视频容器解析之RealMedia File Format

版权声明：转载自http://blog.csdn.net/fangshy/archive/2007/04/14/1564736.aspx

Appendix E: RealMedia File Format (RMFF)

     Helix Architecture supports RealMedia File Format (RMFF), which enables Helix to deliver high- quality multimedia content over a variety of network bandwidths. Third-party developers can convert their media formats into RMFF, enabling Helix Universal Server to deliver the files to RealOne Player or other applications built with the Helix Client and Server Software Development Kit. Third-party developers can thereby use Helix to transport content over the Internet to their own applications.

       RealMedia File Format is a standard tagged file format that uses four-character codes to identify file elements. These codes are 32-bit, represented by a sequence of one to four ASCII alphanumeric characters, padded on the right with space characters. The data type for four-character codes is FOURCC. Use the PN_FOURCC macro to convert four characters into a four-character code.

The basic building block of a RealMedia File is a chunk, which is a logical unit of data, such as a stream header or a packet of data. Each chunk contains the following fields:

 

• Four-character code specifying the chunk identifier

   

• 32-bit value specifying the size of the data member in the chunk

   

• Blob of opaque chunk data

Depending on its type, a top-level chunk can contain subobjects. This document describes the tagged chunks contained in RMFF, as well as the format of the data stored in each type of tagged chunk.

Tagged File Formats

 

Header Section

Because RMFF is a tagged file format, the order of the chunks is not explicit, except that the RealMedia File Header must be the first chunk in the file. However, most applications write the standard headers into the file's header section. The following chunks are typically found in the header section of RMFF:

 

• RealMedia File Header (this must be the first chunk of the file)

   

• Properties Header

   

• Media Properties Header

   

• Content Description Header

After the RealMedia File Header object, the other headers may appear in any order. All headers are required except the Index Header. The following sections describe the individual header objects.

RealMedia File Header

Each RealMedia file begins with the RealMedia File Header, which identifies the file as RMFF. There is only one RealMedia File Header in a RealMedia file. Because the contents of the RealMedia File Header may change with different versions of RMFF, the header structure supports an object version field for determining what additional fields exists. The following pseudo-structure describes the RealMedia File Header:

RealMedia_File_Header { UINT32 object_id; UINT32 size; UINT16 object_version; if (object_version == 0) { UINT32 file_version; UINT32 num_headers; } }

The RealMedia File Header contains the following fields:

object_id: 32 bits

The unique object ID for a RealMedia File (.RMF). All RealMedia files begin with this identifier.

size: 32 bits

The size of the RealMedia header section in bytes.

object_version: 16 bits

The version of the RealMedia File Header object. All files created according to this specification have an object_version number of 0 (zero).

file_version: 32 bits

The version of the RealMedia file in PN Version format. All files created according to this specification have a major version number of 1. This member is present on all RealMedia_File_Header objects with an object_version of 0 (zero).

num_headers: 32 bits

The number of headers in the header section that follow the RealMedia File Header. This member is present on all RealMedia_File_Header objects with an object_version of 0 (zero).

Properties Header

The Properties Header describes the general media properties of the RealMedia File. Components of the RealMedia system use this object to configure themselves for handling the data in the RealMedia file or stream. There is only one Properties Header in a RealMedia file. The following pseudo-structure describes the Properties header:

Properties { UINT32 object_id; UINT32 size; UINT16 object_version; if (object_version == 0) { UINT32 max_bit_rate; UINT32 avg_bit_rate; UINT32 max_packet_size; UINT32 avg_packet_size; UINT32 num_packets; UINT32 duration; UINT32 preroll; UINT32 index_offset; UINT32 data_offset; UINT16 num_streams; UINT16 flags; } }

The Properties Header contains the following fields:

object_id: 32 bits

The unique object ID for a Properties Header ('PROP').

size: 32 bits

The size of the Properties Header in bytes.

object_version: 16 bits

The version of the RealMedia File Header object. All files created according to this specification have an object_version number of 0 (zero).

max_bit_rate: 32 bits

The maximum bit rate required to deliver this file over a network. This member is present on all Properties objects with an object_version of 0 (zero).

avg_bit_rate: 32 bits

The average bit rate required to deliver this file over a network. This member is present on all Properties objects with an object_version of 0 (zero).

max_packet_size: 32 bits

The largest packet size (in bytes) in the media data. This member is present on all Properties objects with an object_version of 0 (zero).

avg_packet_size: 32 bits

The average packet size (in bytes) in the media data. This member is present on all Properties objects with an object_version of 0 (zero).

num_packets: 32 bits

The number of packets in the media data. This member is present on all Properties objects with an object_version of 0 (zero).

duration: 32 bits

The duration of the file in milliseconds. This member is present on all Properties objects with an object_version of 0 (zero).

preroll: 32 bits

The number of milliseconds to prebuffer before starting playback. This member is present on all Properties objects with an object_version of 0 (zero).

index_offset: 32 bits

The offset in bytes from the start of the file to the start of the index header object. This member is present on all Properties objects with an object_version of 0 (zero).

data_offset: 32 bits

The offset in bytes from the start of the file to the start of the Data Section. This member is present on all Properties objects with an object_version of 0 (zero).

num_streams: 16 bits

The number of media streams contained in the file. This member is present on all Properties objects with an object_version of 0 (zero).

flags: 16 bits

Flags indicating characteristics of the RealMedia file. The following flags are defined:

 

• #define PN_SAVE_ENABLED 0x0001

  Allows clients to save a copy of the RealMedia file to disk.

   

• #define PN_PERFECT_PLAY_ENABLED 0x0002

  Allows clients to use extra buffering to ensure Perfect Play.

   

• #define PN_LIVE_BROADCAST 0x0004

  The RealMedia file is being generated by a live broadcast.

Media Properties Header

The Media Properties Header describes the specific media properties of each stream in a RealMedia File. Components of the RealMedia system use this object to configure themselves for handling the media data in each stream. There is one Media Properties Header for each media stream in a RealMedia file. The following pseudo-structure describes the Media Properties header:

Media_Properties { UINT32 object_id; UINT32 size; UINT16 object_version; if (object_version == 0) { UINT16 stream_number; UINT32 max_bit_rate; UINT32 avg_bit_rate; UINT32 max_packet_size; UINT32 avg_packet_size; UINT32 start_time; UINT32 preroll; UINT32 duration; UINT8 stream_name_size; UINT8[stream_name_size] stream_name; UINT8 mime_type_size; UINT8[mime_type_size] mime_type; UINT32 type_specific_len; UINT8[type_specific_len] type_specific_data; } }

The Media Properties Header contains the following fields:

object_id: 32 bits

The unique object ID for a Media Properties Header ("MDPR").

size: 32 bits

The size of the Media Properties Header in bytes.

object_version: 16 bits

The version of the Media Properties Header object.

stream_number: 32 bits

The stream_number (synchronization source identifier) is a unique value that identifies a media stream. Every data packet that belongs to a media stream contains the same STREAM_NUMBER. The STREAM_NUMBER enables a receiver of multiple media streams to distinguish which packets belong to each media stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

max_bit_rate: 32 bits

The maximum bit rate required to deliver this stream over a network. This member is present on all MediaProperties objects with an object_version of 0 (zero).

avg_bit_rate: 32 bits

The average bit rate required to deliver this stream over a network. This member is present on all MediaProperties objects with an object_version of 0 (zero).

max_packet_size: 32 bits

The largest packet size (in bytes) in the stream of media data. This member is present on all MediaProperties objects with an object_version of 0 (zero).

avg_packet_size: 32 bits

The average packet size (in bytes) in the stream of media data. This member is present on all MediaProperties objects with an object_version of 0 (zero).

start_time: 32 bits

The time offset in milliseconds to add to the time stamp of each packet in a media stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

preroll: 32 bits

The time offset in milliseconds to subtract to the time stamp of each packet in a media stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

duration: 32 bits

The duration of the stream in milliseconds. This member is present on all MediaProperties objects with an object_version of 0 (zero).

stream_name_size: 8 bits

The length of the following stream_name field in bytes. This member is present on all MediaProperties objects with an object_version of 0 (zero).

stream_name: variable length

A nonunique alias or name for the stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

mime_type_size: 8 bits

The length of the following mime_type field in bytes. This member is present on all MediaProperties objects with an object_version of 0 (zero).

mime_type: variable length

A nonunique MIME style type/subtype string for data associated with the stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

type_specific_len: 32 bits

The length of the following type_specific_data in bytes. The type_specific_data is typically used by the data type renderer to initialize itself in order to process the media stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

type_specific_data: variable length

The type_specific_data is typically used by the data-type renderer to initialize itself in order to process the media stream. This member is present on all MediaProperties objects with an object_version of 0 (zero).

Content Description Header

The Content Description Header contains the title, author, copyright, and comments information for the RealMedia File. All text data is in ASCII format. The following pseudo-structure describes the Content Description Header:

Content_Description { UINT32 object_id; UINT32 size; UINT16 object_version; if (object_version == 0) { UINT16 title_len; UINT8[title_len] title; UINT16 author_len; UINT8[author_len] author; UINT16 copyright_len; UINT8[copyright_len] copyright; UINT16 comment_len; UINT8[comment_len] comment; } }

The Content Description Header contains the following fields:

object_id: 32 bits

The unique object ID for the Content Description Header ('CONT').

size: 32 bits

The size of the Content Description Header in bytes.

object_version: 16 bits

The version of the Content Description Header object.

title_len: 16 bits

The length of the title data in bytes. Note that the title data is not null-terminated. This member is present on all Content Description Header objects with an object_version of 0 (zero).

title: variable length

An array of ASCII characters that represents the title information for the RealMedia file. This member is present on all Content Description Header objects with an object_version of 0 (zero).

author_len: 16 bits

The length of the author data in bytes. Note that the author data is not null-terminated. This member is present on all Content Description Header objects with an object_version of 0 (zero).

author: variable length

An array of ASCII characters that represents the author information for the RealMedia file. This member is present on all Content Description Header objects with an object_version of 0 (zero).

copyright_len: 16 bits

The length of the copyright data in bytes. Note that the copyright data is not null-terminated. This member is present on all Content Description Header objects with an object_version of 0 (zero).

copyright: variable length

An array of ASCII characters that represents the copyright information for the RealMedia file. This member is present on all Content Description Header objects with an object_version of 0 (zero).

comment_len: 16 bits

The length of the comment data in bytes. Note that the comment data is not null-terminated. This member is present on all Content Description Header objects with an object_version of 0 (zero).

comment: variable length

An array of ASCII characters that represents the comment information for the RealMedia file. This member is present on all Content Description Header objects with an object_version of 0 (zero).

Data Section

The data section of the RealMedia file consists of a Data Section Header that describes the contents of the data section, followed by a series of interleaved media data packets. Note that the size field of the Data Chunk Header is the size of the entire data chunk, including the media data packets.

Data Chunk Header

The Data Chunk Header marks the start of the data chunk. There is usually only one data chunk in a RealMedia file; however, for extremely large files, there may be multiple data chunks. The following pseudostructure describes the Data chunk header:

Data_Chunk_Header { UINT32 object_id; UINT32 size; UINT16 object_version; if (object_version == 0) { UINT32 num_packets; UINT32 next_data_header; } }

The Data Chunk Header contains the following fields:

object_id: 32 bits

The unique object ID for the Data Chunk Header ('DATA').

size: 32 bits

The size of the Data Chunk in bytes.

object_version: 16 bits

The version of the Data Chunk Header object.

num_packets: 32 bits

Number of packets in the data chunk. This member is present on all Data Chunk Header objects with an object_version of 0 (zero).

next_data_header: 32 bits

Offset from start of file to the next data chunk. This field is not typically used. This member is present on all Data Chunk Header objects with an object_version of 0 (zero).

Data Packet Header

The data section of a RealMedia file consists of a series of interleaved data objects. Each data object contains a packet of data for a media stream. Because a RealMedia file can contain numerous media streams, data objects for each stream are interleaved. The following pseudo-structure describes the details of each interleaved packet:

Media_Packet_Header { UINT16 object_version; if (object_version == 0) { UINT16 length; UINT16 stream_number; UINT32 timestamp; UINT8 reserved; UINT8 flags; UINT8[length] data; } }

The Media Packet Header contains the following fields:

object_version: 16 bits

The version of the Media Packet Header object.

length: 16 bits

The length of the packet in bytes. This member is present on all Media Packet Header objects with an object_version of 0 (zero).

stream_number: 16 bits

The 16-bit alias used to associate data packets with their associated Media Properties Header. This member is present on all Media Packet Header objects with an object_version of 0 (zero).

timeStamp: 32 bits

The time stamp of the packet in milliseconds This member is present on all Media Packet Header objects with an object_version of 0 (zero).

reserved: 8 bits

This is not used. This member is present on all Media Packet Header objects with an object_version of 0 (zero).

flags: 16 bits

flags describing the properties of the packet. The following flags are defined:

#define PN_RELIABLE_FLAG 0x0001

If this flag is set, the packet is delivered reliably.

#define PN_KEYFRAME_FLAG 0x0002

If this flag is set, the packet is part of a key frame or in some way marks a boundary in your data stream.

This member is present on all Media Packet Header objects with an object_version of 0 (zero).

data: variable length

The application-specific media data. This member is present on all Media Packet Header objects with an object_version of 0 (zero).

Index Section

The index section of the RealMedia file consists of a Index Chunk Header that describes the contents of the index section, followed by a series of index records. Note that the size field of the Index Chunk Header is the size of the entire index chunk, including the index records.

Index Section Header

The Index Chunk Header marks the start of the index chunk. There is usually one index chunk per stream in a RealMedia file. The following pseudo-structure describes the Index chunk header.

Index_Chunk_Header { u_int32 object_id; u_int32 size; u_int16 object_version; if (object_version == 0) { u_int32 num_indices; u_int16 stream_number; u_int32 next_index_header; } }

The Index Chunk Header contains the following fields:

object_id: 32 bits

The unique object ID for the Index Chunk Header ('INDX').

size: 32 bits

The size of the Index Chunk in bytes.

object_version: 16 bits

The version of the Index Chunk Header object.

num_indices: 32 bits

Number of index records in the index chunk. This member is present on all Index Chunk Header objects with an object_version of 0 (zero).

stream_number: 16 bits

The stream number for which the index records in this index chunk are associated. This member is present on all Index Chunk Header objects with an object_version of 0 (zero).

next_index_header: 32 bits

Offset from start of file to the next index chunk. This field enables RealMedia file format readers to find all the index chunks quickly. This member is present on all Index Chunk Header objects with an object_version of 0 (zero).

Index Record

The index section of a RealMedia file consists of a series of index record objects. Each index record contains information for quickly finding a packet of a particular time stamp for a media stream. The following pseudo-structure describes the details of each index record:

IndexRecord { UINT16 object_version; if (object_version == 0) { u_int32 timestamp; u_int32 offset; u_int32 packet_count_for_this_packet; } }

An Index Record contains the following fields:

object_version: 16 bits

The version of the Index Record object.

timestamp: 32 bits

The time stamp (in milliseconds) associated with this record. This member is present on all Index Record objects with an object_version of 0 (zero).

offset: 32 bits

The offset from the start of the file at which this packet can be found. This member is present on all Index Record objects with an object_version of 0 (zero).

packet_count_for_this_packet: 32 bits

The packet number of the packet for this record. This is the same number of packets that would have been seen had the file been played from the beginning to this point. This member is present on all Index Record objects with an object_version of 0 (zero).