4a encoding (Read 5698 times)