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Full Version: Switching aac mpeg version
Hydrogenaudio Forums > Lossy Audio Compression > AAC > AAC - General
enry2k
Hi Is it correct to state that the only differences between aac mpeg-2 and mpeg-4 versions are: an hex 1 in the 4th position instead of 9
two more zero bits are added at the end of the 10th position?

two little samples of the two versions of an aac file:
FFF9508002FFFC21 aac header mpeg-2 version
FFF1508000BFFF08 aac header mpeg-4 version

(taking samples from somewhere near the end of the two versions of the same file seems not reveal any of such correlations between the two files. I haven't found the two bit shift like in the beginning) Maybe there is a differesnt reason.

Are there further differeces?
If the only differences are those shown above, it should be relatively easy to write an c language program to change the version of an aac file. I would like to try!
I think that an expert like Menno could reply to my questions.
Best Reguards.
P.S. Where audiocoding.com website has gone?
JohnV
The 2bits in headers is the only difference, but it's in every frame, but without the ADTS header MPEG2-AAC LC=MPEG4-AAC LC. This same issue has been discussed in details here:
http://www.hydrogenaudio.org/forums/index....=ST&f=13&t=1527
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