https://www.cnblogs.com/chyingp/p/flv-getting-started.html
https://blog.csdn.net/ai2000ai/article/details/77530741
从整个文件上开看,FLV是由 header 和 Body 组成.
1、Header
Signature: FLV 文件的前3个字节为固定的‘F’‘L’‘V’,用来标识这个文件是flv格式的.在做格式探测的时候，
如果发现前3个字节为“FLV”，就认为它是flv文件.
Version: 第4个字节表示flv版本号.
Flags: 第5个字节中的第0位和第2位,分别表示 video 与 audio 存在的情况.(1表示存在,0表示不存在)
DataOffset : 最后4个字节表示FLV header 长度.
2 FLV body = TagSize|Tag | TagSize | Tag... ...
FLV header之后,就是 FLV File Body.
FLV File Body是由一连串的back-pointers + tags构成.back-pointers就是4个字节数据,表示前一个tag的size.
3.FLV Tag
FLV文件中的数据都是由一个个TAG组成,TAG里面的数据可能是video、audio、scripts.
onMetaData 包含在scripts 中， 含有十分重要的信息， 比如宽高、duration、filesize 等
下表是TAG的结构:
1.FLVTAG
| Field | Type | Comment |
| Reserved | UB [2] | Reserved for FMS, should be 0 |
| Filter | UB [1] | Indicates if packets are filtered. 0 = No pre-processing required. 1 = Pre-processing (such as decryption) of the packet is required before it can be rendered. Shall be 0 in unencrypted files, and 1 for encrypted tags. See Annex F. FLV Encryption for the use of filters. |
| TagType | UB [5] |
Type of contents in this tag. The following types are |
| DataSize | UI24 | Length of the message. Number of bytes after StreamID to end of tag (Equal to length of the tag – 11) |
| Timestamp | UI24 | Time in milliseconds at which the data in this tag applies. This value is relative to the first tag in the FLV file, which always has a timestamp of 0. |
| TimestampExtended | UI8 | Extension of the Timestamp field to form a SI32 value. This field represents the upper 8 bits, while the previous Timestamp field represents the lower 24 bits of the time in milliseconds. |
| StreamID | UI24 | Always 0. |
| AudioTagHeader | IF TagType == 8 AudioTagHeader |
|
| VideoTagHeader | IF TagType == 9 VideoTagHeader |
|
| EncryptionHeader | IF Filter == 1 EncryptionTagHeader |
|
| FilterParams | IF Filter == 1 FilterParams |
|
| Data | IF TagType == 8 AUDIODATA IF TagType == 9 VIDEODATA IF TagType == 18 SCRIPTDATA |
Data specific for each media type. |
TagType: TAG中第1个字节中的前5位表示这个TAG中包含数据的类型,8 = audio,9 = video,18 = script data.
DataSize:StreamID之后的数据长度.
Timestamp和TimestampExtended组成了这个TAG包数据的PTS信息,记得刚开始做FVL demux的时候，并没有考虑TimestampExtended的值,直接就把Timestamp默认为是PTS，后来发生的现 象就是画面有跳帧的现象,后来才仔细看了一下文档发现真正数据的PTS是PTS= Timestamp | TimestampExtended<<24.
StreamID之后的数据就是每种格式的情况不一样了，接下格式进行详细的介绍.
Audio Tags
如果TAG包中的TagType==8时，就表示这个TAG是audio。
StreamID之后的数据就表示是AudioTagHeader，AudioTagHeader结构如下：