[Mp4-tech] [Audio] PS decoding

[Devilal]

Hi,
In the case of Parametric Stereo(PS) in HE-AAC, we get the
bs_extended_data(1 bit)  flag set to 1 and then
bs_extension_id (2 bits) is read. Then the sbr_extension() is called, in
which if bs_extension_id  read is 2 (corresponding to EXTENSION_ID_PS) then
we decode PS data ( ps_data() ) .
All this parsing is done in the function sbr_single_channel_element () which
in turn is called in sbr_data(), which is called independently from the fact
that we get bs_header_flag = 0 or 1. Received bs_header_flag  can be  1 with
the frame( in that case we do HF Generation, HF Adjustment and then QMF
Synthesis) OR can be 0  with the frame ( in that case we do upsampling by
applying QMF Synthesis and we bypass HF Generation and HF Adjustment).
In some of the implementations I have gone through (like 3gpp's fixed point
and ISO's floating point reference code for HE-AAC), the sbr_data() is
called only if the sbr header flag is set (bs_header_flag = 1 ) in the
incoming frame. And hence, if the sbr header flag read is not set (bs_
header_flag = 0 ) then the decoder does not care about bs_extended_data and
bs_extension_id and hence ps_data is not decoded.
So, the decoder does not decode PS data and does not apply all the routines
related to PS (like hybrid analysis, synthesis , decorrelation, rotaion etc.
in the QMF Synthesis) and it simply does upsampling.
In the case of bs_header_flag = 0 (or when we do not decode PS data) , we
apply QMF Synthesis only once . On the other hand when  bs_header_flag = 1 (or
when we decode PS data)  , we apply QMF Synthesis twice, first time with PS
Active flag(so that it goes through PS routines and generate one channel
data) and second  time without PS flag(so, it apply QMF Synthesis as if
there is no PS present in the stream and generate the second channel data).
I am not able to understand how the second channel is generated in the first
case when bs_header_flag  = 0 and we do not do PS decoding.
Please correct me if I am missing anything here.
-- 
Regards,
Devilal
--------------------
Mobile: +91-9986423985
email: devilal gmail.com
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