Block size of various AAC coders, what is AAC block size Options

[lhuisset]

Hello,

I m trying to understand the AAC specification. I understood that the block size was 1024 samples for a long block and 128 samples for a short block.

In a 3GPP paper that describes the enhanced AAC + coder, the block size is 2048 samples for a long block and 256 samples for a short block.

Browsing various web sites I got contradictory info about the block size for the various AAC profiles.

Is there someone who could shed some light here? what is the block size for which AAC variant ?

Thanks
Amities,
Stephane

[SebastianG]

Hello,

I m trying to understand the AAC specification. I understood that the block size was 1024 samples for a long block and 128 samples for a short block.

In a 3GPP paper that describes the enhanced AAC + coder, the block size is 2048 samples for a long block and 256 samples for a short block.

Browsing various web sites I got contradictory  info about the block size for the various AAC profiles.

Is there someone who could shed some light here? what is the block size for which AAC variant ?

Thanks
Amities,
Stephane

Well, it depends on how you look at it and what your definition of "block" is. Since AAC makes use of the MDCT and the MDCT in turn is a block overlapping transform, the transform windows are twice as long as what I would call a "block". So, 2048/256 are the sizes of the transform windows and 1024/128 are actually the block sizes. An AAC frame contains either 1x1024 or 8x128 samples per channel. BUT an AAC frame usually influences 2048 samples (because of the overlapped transform)


Sebastian

[Garf]

The discrepancy comes from the MDCT overlap, whether you consider it or not. There really are only 2 framesizes (SSR profile not counting).