[MultiMedia] FLV format

The FLV header All FLV files begin with the following header:

• Signature(UI8): Signature byte always ‘F’ (0x46)
• Signature(UI8): Signature byte always ‘L’ (0x4C)
• Signature(UI8): Signature byte always ‘V’ (0x56)
• Version(UI8): File version (for example, 0x01 for FLV version 1)
• TypeFlagsReserved(UB[5]): Must be 0
• TypeFlagsAudio(UB[1]): Audio tags are present
• TypeFlagsReserved(UB[1]): Must be 0
• TypeFlagsVideo(UB[1]): Video tags are present
• DataOffset(UI32): Offset in bytes from start of file to start of body (that is, size of header)

The DataOffset field usually has a value of 9 for FLV version 1. This field is present to accommodate larger headers in future versions.

Example: 
0x46 0x4C 0x56 0x01 0x05 0x00 0x00 0x00 0x09 means 
TypeFlagsAudio = 1 
TypeFlagsVideo = 1 
DataOffset = 0x00 0x00 0x00 0x09

The FLV file body After the FLV header, the remainder of an FLV file consists of alternating back-pointers and tags. They interleave as shown in the following table:

• PreviousTagSize0(UI32): Always 0
• Tag1(FLVTAG): First tag
• PreviousTagSize1(UI32): Size of previous tag, including its header. For FLV version 1, this value is 11 plus the DataSize of the previous tag.
• Tag2(FLVTAG): Second tag
• …
• PreviousTagSizeN-1(UI32): Size of second-to-last tag
• TagN(FLVTAG): Last tag
• PreviousTagSizeN(UI32): Size of last tag

FLV tags FLV tags have the following format:

• TagType(UI8): Type of this tag. Values are: 0x8: audio, 0x9: video, 0x12: script data and all others: reserved.
• DataSize(UI24): Length of the data in the Data field
• 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.
• Data: If TagType == 0x08 for AUDIODATA; If TagType == 0x09 for VIDEODATA; If TagType == 0x12 for SCRIPTDATAOBJECT

Example: 
0x12 0x00 0x03 0x15 0x00 0x00 0x00 0x00 0x00 0x00 0x00 means 
TagType = 0x12
DataSize = 0x00 0x03 0x15
TimeStamp = 0x00 0x00 0x00
TimeStampExtend = 0x00
StreamID = 0x00 0x00 0x00

Originally published at vincewiki.blogspot.com on June 27, 2017.