MPEG-4 Part 2
MPEG-4 Part 2 is a Moving Picture Experts Group standard belonging to the MPEG-4 group of standards that also includes AVC. However, despite allowing much more efficient Compression than MPEG-2 (as used in DVDs and most DTV), it lacks the advanced technology for which AVC (Advanced Video Coding) is named, including CABAC, InLoop Deblocking, and Partitions. The best known implementations of MPEG-4 Part 2 are the DivX and XviD codecs, which began primarily as computer-bound formats that required a computer to watch but are now playable by a large number of standalone DVD players.
Like most video standards, MPEG-4 Part 2 is divided into different Profiles. The two most common are Simple Profile (SP) and Advanced Simple Profile (ASP). Simple Profile is suited primarily for applications such as mobile phones and video conferencing where quality is less important than size. Video encoded to Simple Profile specifications is suitable for Muxing into a 3GP Container for use on a 3G mobile phone network. Advanced Simple Profile is better suited for home video applications, making use of features such as B Frames, Quarter Pixel Motion Estimation, and Global Motion Compensation. The two profiles are generally referred to as simply MPEG-4 ASP or MPEG-4 SP.
Because the MPEG-4 Part 2 standards were designed with applications like mobile video conferencing and Streaming video across the internet, the most obvious difference between it and predecessors like MPEG-2 is stream size. A video stream encoded with a MPEG-4 ASP compliant encoder, when compared to a MPEG-2 file with comparable quality, will have a significantly smaller size.
As there was no official MPEG-4 container designated before or with MPEG-4 Part 2, a number of different containers are commonly used. The one that's probably most familiar is the Windows AVI (Audio Video Interleave) container. Although it's not designed for MPEG-style decoding, where B Frames, which must be decoded out of playback order, are used, the earliest ASP-based encoder readily available to the public included a hack to use the AVI container. With a variety of ASP and SP compliant codecs available written around the Video for Windows interface that uses AVI natively, it remains the dominant container for MPEG-4 ASP encoding. It also has the widest support among standalone DVD players.
After DivX and XviD, along with a handful of other MPEG-4 ASP compatible codecs, had made the format popular the MP4 container became standard for all MPEG-4 formats. Along with the rise in popularity of AVC encoding, support for the MP4 container has become fairly common for both MPEG-4 Part 10 (AVC), and increasingly also MPEG-4 ASP as well. This is also due in part to the growth of the encoder market segment using non-Windows operating systems, and therefore unable to make use the AVI container. Although the MP4 container makes it easier to ensure hardware compatibility, it also provides challenges because of the lack of native support at the operating system level. While Windows comes with support for reading files from MPEG-2 and AVI containers built in, additional software must be installed to read MP4 files. You can find more information on this in our guide on How to play MP4 files
Probably due to MPEG-4 Part 2's video conferencing heritage, where suitable audio standards already existed, there's no audio format defined. In reality the audio format is often dependent on the container used to hold the muxed video and audio. Simple Profile video is often stored in a (MP4 based) 3GP container, along with either MP3 or AAC LC audio. For Advanced Simple Profile video, which is normally encoded from a standard consumer video format like DVD or DTV (MPEG-2 generally), the audio used in the AVI container is generally either AC-3 (Dolby Digital) taken directly from the original source or VBR MP3. Like AVI, the MKV container may be used to mux original AC-3 streams with ASP video, but also allows audio in a variety of other formats.
In the MP4 container it's usually AAC LC or HE AAC, transcoded from the original source. Like the standard MPEG-2 container, MP4 allows any type of audio as a private stream, but unless you have a decoder available that can Decode AC-3 from MP4 you'll end up with a file that has no useable audio. If you do have the ability to playback AC-3 audio from an MP4 container, be aware that the same file may not play with every player, software or standalone.
With MPEG-4 ASP's popularity among consumers, and the obvious advantage of smaller video files for upcoming HD sources, the International Telecommunications Union and Motion Pictures Experts Group collaborated on a new standard, specifically intended to improve both quality and compression over both MPEG-2 and MPEG-4 Part 2. The resulting standard, known as H.264, Advanced Video Coding, or simply MPEG-4 Part 10 has become a cornerstone of both Blu-ray and HD DVD, as well as working its way into Digital Television broadcasting, where the increasing number of HD channels makes improved compression a necessity for television services that have a Limited amount of Bandwidth. It's also begun displacing MPEG-4 ASP as the dominant high compression consumer format.
How to play AVI
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