深入解析：MP4与M4A文件格式

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"该资源主要涉及MP4和M4A文件格式的分析，参考了QuickTime File Format Specification，这是Apple Inc.在2007年发布的一份文档，详细介绍了这两种广泛使用的数字媒体容器格式。" MP4 (MPEG-4 Part 14)是一种数字多媒体容器格式，被广泛用于存储音频、视频和其他时间同步数据。它基于Apple的QuickTime文件格式，并且符合ISO/IEC 14496-12标准。MP4格式的特点包括高效的编码和压缩，支持流媒体，以及包含元数据的能力，例如轨道信息、字幕和章节标记。 M4A（MPEG-4 Audio）是MP4格式的一个子集，专门用于音频内容。它通常用于存储无损或有损音频编码，如AAC (Advanced Audio Coding) 或ALAC (Apple Lossless Audio Codec)。M4A文件的主要优点在于其小巧的文件大小与高质量的音频体验之间的平衡，使得它们在网络传输和存储时非常方便。 QuickTime File Format Specification详细阐述了文件的结构，包括如何组织音视频数据、时间码、轨道、以及如何定义元数据。文件由一系列的'boxes'组成，每个box都包含了特定的信息，比如'mdat' box用于存储实际的媒体数据，而'moov' box则包含文件的元数据，如时间线信息。文档还可能涵盖了文件的解析方法，包括如何读取和写入这些格式，这对于开发者来说非常重要，因为他们需要理解如何正确处理和操作这些文件。此外，对于版权保护，文档可能会提及如何嵌入数字权限管理（DRM）信息，以限制内容的非法复制和分发。关于M4A，它与MP4的区别在于不包含视频数据，而是专注于音频质量的优化。这使得M4A文件通常比同等质量的MP3文件更小，同时提供了更好的音质。由于M4A格式主要与Apple的产品和服务相关联，因此在iPods、 iPhones、以及iTunes等平台上，M4A格式得到了广泛支持和应用。在版权方面，文档提醒读者，虽然可以为了个人使用存储和打印文档，但未经许可，不得用于商业目的，以免触犯商标侵权和不公平竞争的法律。所有关于技术的知识产权都归Apple Inc.所有。这个资源对理解和处理MP4和M4A文件格式的开发者、媒体工程师以及数字内容创作者非常有价值，它提供了一套详细的规范来指导如何有效地操作和利用这些文件格式。

QuickTime files are described in general, rather than how they are supported on a specific computing platform

or in a specific programming language. As a result, the file format information is presented in a tabular

manner, rather than in coded data structures. Similarly, field names are presented in English rather than as

programming language tags. Furthermore, to the extent possible, data types are described generically. For

example, this book uses “32-bit signed integer” rather than “long” to define a 32-bit integer value.

QuickTime files are used to store QuickTime movies, as well as other data. If you are writing an application

that parses QuickTime files, you should recognize that there may be non-movie data in the files.

Licensing Information

The QuickTime File Format Specification is provided for informational purposes. Apple may have patents,

patent applications, trademarks, copyrights, or other intellectual property rights covering subject matter in

this document. The furnishing of this document does not give you a license to any patents, trademarks,

copyrights, or other intellectual property.

Important: For more information about licensing the QuickTime File Format, contact: Apple, Inc., Software

Licensing Department, 12545 Riata Vista Circle, MS 198 3-SWL, Austin, TX 78727. Email Address:

sw.license@apple.com

Special Fonts

All code listings, reserved words, and the names of actual data structures, constants, fields, parameters, and

routines are shown in Letter Gothic (this is Letter Gothic).

Words that appear in boldface are key terms or concepts and are defined in the glossary.

Updates to This Specification

For updates or changes to this specification, go to the QuickTime documentation site at

QuickTime Reference Library

and click the File Format Specification link.

For More Information

For information about membership in Apple’s developer program and developer technical support, you

should go to this URL:

Apple Developer Connection

For information on registering signatures, file types, and other technical information, contact

Licensing Information

INTRODUCTION

Introduction to QuickTime File Format Specification

QuickTime movies are stored on disk, using two basic structures for storing information: atoms (also known

as simple atoms or classic atoms), and QT atoms. To understand how QuickTime movies are stored, you need

to understand the basic atom structures described in this chapter. Most atoms you encounter in the QuickTime

File Format are simple or classic atoms. Both simple atoms and QT atoms, however, allow you to construct

arbitrarily complex hierarchical data structures. Both also allow your application to ignore data they don’t

understand.

Metadata

A QuickTime file stores the description of its media separately from the media data. The description, or

metadata, is called the movie resource, movie atom, or simply the movie, and contains information such as

the number of tracks, the video compression format, and timing information. The movie resource also contains

an index describing where all the media data is stored.

The media data is the actual sample data, such as video frames and audio samples, used in the movie. The

media data may be stored in the same file as the QuickTime movie, in a separate file, in multiple files, in

alternate sources such as databases or real-time streams, or in some combination of these.

Atoms

The basic data unit in a QuickTime file is the atom. Each atom contains size and type fields that precede any

other data. The size field indicates the total number of bytes in the atom, including the size and type fields.

The type field specifies the type of data stored in the atom and, by implication, the format of that data. In

some cases, the size and type fields are followed by a version field and a flags field. An atom with these

version and flags fields is sometimes called a full atom.

Note: An atom, as described in this document, is functionally identical to a box, as described in the ISO

specifications for MPEG-4 and JPEG-2000. An atom that includes version and flags fields is functionally identical

to a full box as defined in those specifications.

Atom types are specified by a 32-bit unsigned integer, typically interpreted as a four-character ASCII code.

Apple, Inc. reserves all four-character codes consisting entirely of lowercase letters. Unless otherwise stated,

all data in a QuickTime movie is stored in big-endian byte ordering, also known as network byte ordering,

in which the most significant bytes are stored and transmitted first.

Atoms are hierarchical in nature. That is, one atom can contain other atoms, which can contain still others,

and so on. This hierarchy is sometimes described in terms of a parent, children, siblings, grandchildren, and

so on. An atom that contains other atoms is called a container atom. The parent atom is the container atom

exactly one level above a given atom in the heirarchy.

Metadata 19

CHAPTER 1

Overview of QTFF

For example, a movie atom contains several different kinds of atoms, including one track atom for each track

in the movie. The track atoms, in turn, contain one media atom each, along with other atoms that define

other track characteristics. The movie atom is the parent atom of the track atoms. The track atoms are siblings.

The track atoms are parent atoms of the media atoms. The movie atom is not the parent of the media atoms,

because it is more than one layer above them in the hierarchy.

An atom that does not contain other atoms is called a leaf atom, and typically contains data as one or more

fields or tables. Some leaf atoms act as flags or placeholders, however, and contain no data beyond their size

and type fields.

The format of the data stored within a given atom cannot always be determined by the type field of the

atom alone; the type of the parent atom may also be significant. In other words, a given atom type can

contain different kinds of information depending on its parent atom. For example, the profile atom inside a

movie atom contains information about the movie, while the profile atom inside a track atom contains

information about the track. This means that all QuickTime file readers must take into consideration not only

the atom type, but also the atom’s containment hierarchy.

Atom Layout

Figure 1-1 (page 20) shows the layout of a sample atom. Each atom carries its own size and type information

as well as its data. Throughout this document, the name of a container atom (an atom that contains other

atoms, including other container atoms) is printed in a gray box, and the name of a leaf atom (an atom that

contains no other atoms) is printed in a white box. Leaf atoms contain data, usually in the form of tables.

Figure 1-1 A sample atom

Container atom

Atom size

Atom type

Container atom

Leaf atom

Atom size

Atom type

Atom data

Atom size

Atom type

A leaf atom, as shown in Figure 1-1 (page 20), simply contains a series of data fields accessible by offsets.

Atoms within container atoms do not generally have to be in any particular order, unless such an order is

specifically called out in this document. One such example is the handler description atom, which must come

before the data being handled. For example, a media handler description atom must come before a media

information atom, and a data handler description atom must come before a data information atom.

Atoms

CHAPTER 1

Overview of QTFF

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