Tremor's mdct backward function 
Tremor's mdct backward function 
Jun 6 2009, 14:49
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#1


Group: Members Posts: 5 Joined: 28May 09 Member No.: 70211 
Hi to all,
I'm trying to understand the Tremor's code that implements the iMDCT. First of all.. Does it implement iMDCT throught a mechanism of rotation and FFT? I'm not sure of it.. If it's true.. where does implement the presorting? Then.. the parameter "n" defines the dimension on witch the iMDCT is calculated (I know that the range on witch n is defined could be only between 64 and 8192 , multiples of 2). Is it the input's array dimension? Thanks for answers 


Jun 6 2009, 15:50
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#2


Group: Members Posts: 5598 Joined: 2September 02 Member No.: 3264 
First of all.. Does it implement iMDCT throught a mechanism of rotation and FFT? I'm not sure of it.. It does not. FFT based iMDCTs are very easy to spot since they nearly all look the same. See mdct.c in ffmpeg (or the unrelated but nearly identically written mdct.c in libfaad) for good examples of these. Then.. the parameter "n" defines the dimension on witch the iMDCT is calculated (I know that the range on witch n is defined could be only between 64 and 8192 , multiples of 2). Is it the input's array dimension? Correct. Its the input array's dimension (number of frequency domain samples), typically 256 or 2048 for vorbis. 


Jun 6 2009, 17:40
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#3


Group: Members Posts: 5 Joined: 28May 09 Member No.: 70211 
It does not. FFT based iMDCTs are very easy to spot since they nearly all look the same. See mdct.c in ffmpeg (or the unrelated but nearly identically written mdct.c in libfaad) for good examples of these. So.. what does implement?? How is it implemented? I thought that it was following the algorithm to implement a MDCT thorugh an FFT in the reverse way... 


Jun 7 2009, 00:43
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#4


Group: Members Posts: 5598 Joined: 2September 02 Member No.: 3264 
It does not. FFT based iMDCTs are very easy to spot since they nearly all look the same. See mdct.c in ffmpeg (or the unrelated but nearly identically written mdct.c in libfaad) for good examples of these. So.. what does implement?? How is it implemented? Check the comments in the code. The author gives a reference for his particular MDCT. Unfortunately, its a book which I've never bothered to track down so I can't say anything specific about it. However, if you search for IMDCTs, you'll see that theres quite a few different algorithms out there for computing them (though the FFT method is most popular). I thought that it was following the algorithm to implement a MDCT thorugh an FFT in the reverse way... The forward and backward MDCTs are nearly identical, and are extremely simple. Most are just a few lines of code for rotations, then an FFT, then a few more lines of code for post rotations. The algorithm in Tremor is clearly quite different since its both very long and has no FFT 


Jul 8 2009, 09:24
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#5


Group: Members Posts: 13 Joined: 8July 09 From: Brussels Member No.: 71300 
It does not. FFT based iMDCTs are very easy to spot since they nearly all look the same. See mdct.c in ffmpeg (or the unrelated but nearly identically written mdct.c in libfaad) for good examples of these. So.. what does implement?? How is it implemented? Check the comments in the code. The author gives a reference for his particular MDCT. Unfortunately, its a book which I've never bothered to track down so I can't say anything specific about it. However, if you search for IMDCTs, you'll see that theres quite a few different algorithms out there for computing them (though the FFT method is most popular). I thought that it was following the algorithm to implement a MDCT thorugh an FFT in the reverse way... The forward and backward MDCTs are nearly identical, and are extremely simple. Most are just a few lines of code for rotations, then an FFT, then a few more lines of code for post rotations. The algorithm in Tremor is clearly quite different since its both very long and has no FFT Hmmm, , actually the mdct_backward function in Tremor's mdct.c file does implement an FFT. I see bitreverse, butterfly etc methods, all the building blocks of a good old Fast Fourier Transform: http://www.dspguide.com/ch12/2.htm 


Jul 8 2009, 16:06
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#6


Group: Members Posts: 5598 Joined: 2September 02 Member No.: 3264 
Hmmm, , actually the mdct_backward function in Tremor's mdct.c file does implement an FFT. I see bitreverse, butterfly etc methods, all the building blocks of a good old Fast Fourier Transform: http://www.dspguide.com/ch12/2.htm If you can pick out a DFT in there, then good work! But don't assume that just because theres butterflies and bit reversal that its an FFT. The MDCT is itself a discretized Fourier transform, so it can be computed using much the same decomposition methods. 


Oct 14 2009, 11:43
Post
#7


Group: Members Posts: 10 Joined: 9May 06 From: Mumbai Member No.: 30588 
Hi to all, I'm trying to understand the Tremor's code that implements the iMDCT. First of all.. Does it implement iMDCT throught a mechanism of rotation and FFT? I'm not sure of it.. If it's true.. where does implement the presorting? Then.. the parameter "n" defines the dimension on witch the iMDCT is calculated (I know that the range on witch n is defined could be only between 64 and 8192 , multiples of 2). Is it the input's array dimension? Thanks for answers Hi, i am working on MDCT Backward of tremor's code. i need to replace this to Duhamels algorithim. can u plz help me out by any suggesstions or inputs?? i need to replace existing MDCT to duhamels FFT so as to increase the performance. Thanks a lot. Regards, Ruchi 


Oct 14 2009, 14:26
Post
#8


Group: Members Posts: 5598 Joined: 2September 02 Member No.: 3264 
Hi, i am working on MDCT Backward of tremor's code. i need to replace this to Duhamels algorithim. can u plz help me out by any suggesstions or inputs?? i need to replace existing MDCT to duhamels FFT so as to increase the performance. Thanks a lot. I tried several FFT based IMDCTs in place of tremor's and all were slower. You will need a very fast FFT algorithm to beat Tremor. Lately I've been working on a slit radix version, but I have no idea if its going to be any faster If you want to try, take a look at one of the mdct.c files mentioned above and use your own FFT. This post has been edited by Mike Giacomelli: Oct 14 2009, 14:26 


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