Files and Storage Formats - A Practical Guide to Video and Audio Compression - page 286

streamed through a variety of different services from the same source file. By the time the
video is being streamed, any organization of the internal file structure as atoms or chunks
is long gone, and the client player just sees a continuous stream of bits, which it must syn-
chronize with, buffer, and then unwrap and de-interleave before decoding.

16.5.4 Atoms

Table 16-2 summarizes the terms that are commonly used to describe the different com-
ponent parts of a video stream.
Accessing a file is accomplished (these days) by loading a chunk or atom of content.
Sometimes these are organized into objects that are collections of video chunks. The video
and audio streams would not be interleaved for the purpose of storage because it would
not serve any useful purpose. The chunks merely have to be accessed in the correct order.
Only those that are part of the current playback session have to be read from the file.
There is more information about atomic structures in the QuickTime file-format book
published through the Apple Developer Connection, details of which are available on the
Apple developer web site. Although it describes the QuickTime file format, the concepts
are relevant to other video formats.
The ISO standards document ISO_IEC_14496-14_2003 describes the MPEG-4 file for-
mat but is rather thinner in scope. The URLs within the ISO web site are long and complex,

Table 16-2 Atoms and Chunks

Terminology Description

Atom An atom is used to describe the object-oriented breakdown of the video

and multi-track audio when stored in the highly structured MPEG-4 and
QuickTime files. These atoms also describe other kinds of data. A series of
atoms would be assembled to form a stream containing one kind of
content or another. Note also that atoms can be nested whereas none of
the other component forms can be used in this way.

Chunk Historically, the video is broken into chunks when it is stored in AVI files.

These don't necessarily correspond to packets or groups of pictures. It is
unlikely you would break a group of pictures across several chunks.

GOP A group of pictures is the logical grouping of a series of frames that are

treated as a unit when running a compressor over them. The compressed
output is then divided into packets.

Packet Normally taken to describe a fragment of transported content. The

packetization serves to split the larger data items into pieces that fit
within TCP/IP networking protocols, for example.

Apple Developer Connection: http://developer.apple.com/
Apple QuickTime developer information: http://developer.apple.com/quicktime/
ISO Standards: http://www.iso.org/