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

Shreya Pathak shreya_pathak rediffmail.com
Wed Aug 25 13:02:29 EDT 2004


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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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