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Topic: Switching aac mpeg version (Read 2811 times) previous topic - next topic
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Switching aac mpeg version

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?

Switching aac mpeg version

Reply #1 –
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
Juha Laaksonheimo