MP4 is the official multimedia container for MPEG-4 video and audio. Although such video can be stored in a number of modern containers, including MPEG-2 PS/TS and Matroska, the development of MP4 is important because it gives hobbyists a replacement for the outdated AVI container which became the standard for MPEG-4 ASP (DivX, XviD, 3IVX, etc,...) video.
While MP4 works fine as a container format, it does have some issues that need to be resolved if its to become any kind of standard for hobbyists. Its primary problem is in audio support. Like the MPEG-2 container formats (Program Stream and Transport Stream) MP4 allows both native audio streams (encoded to a standard defined as part of MPEG-4) and private streams (those not part of the MPEG-4 specs). Unlike those formats, however, there's very little support among software and hardware vendors reading private streams. As a result, if your MP4 file contains audio in a format other than AAC there's a good chance your player won't be able to read it.
With the dominance of Dolby Digital audio for formats from DVD to DTV to Blu-ray, and lack of AAC surround decoders in home theater receivers, many people consider AC-3 support essential for encoding. This has led to a boom for Matroska (MKV) technology, which has no problem muxing or splitting (reading) nearly any audio format your computer supports, including AC-3. Unfortunately hardware (DVD player) support for MKV is lacking at the moment. However, unless support for AC-3 in MP4 containers becomes standard, it's possible Matroska will eventually become the de facto standard, if not an official (MPEG sanctioned) option.
Windows SupportUnlike the more common AVI and MPEG (or MPG) containers, Windows doesn't include built-in support for reading MP4 files. In order to read video and audio from MP4 files you'll need to install a special program called a media splitter. Haali Media Splitter adds support for both MP4 and MKV files to Windows (DirectShow), and is free.
MP4 and MOVMP4 was based on Apples MOV (Quicktime) container, which they were already using for MPEG-4 content. Rather than using a different type of file for elementary (separate video and audio) streams, MOV and MP4 use the same container regardless of how many or what type of streams are stored in it. Apple's iTunes online music store made audio only MP4 files a common sight for consumers. Although not all portable media players can read them, Apple's M4A extension (for audio only MP4 files) has become Synonymous with audio.
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Haali Media Splitter (Freeware)
Haali Media Splitter is a DirectShow splitter for .MKV (matroska) .MP4 and .AVI