[時間：2016-07] [狀態(tài)：Open]

RM/RMVB是Real公司私有的封裝格式，常見的后綴形式是rm、ra、rmvb。
通常封裝的都是real轉(zhuǎn)悠的編碼格式，比如音頻中的sipro、cook、atrc、ralf、raac，視頻的RV10、RV20、RV30、RV40。

0. 學習多媒體容器格式的目的

主要是為了回答以下問題：

該容器中數(shù)據(jù)是如何組織的？

該容器包含哪些編碼格式的數(shù)據(jù)？這些數(shù)據(jù)是如何存儲的？

該容器包含哪些元數(shù)據(jù)信息？包含哪些節(jié)目信息？

對于支持多節(jié)目的容器格式，如何找到對應(yīng)的音頻流、視頻流、字幕流？

如何確定該容器的節(jié)目播放時長？

如何從該容器中提取音頻、視頻、字幕數(shù)據(jù)，并交給解碼器解碼，有時間戳否？

該容器是否支持seek？有哪些輔助信息？

是否支持直接流化？

哪里可以找到該容器格式最標準的文檔資料？

有哪些可用的工具，方便分析容器格式異常或者錯誤？

1. RM文件格式概述

RealMedia File Format（RMFF）是一種基于TAG的文件格式，每個TAG有四個字節(jié)（FOURCC）用于標識元素類型。
RM文件的基本構(gòu)成塊是chunk。每個chunk構(gòu)成如下：

==============ID(FOURCC) --------------size(4 byte) --------------data([size]) ==============

每個chunk的ID決定了data域如何解析。頂層的chunk可以包含sub-chunk。
一個常見的chunk構(gòu)成如下圖：

通常RM文件有三部分構(gòu)成：header section、data section、index section。每個section都是多個chunk構(gòu)成，具體可以參考下圖：

后續(xù)部分詳細介紹各個section。

2. RM文件頭（header section）

RMFF是基于TAG的格式，在header section中各個chunk出現(xiàn)的順序并不是固定的，但RealMedia File Header（文件頭）必須是第一個chunk。其他后續(xù)chunk包括：Properties Header（屬性頭）、Media Properties Header（媒體屬性頭）、Content Description Header（內(nèi)容描述頭）。

RealMedia文件頭

RealMedia文件頭通常用于識別文件格式，并且每個RM文件只有一個文件頭。其中包含的字段如下：

RealMedia_File_Header {UINT32 object_id;UINT32 size;UINT16 object_version;if ((object_version == 0) || (object_version == 1)){UINT32 file_version;UINT32 num_headers;} }

各字段具體含義見下表：

fieldtypedescription

object_id	UINT32	The unique object ID for a RealMedia File (.RMF). All RealMedia files begin with this identifier.
size	UINT32	The size of the RealMedia header section in bytes.
object_version	UINT16	The version of the RealMedia File Header object. All files created according to this specification have an object_version number of 0 (zero) or 1.
file_version	UINT32	The version of the RealMedia file. This member is present on all RealMedia_File_Header objects with an object_version of 0 (zero) or 1.
num_headers	UINT32	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) or 1.

注：后續(xù)表格中將不會出現(xiàn)關(guān)于object_version的限制，具體建議參考標準文檔。

RM屬性頭

Properties Header描述RMF的一般媒體屬性。
RM系統(tǒng)會參考這個對象中的數(shù)據(jù)處理RM文件或流中的數(shù)據(jù)。在RMF中只有一個屬性頭。其中包含的字段如下：

Properties_Header {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;} }

各字段具體含義見下表：

fieldtypedescription

object_id	UINT32	The unique object ID for a Properties Header ('PROP').
size	UINT32	The 32-bit size of the Properties Header in bytes.
object_version	UINT16	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	UINT32	The maximum bit rate required to deliver this file over a network.
avg_bit_rate	UINT32	The average bit rate required to deliver this file over a network.
max_packet_size	UINT32	The largest packet size (in bytes) in the media data.
avg_packet_size	UINT32	The average packet size (in bytes) in the media data.
num_packets	UINT32	The number of packets in the media data.
duration	UINT32	The duration of the file in milliseconds.
preroll	UINT32	The number of milliseconds to prebuffer before starting playback.
index_offset	UINT32	The offset in bytes from the start of the file to the start of the index header object. This value can be 0 (zero), which indicates that no index chunks are present in this file.
data_offset	UINT32	The offset in bytes from the start of the file to the start of the Data Section. Note: There can be a number of Data_Chunk_Headers in a RealMedia file. The data_offset value specifies the offset in bytes to the first Data_Chunk_Header. The offsets to the other Data_Chunk_Headers can be derived from the next_data_header field in a Data_Chunk_Header.
num_streams	UINT16	The total number of media properties headers in the main headers section.
flags		UINT16

RM媒體屬性頭（Media Properties Header）

Media Properties Header描述了RM文件中每個流的特定媒體屬性。每一個流都有一個媒體屬性頭。其中包含的字段如下：

Media_Properties_Header {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;} }

各字段含義如下表：

fieldtypedescription

object_id	UINT32	The unique object ID for a Media Properties Header ("MDPR").
size	UINT32	The size of the Media Properties Header in bytes.
object_version	UINT16	The version of the Media Properties Header object.
stream_number	UINT16	The stream_number (synchronization source identifier) is a unique value that identifies a physical stream. Every data packet that belongs to a physical stream contains the same STREAM_NUMBER. The STREAM_NUMBER enables a receiver of multiple physical streams to distinguish which packets belong to each physical stream.
max_bit_rate	UINT32	The maximum bit rate required to deliver this stream over a network.
avg_bit_rate	UINT32	The average bit rate required to deliver this stream over a network.
max_packet_size	UINT32	The largest packet size (in bytes) in the stream of media data.
avg_packet_size	UINT32	The average packet size (in bytes) in the stream of media data.
start_time	UINT32	The time offset in milliseconds to add to the time stamp of each packet in a physical stream.
preroll	UINT32	The time offset in milliseconds to subtract from the time stamp of each packet in a physical stream.
duration	UINT32	The duration of the stream in milliseconds.
stream_name_size	UINT8	The length of the following stream_name member in bytes.
stream_name	UINT8[]	A nonunique alias or name for the stream. This size of this member is variable.
mime_type_size	UINT8	The length of the following mime_type field in bytes.
mime_type	UINT8[]	A nonunique MIME style type/subtype string for data associated with the stream.This size of this member is variable.
type_specific_len	UINT32	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 physical stream.
type_specific_data	UINT8[]	The type_specific_data is typically used by the data type renderer to initialize itself in order to process the physical stream.The size of this member is variable.

RM邏輯流屬性頭

RM中可以包含多節(jié)目流，一般RM文件中通過RM邏輯流（logical stream）將多個物理流構(gòu)成。邏輯流包含以下信息：有哪些物理流構(gòu)成的，以及一些用于識別邏輯流的屬性（比如語言、包組等）。
邏輯流也是保存在Media Properties Header中，其mime type的前綴是"logical-"。舉個例子，一個RealAudio流（physical stream）的mime type是audio/x-pn-multirate-realaudio，那么對應(yīng)的邏輯流（logical stream）的mime type是logical-audio/x-pn-multirate-realaudio。下圖是一個邏輯流的構(gòu)成示例：

對于邏輯流對應(yīng)的屬性頭，其type_specific_data字段包含LogicalStream結(jié)構(gòu)。
文件中也有一個特殊的邏輯流，其MIME type是logical-fileinfo，包含整個文件的信息，而且只能有一個類似的文件。

LogicalStream Structure

其中包含的字段如下：

LogicalStream {ULONG32 size;UINT16 object_version;if (object_version == 0){UINT16 num_physical_streams;UINT16 physical_stream_numbers[num_physical_streams];ULONG32 data_offsets[num_physical_streams];UINT16 num_rules;UINT16 rule_to_physical_stream_number_map[num_rules];UINT16 num_properties;NameValueProperty properties[num_properties];} };

各字段含義如下表：

fieldtypedescription

size	UINT32	The size of the LogicalStream structure in bytes.
object_version	UINT16	The version of the LogicalStream structure.
num_physical_streams	UINT16	The number of physical streams that make up this logical stream. The physical stream numbers are stored in a list immediately following this field. These physical stream numbers refer to the stream_number field found in the Media Properties Object for each physical stream belonging to this logical stream.
physical_stream_numbers	UINT16[]	The list of physical stream numbers that comprise this logical stream. The size of this structure member is variable.
data_offsets	UINT32[]	The list of data offsets indicating the start of the data section for each physical stream. The size of this structure member is variable.
num_rules	UINT16	The number of ASM rules for the logical stream. Each physical stream in the logical stream has at least one ASM rule associated with it or it will never get played. The mapping of ASM rule numbers to physical stream numbers is stored in a list immediately following this member. These physical stream numbers refer to the stream_number field found in the Media Properties Object for each physical stream belonging to this logical stream.
rule_to_physical_stream_map	UINT16[]	The list of physical stream numbers that map to each rule. Each entry in the map corresponds to a 0-based rule number. The value in each entry is set to the physical stream number for the rule. For example: rule_to_physical_stream_map[0] = 5 This example means physical stream 5 corresponds to rule 0. All of the ASM rules referenced by this array are stored in the first name-value pair of this logical stream which must be called "ASMRuleBook" and be of type "string". Each rule is separated by a semicolon. The size of this structure member is variable.
num_properties	UINT16	The number of NameValueProperty structures contained in this structure. These name/value structures can be used to identify properties of this logical stream (for example, language).
properties	NameValueProperty[]	The list of NameValueProperty structures (see NameValueProperty Structure below for more details). As mentionied above, it is required that the first name-value pair be a string named "ASMRuleBook" and contain the ASM rules for this logical stream. The size of this structure member is variable.

NameValueProperty Structure

其中包含的字段如下：

NameValueProperty {ULONG32 size;UINT16 object_version;if (object_version == 0){UINT8 name_length;UINT8 name[namd_length];INT32 type;UINT16 value_length;UINT8 value_data[value_length];} }

各字段含義如下表：

fieldtypedescription

size	UINT32	The size of the NameValueProperty structure in bytes.
object_version	UINT16	The version of the NameValueProperty structure.
name_length	UINT8	The length of the name data.
name	UINT8[]	The name string data.
type	UINT32	The type of the value data. This member can take on one of three values (any other value is undefined), as shown in the following table: =0 32-bit unsigned integer property =1 buffer =2 string
value_length	UINT16	The length of the value data.
value_data	UINT8[]	The value data.

RM內(nèi)容描述頭（Content Description Header）

Content Description Header包含了RM文件的title、author、copyright、comments information等信息。其中包含的字段如下：

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;} }

各字段含義如下表：

fieldtypedescription

object_id	UINT32	The unique object ID for the Content Description Header ("CONT").
size	UINT32	The size of the Content Description Header in bytes.
object_version	UINT16	the version of the Content Description Header object.
title_len	UINT16	The length of the title data in bytes. Note that the title data is not null-terminated.
title	UINT8[title_len]	An array of ASCII characters that represents the title information for the RealMedia file. The size of this member is variable.
author_len	UINT16	The length of the author data in bytes. Note that the author data is not null-terminated.
author	UINT8[author_len]	An array of ASCII characters that represents the author information for the RealMedia file. The size of this member is variable.
copyright_len	UINT16	The length of the copyright data in bytes. Note that the copyright data is not null-terminated.
copyright	UINT8[]	An array of ASCII characters that represents the copyright information for the RealMedia file. The size of this member is variable.
comment_len	UINT16	The length of the comment data in bytes. Note that the comment data is not null-terminated.
comment	UINT8[]	An array of ASCII characters that represents the comment information for the RealMedia file.The size of this member is variable.

3. RM數(shù)據(jù)段（Data Section）

Data Section的起始位置可以通過Properties Header的data_offset字段獲取。通常RM數(shù)據(jù)段包括一個Data Chunk Header和多個交織的媒體數(shù)據(jù)包（data packet）構(gòu)成。

Data Chunk Header

標記數(shù)據(jù)塊的開始位置。一般RM文件只有一個數(shù)據(jù)塊。特別大的文件，可能有多個數(shù)據(jù)塊。其中包含的字段如下：

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

各字段含義如下表：

fieldtypedescription

object_id	UINT32	The unique object ID for the Data Chunk Header ('DATA').
size	UINT32	The size of the Data Chunk in bytes. The size includes the size of the header plus the size of all the packets in the data chunk.
object_version	UINT16	The version of the Data Chunk Header object.
num_packets	UINT32	Number of packets in the data chunk.
next_data_header	UINT32	Offset from start of file to the next data chunk. A non-zero value refers to the file offset of the next data chunk. A value of zero means there are no more data chunks in this file. This field is not typically used.

Data Packet

data chunk header之后緊跟著是num_packets個數(shù)據(jù)包。這些packet可能來自多個流，但是其時間戳是按照升序順序存儲的。每一個數(shù)據(jù)包的構(gòu)成如下：

Media_Packet_Header {UINT16 object_version;if ((object_version == 0) || (object_version == 1)){UINT16 length;UINT16 stream_number;UINT32 timestamp;if (object_version == 0){UINT8 packet_group;UINT8 flags;}else if (object_version == 1){UINT16 asm_rule;UINT8 asm_flags;}UINT8[length] data;}else{StreamDone();} }

各字段含義如下表：

fieldtypedescription

object_version	UINT16	The version of the Media Packet Header object.
length	UINT16	The length of the packet in bytes.
stream_number	UINT16	The 16-bit alias used to associate data packets with their associated Media Properties Header.
timeStamp	UINT32	The time stamp of the packet in milliseconds.
packet_group	UINT8	The packet group to which the packet belongs. If packet grouping is not used, set this field to 0 (zero).
flags	UINT8	Flags describing the properties of the packet. The following flags are defined: HX_RELIABLE_FLAG=1 If this flag is set, the packet is delivered reliably. HX_KEYFRAME_FLAG=2 If this flag is set, the packet is part of a key frame or in some way marks a boundary in your data stream.
asm_rule	UINT16	The ASM rule assigned to this packet.
asm_flags	UINT8	Contains HX_ flags that dictate stream switching points.
data	UINT8[length]	The application-specific media data. The size of this member is variable.

4. RM索引段（Index Section）

Index Section存儲了音視頻關(guān)鍵幀相關(guān)的時間到偏移量的映射。
通常索引塊包含一個Index Chunk Header和一系列的index records。

Index Chunk Header

Index Chunk Header標識索引塊的開始位置，其偏移量可以通過Properties Header的index_offset字段獲取。其中保存了索引段的屬性信息。其中包含的字段如下：

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;} }

各字段含義如下表：

fieldtypedescription

object_id	UINT32	The unique object ID for the Index Chunk Header ("INDX").
size	UINT32	The size of the Index Chunk in bytes.
object_version	UINT16	The version of the Index Chunk Header object.
num_indices	UINT32	Number of index records in the index chunk.
stream_number	UINT16	The stream number for which the index records in this index chunk are associated.
next_index_header	UINT32	Offset from start of file to the next index chunk. This member enables RealMedia file format readers to find all the index chunks quickly. A value of zero for this member indicates there are no more index headers in this file.

index records

index record中記錄時間戳到數(shù)據(jù)包偏移量的映射。其中包含的字段如下：

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

各字段含義如下表：

fieldtypedescription

object_version	UINT16	The version of the Index Record object.
timestamp	UINT32	The time stamp (in milliseconds) associated with this record.
offset	UINT32	The offset from the start of the file at which this packet can be found.
packet_count_for_this_packet	UINT32	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.

注意，通常情況下每個stream對應(yīng)一個索引段，也就是說index section可能會出現(xiàn)多次。

6. RM元數(shù)據(jù)段（Metadata Section）

RealMedia元數(shù)據(jù)中只有一個tag，這個tag里面包含一系列的命名metadata，這些metadata描述了媒體文件的屬性。這些metadata可以是文本、整型或二進制數(shù)據(jù)。Metadata Section包含一個Header和Tag Body。

Metadata Section Header

定義如下：

MetadataSectionHeader {u_int32 object_id; // The unique object ID for the Metadata Section Header ("RMMD")u_int32 size; }

Metadata Tag

metadata tag有多個properties構(gòu)成。這些properties通過樹形結(jié)構(gòu)組織，每一個property包含一個類型和值，也可能包括多個sub-properties。其中包含如下字段：

MetadataTag {u_int32 object_id;u_int32 object_version;u_int8[] properties; }

各字段含義如下表：

fieldtypedescription

object_id	UINT32	The unique object ID for the Metadata Tag ("RJMD").
object_version	UINT32	The version of the Metadata Tag.
properties	UINT8[]	The MetadataProperty structure that makes up the metadata tag (see "Metadata Property Structure" for more details). As mentioned above, the properties will be represented as one unnamed root metadata property with multiple sub-properties, each with their own optional sub-properties. These will be nested, as in a tree.

Metadata Property Structure

該部分包含如下字段：

MetadataProperty {u_int32 size;u_int32 type;u_int32 flags;u_int32 value_offset;u_int32 subproperties_offset;u_int32 num_subproperties;u_int32 name_length;u_int8[name_length] name;u_int32 value_length;u_int8[value_length] value;PropListEntry[num_subproperties] subproperties_list;MetadataProperty[num_subproperties] subproperties; }

各字段含義如下表：

fieldtypedescription

size	UINT32	The size of the MetadataProperty structure in bytes.
type	UINT32	The type of the value data. The data in the value array can be one of the following types: MPT_TEXT The value is string data. MPT_TEXTLIST The value is a separated list of strings, separator specified as sub-property/type descriptor. MPT_FLAG The value is a boolean flag either 1 byte or 4 bytes, check size value. MPT_ULONG The value is a four-byte integer. MPT_BINARY The value is a byte stream. MPT_URL The value is string data. MPT_DATE The value is a string representation of the date in the form: YYYYmmDDHHMMSS (m = month, M = minutes). MPT_FILENAME The value is string data. MPT_GROUPING This property has subproperties, but its own value is empty. MPT_REFERENCE The value is a large buffer of data, use sub-properties/type descriptors to identify mime-type.
flags	UINT32	Flags describing the property. The following flags are defined these can be used in combination: MPT_READONLY Read only, cannot be modified. MPT_PRIVATE Private, do not expose to users. MPT_TYPE_DESCRIPTOR Type descriptor used to further define type of value.
value_offset	UINT32	The offset to the value_length , relative to the beginning of the MetadataProperty structure.
subproperties_offset	UINT32	The offset to the subproperties_list , relative to the beginning of the MetadataProperty structure.
num_subproperties	UINT32	The number of subproperties for this MetadataProperty structure.
name_length	UINT32	The length of the name data, including the null-terminator.
name	UINT8[]	The name of the property (string data). The size of this member is designated by name_length.
value_length	UINT32	The length of the value data.
value	UINT8[]	The value of the property (data depends on the type specified for the property). The size of this member is designated by value_length.
subproperties_list	PropListEntry[]	The list of PropListEntry structures. The PropListEntry structure identifies the offset for each property (see "PropListEntry Structure" for more details. The size of this member is num_subproperties * sizeof(PropListEntry).
subproperties	MetadataProperty[]	The sub-properties. Each sub-property is a MetadataProperty structure with its own size, name, value, sub-properties, and so on. The size of this member is variable.

PropListEntry Structure

該部分包含如下字段：

PropListEntry {u_int32 offset;u_int32 num_props_for_name; }

各字段含義如下表：

fieldtypedescription

offset	UINT32	The offset for this indexed sub-property, relative to the beginning of the containing MetadataProperty.
num_props_for_name	UINT32	The number of sub-properties that share the same name. For example, a lyrics property could have multiple versions as differentiated by the language sub-property type descriptor.

Metadata Section Footer

metadata section footer標志著RealMedia文件的metasection的結(jié)束。由于位于RM文件末尾，section footer的位置是固定的，即相對文件結(jié)尾偏移量-140字節(jié)。section footer中的size字段表示metadata tag的長度，這個字段可以用于快速定位metadata數(shù)據(jù)。其中包含的字段如下：

MetadataSectionFooter {u_int32 object_id; // The unique object ID for the Metadata Section Footer ("RMJE").u_int32 object_version; // The version of the metadata tag.u_int32 size; // The size of the preceding metadata tag }

ID3v1 Tag

ID3v1 Tag位于metadata section最后一部分，其長度固定為128字節(jié)。格式可參考ID3v1 standard。

7. 其他問題

整個rm/rmvb文件結(jié)構(gòu)相對比較簡單。
比較官方的文檔可以從Helix DNA Common Components找到。
目前來說只有一個工具可以分析RM/RMVB文件，名字是RM文件分析器。

參考文獻

multimedia-wiki-RealMedia

RealMedia File Format-helix

RM文件解析

轉(zhuǎn)載于:https://www.cnblogs.com/tocy/p/media_container_7-rmvb.html

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