The MPEG-4 Part-14 video container, commonly known as
MP4, was developed by Apple to replace their aging Quicktime format. MPEG-4
Part-14 was approved in 1998, and became a container standard by 2000.
Quicktime (MOV)
MP4 and MOV containers are essentially the same thing with a few minor
differences. They both have the same file structure, but MP4 containers have
more tagging options such as scene descriptions. Both formats are encoded with
MPEG-4 codecs, which make them very similar. The biggest of differences comes
in hardware and software support for MP4 - where with MOV you are much more
limited in your playback options.
Native Streams and
Private Streams
The MPEG-4 Part-14 container was designed to use an MPEG-4 video codec. By
requiring the native stream to use MPEG-4 technology, it is much simpler to
ensure playback on all devices. The limitation of this is that as newer and
better coding methods are developed, the MP4 format is not as versatile. To
help alleviate this problem, the use of private data streams are also available
in the container which makes alternative codecs available.
Although MP4 supports both native and private media streams,
there is very little hardware support for private audio streams. This means
that many players with MP4 support won't be able to play MP4 files if they
contain any private streams.
Variable Bitrate
(VBR)
One of the benefits of the MP4 container is the option of using variable
bitrates for both video and audio. By having a variable bitrate, the streams
can be encoded at a higher quality during the complex parts of the video and allow
the encoding process to lower the bitrate during the less demanding parts.
Using this method helps keep the file size to a minimum; VBR
has also become the preferred method of encoding MP3 files for the same reason.
Interactivity
Another benefit of the MP4 container is the inclusion of an interactive DVD-style
menu system. This is another feature that is not available in the AVI container format.
(Lack of) Audio
Format Support
Here is where some of the holes in the MP4 format start to get noticed. There
are very limited options when it comes to using different audio formats inside
the MP4 container.
Although the container supports the high quality Advanced Audio
Coding (AAC) format, they do not support the widely used AC-3 audio format.
AC-3 is commonly known as Dolby Digital 5.1, which is included on almost every
DVD released today. The ACC standard, although more efficient, is not widely supported
by most home theater receivers.
 M4V, MVA and M4P
Sometimes you will see MP4 files with different file extensions. Although these
extensions are technically not correct as defined by the official MP4 standard,
Apple sells videos from its iTunes store in M4V and audio in M4A. They also
sell copy protected AAC files with a M4P file extension.
Although Apple has been moving in the right direction in
eliminating Digital Rights Management (DRM) from many audio files, the video
files they sell are still infested with the copy protection system. For this
reason, I couldn't suggest purchasing any of these files from iTunes until they
change their practices.
We're still looking for the perfect format. Next week, we'll
take a look at the Matroska Container and then you'll hopefully have a video
format you won't be afraid to take home to Mom.
Share your experiences with MP4 files or their illegitimate
iTunes counterparts below!
More Info:
MPEG-4 - The Container for All Digital Media
Afterdawn: MP4
MPEG Standards Explained
AC3 in MP4 Without the Use of a Private Stream? (Doom9 forums)
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