[Mp4-tech] [Audio] Understanding SBR Decoding

[Andreas Schneider]

Hello Garima,
Garima Singh <garima.singh tivr.co.in> wrote on 04.01.2024 10:11:58:
> Hi All,
> 
> I have some questions regarding SBR decoding.
> 
> In AAC-LC each channel data of one frame is of 1024 samples , but in
> HE-AAC decoder, 1024 samples output of AAC-LC produces 2048 samples 
> output from QMF synthesis bank. Could you please explain how are 
> these mapped to channels? 

Not sure I understand you. These 2048 samples belong to the same channel 
as the 1024 samples from the AAC decoder. I.e. they have twice the 
sampling rate.
> Also in the SBR decoder, if core AAC decoder works on X sampling 
> rate then SBR decoder works on 2X sampling rate.My question here is 
> what will be the sampling rate of input signal (X or 2X) ?

If the input signal you are refering to is the original wave-file before 
encoding, then the answer is that this will have the sampling rate 2X.
The encoder - decoder chain looks like this (this example assumes a 
sampling rate of 48 kHz of the original file):
                       => SBR encoder (fs=48kHz) => SBR bitstream
PCM-file (fs=48 kHz) <                                               > 
bitstream
                       => 2:1 downsampler => AAC encoder (fs=24 kHz)
bitstream => AAC decoder (fs=24 kHz) => SBR decoder => output file (fs=48 
kHz)
The upsampling in the SBR decoder is achieved by using a 32 band analysis 
filterbank and a 64 band synthesis filterbank.
I hope this helps.
Kind regards,
Andreas
> 
> Thanks,
> 
> 

> Regards,
> Garima Singh 
> 
> TIVR Communications - Delivering Efficient Solutions for Mobile 
Multimedia 
> www.tivr.co.in
--
Andreas Schneider
Senior Research Engineer
mailto:snd CodingTechnologies.com
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