[Mp4-tech] frame size in AAC

[Tracy Yang]

I found that in AAC, the frame size is fixed to 1024 samples for long
windows regardless of input sampling frequency. I understand that the
frame length should be approximately 20ms, because this is the time
frame that we can consider the audio signal is quasi-stationary. For
input sound with 44.1 or 48 kHz sampling frequency, each frame of 1024
samples is about 20ms. However, for lower sampling frequencies, this
frame length will make each frame to be much longer than 20 ms. I know
that the coding quality for files with lower sampling frequencies was
not bad. Is this because other coding tools, such as window type
switching and TNS, take care of it, or is there any theoretically
background stating that it is not necessary to have 20 ms as frame
length for quasi-stationary audio signals.
------------------------------------------------------
Dai Tracy Yang, Ph.D.
Audio Technologist
Vidiator Technology (US) Inc.
10900 NE Eighth Street, Suite 600
Bellevue, WA 98004, U.S.A.
 <http://www.vidiator.com/> http://www.vidiator.com
Direct: +1 (425) 278 1311
Email: tracy.yang AT vidiator.com 
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