Understanding the MPEG-4 Movie Atom
An atom is a self-contained data unit that contains information about an MP4 file. The moov atom, also referred to as the movie atom, defines the timescale, duration, display characteristics of the movie, as well as subatoms containing information for each track in the movie. The optimal location of the moov atom depends on the selected delivery method.
MPEG-4 Stream Packaging
For Flash Player to be able to play back an MPEG-4 (MP4) file, the file must be packaged in a specific type of container—one that follows the MPEG-4 Part 12 (ISO/IEC 14496-12) specification. Stream packaging is the process of making a multiplexed media file. Also known as muxing, this procedure combines multiple elements that enable control of the distribution delivery process into a single file. Some of these elements are represented in self-contained atoms.
As mentioned at the outset, an atom is a basic data unit that contains a header and a data field. The header contains referencing metadata that describes how to find, process, and access the contents of the data field, which may include (but is not limited to) the following components:
- Video frames
- Audio samples
- Interleaving AV data
- Captioning data
- Chapter index
- Title
- Poster
- User data
- Various technical metadata: codec, timescale, version, preferred playback rate, preferred playback volume, movie duration, etc.
In an MPEG-4–compliant container, every movie contains a moov atom. Normally, a movie atom contains a movie header atom (an mvhd atom) that defines the timescale and duration information for the entire movie, as well as its display characteristics. The movie atom also contains one track atom (a trak atom) for each track in the movie. Each track atom contains one or more media atoms (an mdia atom) along with other atoms that define other track and movie characteristics.
In this tree-like hierarchy, the moov atom acts as an index of the video data. It is here that the MPEG-4 muxer stores information about the file to enable the viewer to play and scrub the file. The file will not start to play until the player can access this index.
Unless specified otherwise, the moov atom is normally stored at the end of the file in on-demand content, after all of the information describing the file has been generated. Depending on the type of on demand delivery method selected—progressive download, streaming, or local playback—the location will need to move either to the end or to the beginning of the file.
If the planned delivery method is progressive download or streaming (RTMP or HTTP), the moov atom will have to be moved to the beginning of the file. This ensures that the required movie information is downloaded first, enabling playback to start right away.
If the moov atom is located at the end of the file, it forces the download of the entire file first before it will start playback. If the file is intended for local playback, then the location of the moov atom will not impact the start time, since the entire file is available for playback right away.
The placement of the moov atom is specified in various software packages through settings such as "progressive download," "fast start," "use streaming mode," or similar options. Software packages such as MP4creator or AtomicParsley enable you to analyze the location of the moov atom in your encoded files (see Figures 1 and 2).
Some tools enable relocation of the moov atom to the beginning of the file's structure through post processing of the compressed MPEG-4 (MP4) file. One such tool is MP4Creator, mentioned earlier, and another is MP4 FastStart. The best way to handle the moov atom location, however, is to set it during the compression and muxing portion of the encoding process. This minimizes the probability of the moov atom inadvertently being placed at the end.
Issues with edts Atom Handling
An edts atom contained in the trak atom of a moov atom located within an MP4 container hierarchy is responsible for tracking times and durations of the media. Flash Player architecture is designed to ignore the existence of an edts atom; however, an edts atom containing invalid or broken data may interfere with smooth and stable switching of HTTP packaged streams. Therefore, it is important to repair or remove an invalid edts atom prior to packaging the file for HTTP Dynamic Streaming.
The broken edts atom can be eliminated from a file using tools such as FLVCheck for file conformance, MP4Creator for structure analysis, and AtomicParsley for removal of metadata (see Figures 3 and 4).
The AtomicParsley command responsible for removing atoms is represented in the following string:
AtomicParsley.exe filename.mp4 --manualAtomRemove "moov.trak.edts" --manualAtomRemove "moov[2].trak.edts" --overWrite
Here, filename.mp4 is the name of the file being processed and --manualAtomRemove is a command that initiates the removal of the specific atom, edts, which is hierarchically located within the trak atom, which is within the moov atom. If the file contains more than one trak atom, such as audio and video media elements, then the track number is added to the "moov.trak.edts" as shown above. By default, AtomicParsley removes the atom from the first moov atom track. Adding the next track number in sequence, or a track number of your choice, forces AtomicParsley to proceed to that atom number next (for example, moov[2].trak.edts). To edit all track numbers, repeat the command for each track. Adding the command string --overWrite overwrites your original processed file.
Tools
- AtomicParsley (command-line program for reading and parsing MPEG-4 files)
- FLVCheck (MPEG-4 checker)
- MP4Box (multimedia packager)
- MP4Creator (MPEG-4 file creater and modifier)
- MP4 FastStart (starts MPEG-4 progressive downloads immediately)
- QTIndexSwapper (starts H.264 progressive downloads immediately)
By Maxim Levkov, Adobe
