[Mp4-tech] [MPEG-4 AAC]

[Andrzej Kaczor]

Hi all,
I want to test SBR encoder and SBR decoder. First I must to encode and then decode using main profil of AAC, because I want to compare it to coding with SBR.
I have encoded and then decoded the WAV-file, which sampling rate is 48 kHz (mono). First I have tried it to encode to 24 kbps, and then to 48 kbps. Below are my command lines (linux):
24 kbps
     ./enc wav_48000Hz.wav      -r 24000           -c "-main"     -o wav_48000Hz.wav.main.24kbps.raw      -d 1
     ./adif2mp4    -bi 24000     -Dn      -s 48000     -p Main         wav_48000Hz.wav.main.24kbps.raw    wav_48000Hz.wav.main.24kbps.mp4
     ./mp4audec  -d 1   wav_48000Hz.wav.main.24kbps.mp4   -pd wav_output      -ed .main.24kbps.wav 
result:
http://at31.ant.uni-hannover.de/~kaczor/master_thesis/matrix_krotkie.main.24kbps.wav
48 kbps
     ./enc wav_48000Hz.wav      -r 48000           -c "-main"     -o wav_48000Hz.wav.main.48kbps.raw      -d 1
     ./adif2mp4    -bi 48000     -Dn      -s 48000     -p Main         wav_48000Hz.wav.main.48kbps.raw    wav_48000Hz.wav.main.48kbps.mp4
     ./mp4audec  -d 1   wav_48000Hz.wav.main.48kbps.mp4   -pd wav_output      -ed .main.48kbps.wav 
result:
http://at31.ant.uni-hannover.de/~kaczor/master_thesis/matrix_krotkie.main.48kbps.wav
The original wav file (48000 Hz, mono):
http://at31.ant.uni-hannover.de/~kaczor/master_thesis/matrix_krotkie.wav
Why sounds the file "matrix_krotkie.main.24kbps.wav" poorly? Maybe it's clipping? Maybe 24 kbps is too little bitstream for main profile of AAC encoder?
Best Regards,
Andrzej Kaczor
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