codecs 组成

The 

codecs

 parameter
is specified by RFC 6381. Specifically, see section
3.3 for the meaning of 

avc1

 and 

mp4a

 values.

In the case of 

avc1.4D401E

, 

avc1

 indicates
H.264 video, and this is followed by a dot and three 2-digit hexadecimal numbers defined by the H.264 standard:

profile_idc

the byte containing the 

constraint_set

 flags
(currently 

constraint_set0_flag

 through 

constraint_set5_flag

,
and the 

reserved_zero_2bits

)

level_idc

Some examples:

avc1.42E01E

:
H.264 Constrained Baseline Profile Level 3

avc1.4D401E

:
H.264 Main Profile Level 3

avc1.64001E

:
H.264 High Profile Level 3

These are also the second, third, and fourth bytes of the Sequence Parameter Set and the AVC Configuration Box in an MP4 file. You can dump these bytes using a program such as 

mp4file

: 

mp4file
--dump movie.mp4

. Look for the 

avcC

 (AVC
Configuration) Box and the hexadecimal values for 

AVCProfileIndication

, 

profile_compatibility

,
and 

AVCLevelIndication

.

As for 

mp4a.40.2

, 

mp4a

 indicates
MPEG-4 audio. It is followed by a dot and a hexadecimal 

ObjectTypeIndication

 (

objectTypeId

 in 

mp4file

 output),
which can be looked up on the MPEG4 registration site. If this hexadecimal value is 

40

 (ISO/IEC
14496-3 Audio), it is followed by another dot and an audio object type in decimal. These are listed in the ISO/IEC 14496-3 standard and onWikipedia,
and correspond to the first 5 bits of the 

DecoderSpecificInfo

 (

decSpecificInfo

)
(unless these bits equal 31, in which case add 32 to the next 6 bits). 

mp4a.40.2

 indicates
AAC LC audio, which is what is usually used with H.264 HTML5 video.

For example, 

codecs="avc1.42E01E,
mp4a.40.2"

 would be correct for the movie below:

$ mp4file --dump movie.mp4
...
type avcC (moov.trak.mdia.minf.stbl.stsd.avc1.avcC)  ◀━━ avc1
configurationVersion = 1 (0x01)
AVCProfileIndication = 66 (0x42)    ◀━━ 42
profile_compatibility = 224 (0xe0)  ◀━━ E0
AVCLevelIndication = 30 (0x1e)      ◀━━ 1E
...
type esds (moov.trak.mdia.minf.stbl.stsd.mp4a.esds)  ◀━━ mp4a
version = 0 (0x00)
flags = 0 (0x000000)
ESID = 2 (0x0002)
streamDependenceFlag = 0 (0x0) <1 bits>
URLFlag = 0 (0x0) <1 bits>
OCRstreamFlag = 0 (0x0) <1 bits>
streamPriority = 0 (0x00) <5 bits>
decConfigDescr
objectTypeId = 64 (0x40)           ◀━━ 40
streamType = 5 (0x05) <6 bits>
upStream = 0 (0x0) <1 bits>
reserved = 1 (0x1) <1 bits>
bufferSizeDB = 0 (0x000000) <24 bits>
maxBitrate = 78267 (0x000131bb)
avgBitrate = 78267 (0x000131bb)
decSpecificInfo
info = <2 bytes>  11 90  |..|     ◀━━ 2 (first 5 bits in decimal)
...