Detailed explanation of FLV format

FLV Format Overview ：

FLV（Flash Video） yes Adobe A popular streaming media format designed and developed by the company , Because of its lightweight video file size 、 Simple package and so on , Make it very suitable for application on the Internet . Besides ,FLV have access to Flash Player Play it , and Flash Player Plug ins have been installed in most browsers around the world , This makes it possible to play through the web FLV The video is very easy .

Flv File format

Flv By file header （File Header） and The file body （File Body） form .

Flv Body By a series of Tag form , Every Tag Another one preTagSize Field , Mark the front one Tag Size .

Flv Header

The file header consists of the following parts ：

Signature(3 Byte)+Version(1 Byte)+Flags(1 Bypte)+DataOffset(4 Byte)

Signature Occupy 3 Bytes ：
Fix FLV Three characters as a symbol . It is generally found that the first three characters are FLV I thought he was flv file .
Version Occupy 1 Bytes ：
Mark FLV Version number of . Here we see 1
Flags Occupy 1 Bytes ：
Content label . The first 0 Position and number 2 position , respectively video And audio Existing situation .(1 Indicates presence ,0 Does not exist ). notice 0x05, That is to say 00000101, Represents an existing video , There is also audio .
DataOffset Occupy 4 Bytes ：
Express FLV Of header length .

The picture below is Flv Header The content of ,（UI8 Indicates no sign 8 position , That's one byte ;UB[5] Represents... In a byte 5 position ）

Flv Body

Flv Body One by one Tag form , Every Tag There is one. preTagSize Field , Mark the front one Tag Size .

Tag There are three types ,Audio Tag（ Audio Tag）,Video Tag（ video Tag）,script Tag（ also called Metadata Tag）

Every Tag from “Tag Header” and “Tag Data” form , For different types of Tag,“Tag Header” The format of is the same ,“Tag Body” The format of is different .

The following figure summarizes the above content , After watching it, right flv There should be an overall understanding .

Generally one flv The file consists of a header message , One script Tag, And several video Tag and audio Tag form .

Let's explain each type of Tag Data

Tag Data

Flv There are three kinds of tag：“Audio Tag Data”、“Video Tag Data”、“Script Tag Data”

Aideo tag By tag Header + Tag Data form ：

tag Header：
TagType: 8 (1 byte)
TagType The meaning of the representative is as follows ：

• 8: audio

• 9 : video

• 18 : script data

DataSize: 7 (3 bytes)
tag Data size
tag Header size = Previous Tag Size - DataSize
TimeStamp: 0 (3 bytes) Time stamp
TimestampExtended: 0 (1 byte) Extended timestamp
StreamId: 0 (3 bytes) flow ID

Audio Tag Data:

Video tag analysis ：

video tag By tag Header + Tag Data form ：

tag Header：

TagType: 9 (1 byte)
TagType The meaning of the representative is as follows ：

• 8: audio
• 9 : video
• 18 : script data

DataSize: 50 (3 bytes)

• tag Data size

• tag Header size = Previous Tag Size - DataSize

TimeStamp: 0 (3 bytes) Time stamp
TimestampExtended: 0 (1 byte) Extended timestamp
StreamId: 0 (3 bytes) flow ID

Video Tag Data

scripts tag analysis ：

This type of Tag Also commonly known as MetadataTag, Will put some about FLV Metadata information of video and audio, such as ：duration、width、height etc. . Usually this type Tag Will follow FileHeader Later as the first Tag appear , And there's only one .

tag Header：

TagType: 18 (1 byte)
TagType The meaning of the representative is as follows ：

• 8: audio

• 9 : video

• 18 : script data

DataSize: 293 (3 bytes)

• tag Data size

• tag Header size = Previous Tag Size - DataSize

TimeStamp: 0 (3 bytes) Time stamp
TimestampExtended: 0 (1 byte) Extended timestamp
StreamId: 0 (3 bytes) flow ID

tag Data：

AMF package ： The first byte indicates AMF Packet type

first AMF package ：
The first byte is generally 0x02, Representation string , The first 2-3 Bytes represents the length of the string , It's usually 0x000A, Followed by a string , It's usually "onMetaData".

second AMF package ：
The first byte is 0x08, Represents an array , The first 2-5 Bytes represent the number of array elements , Followed by the elements of the array , The format is ： Element name length （UI16） + Element name （UI8[n]） + The value of the element (double), Finally “009” ending .