Spreading function slopes 
Spreading function slopes 
Aug 21 2004, 07:23
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#1


MPEG4 AAC developer Group: Developer Posts: 398 Joined: 1June 03 Member No.: 6943 
About a year or 2 ago, there was a posting by someone by the name of Yan..
It was a very interesting issue.. which is to switch the slope of the spreading function so that the lower slope is less steeper than the upper slope.. Initially, I thought I found something interesting when I tried calculating the r_normalization factors and compared to the R_norm of the MP3 ISO specs.. I now think that Yan is wrong.. and MP3 ISO specs is wrong about the r_normal.. Both slopes should be identical in steepness.. Yan's proposal would result in audible artifacts.. Not only that.. refering to the orginal SNR equation : SNR = (14.5 + i) * tb +(1tb)* 5.5 this is when the spreading function slope is steeper in the upper slope than in the lower slope.. just as in Layer II and Layer I psychoacoustic model II.. when it was changed to SNR = 29 *tb + (1tb)*6.0 in MP3, both slope of the spreading function has to be modified to make them identical in steepness to compensate for the constant TMN.. Try working out the mathematical equations.. wkwai 


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