[Mp4-tech] Mpeg4 AAC + why exp-=4 (for short windows) else exp-=7

[Mody, Mihir]

Hi Shreya,
It is due to dynamic range range difference between length of IMDCT in short window Vs other window (e.g long window) case. In case of short window , the transform length  is 128 ( 2 power 7), while in case of long it is 1024 ( 2 power 10). The difference in two dynamic range is 3, which will explain different scaling.
Regards,
Mihir Mody,
Multimedia codecs group,
Texas Instruments India, Ltd,
Email : mihir ti.com
Phone : +91-80-25099307
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org]On Behalf Of Shreya Pathak
Sent: Wednesday, August 25, 2023 2:36 PM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] Mpeg4 AAC + why exp-=4 (for short windows) else exp-=7
Hi,
  In Mpeg4 AAC publicly available source code in file specrec.c and 
in function apply_scalefactors() 
#ifdef FIXED_POINT
            exp -= 25;
            /* IMDCT pre-scaling */
            if (hDecoder->object_type == LD)
            {
                exp -= 6 /*9*/;
            } else {
                if (ics->window_sequence == EIGHT_SHORT_SEQUENCE)
                    exp -= 4 /*7*/;
                else
                    exp -= 7 /*10*/;
            }
#endif
I got exp-=25, but I didn't get why this 
                if (ics->window_sequence == EIGHT_SHORT_SEQUENCE)
                    exp -= 4 /*7*/;
                else
                    exp -= 7 /*10*/;
is done.
Does any one have any idea on this ?
Please reply asap.
Thanks and Regards
Shreya
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