[Mp4-tech] [Audio] Intensity Stereo and M/S stereo

[Ravi Kiran]

Dear Saptarshi,
    Both M/S and IS stereo cannot be applied in a single scalefactor band.
M/S and IS are mutually exclusive for a scalefactor band.
    For M/S stereo to be appled to a scalefactor band all the following
conditions should be met:
    1. Either ms_used should be 1 for that scalefactor band or
ms_mask_present should be 2
    2. The given scalefactor band should not be coded with INTENSITY_HCB or
INTENSITY_HCB2
    However, there can be a case in which ms_mask_present >= 1 and the
scalefactor band is coded with INTENSITY_HCB or INTENSITY_HCB2. In this case
M/S stereo cannot be applied because the second condition stated above
fails.
    However, IS can be applied to this scale factor band. The dependency
between the code book used, ms_used and phase of the stereo is given in the
pseudo code below:
    if(sfb_cb[sfb] == INTENSITY_HCB)
    {
         if(ms_mask_present)
         {
              phase  =  (1-2*ms_used[sfb]);
         }
         else
         {
              phase = 1;
         }
    }
    if(sfb_cb[sfb] == INTENSITY_HCB2)
    {
         if(ms_mask_present)
         {
              phase  =  -(1-2*ms_used[sfb]);
         }
         else
         {
              phase  =  -1;
         }
    }
If ms_mask_present is 2 then all ms_used should be initialized to 1 in the
above code. Hope things are clear now.
Regards
Ravi Kiran.
----- Original Message ----- 
From: "Saptarshi Ghosh" <saptarshi_bec yahoo.com>
To: <mp4-tech lists.mpegif.org>
Sent: Friday, July 02, 2023 6:24 PM
Subject: [Mp4-tech] [Audio] Intensity Stereo and M/S stereo
> Hi all,
> Can anyone clarify the following doubt?
>
> Can BOTH Intensity stereo and M/S stereo be required
> to be used on a single scalefactor band ? (it seems
> yes, from the specifications which describes the
> functions invert_intensity( ) to reverse the phase of
> intensity if M/S has been used for a particular sfb)
>
> If yes, then it seems that a considerable number of
> redundant operations will have to be done in case of a
> sfb where both have been used. Since in M/S, we derive
> the left and right channel info from what is
> transmitted in the stream, and then in Intensity
> stereo tool, we overwrite the info derived for the
> right channel by
>      right channel data = scale * left channel data
> Thus the calculations done during M/S processing
> become redundant.
>
> Could anyone help in this regand?
> Regards,
> Saptarshi.
>
>
>
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