Files and Storage Formats - A Practical Guide to Video and Audio Compression - page 289

are applicable to the movie in a synchronized and time-sensitive way, they are stored as
an additional track. Hence the term hint track or hinted movie.
There will be more than one hint track added to a movie, typically, one for each of
the target services the content is streamed to. The technical approach is to create a hint
track for every stream. There would be one for the video and one for each audio track.
This is a useful approach to preparing videos for streaming because the same movie
file can be downloaded and played locally. The player will just open the video and audio
tracks in the normal way. The hint data will be ignored because it is concerned with trans-
port and not playback.
A media packetizer tool adds the hints. This might be built into your compression
software or it might be an additional tool that you must run over the file after it has been
compressed. If you are buying a compression tool to use for creating streamed content,
then you should check that the hinting support is available as part of the toolkit.

Be careful when flattening movies (a QuickTime operation) or doing other
processing on them, as you might accidentally remove the hint tracks.

16.6 MXF Files

The MXF file format is emerging as an important interchange format for professional
users. MXF encapsulates a variety of media formats in a container with the edit decision
list (EDL) information and any relevant metadata. This is transferred to another system
that understands the EDL and extracts the essence assets for further processing. MXF is
format agnostic. That is to say, it does not care what the essence format is when you fill an
MXF container with assets. There is no actual MXF video format because the standard only
describes a container.
The MXF container format is derived from the AAF file format. This standard is man-
aged by the AAFA . Data is wrapped in a three-part structure as shown in Figure 16-7.
The MXF file format specification allows for these key-length-value (KLV) fragments
to be nested as shown in Figure 16-8.
This is very similar to the way that QuickTime and MPEG-4 files are structured with
atoms and chunks. The metadata is described using an XML format so that implementa-
tions can use some already existing utility-code libraries to import and export the MXF

Key Length Value

Figure 16-7 A KLV fragment in an MXF file.