This is the first time Ive implemented a Reed Solomon decoder, and Im having some strange problems , which I have not been able to debug. Ive implemented a RS(255,191) code. That makes it a lil hard to verify all steps of my C code by calculations on paper. Anyways, Ive tried Euclids and Berlekamp in C, I always get the correct error locations, but my magnitudes are not correct. If my Error locator polynomial is correct, then my magnitude should be correct too,(with Euclid, magnitude poly is also available at the end of calculations, I have also verified by calculating magnitude polynomial from error locator poly using (1 + S(x) )(error locator poly) mod x^(2t +1)). I even calculated Forney , on paper, since I thought if error locations are correct, the error must be in the next step.I get an incorrect answer even on paper. Any suggestions, on where to debug.Error locations are the roots of the error locator polynomial, so is it possible I get the first step correct answer because Im using some multiple of error locator polynomial.

# Reed Solomon decoder -help

Started by ●December 4, 2007

Reply by ●December 4, 20072007-12-04

Are you using the correct Forney formula? There are different formulas depending on the sequence of roots of the generator polynomial. See, for example, http://www.ifp.uiuc.edu/~sarwate/decoder.ps Your "wrong" error magnitude Z might just be equal to the correct error magnitude Y times X^i where X is the error location at which the error magnitude is Y. --Dilip Sarwate "Zeph80" <surabhi_talwar@hotmail.com> wrote in message news:7a6dncj7X75T5cjanZ2dnUVZ_tWtnZ2d@giganews.com...> This is the first time Ive implemented a Reed Solomon decoder, and Im > having some strange problems , which I have not been able to debug. > Ive implemented a RS(255,191) code. That makes it a lil hard to verify all > steps of my C code by calculations on paper. Anyways, Ive tried Euclids > and > Berlekamp in C, I always get the correct error locations, but my > magnitudes > are not correct. > If my Error locator polynomial is correct, then my magnitude should be > correct too,(with Euclid, magnitude poly is also available at the end of > calculations, I have also verified by calculating magnitude polynomial > from error locator poly using (1 + S(x) )(error locator poly) mod x^(2t > +1)). > > I even calculated Forney , on paper, since I thought if error locations > are correct, the error must be in the next step.I get an incorrect answer > even on paper. > > Any suggestions, on where to debug.Error locations are the roots of the > error locator polynomial, so is it possible I get the first step correct > answer because Im using some multiple of error locator polynomial.

Reply by ●December 6, 20072007-12-06

>Are you using the correct Forney formula? There >are different formulas depending on the sequence of >roots of the generator polynomial. See, for example, >http://www.ifp.uiuc.edu/~sarwate/decoder.ps >Your "wrong" error magnitude Z might just be >equal to the correct error magnitude Y times X^i >where X is the error location at which the error >magnitude is Y. > >--Dilip Sarwate > >"Zeph80" <surabhi_talwar@hotmail.com> wrote in message >news:7a6dncj7X75T5cjanZ2dnUVZ_tWtnZ2d@giganews.com... >> This is the first time Ive implemented a Reed Solomon decoder, and Im >> having some strange problems , which I have not been able to debug. >> Ive implemented a RS(255,191) code. That makes it a lil hard to verifyall>> steps of my C code by calculations on paper. Anyways, Ive tried Euclids>> and >> Berlekamp in C, I always get the correct error locations, but my >> magnitudes >> are not correct. >> If my Error locator polynomial is correct, then my magnitude shouldbe>> correct too,(with Euclid, magnitude poly is also available at the endof>> calculations, I have also verified by calculating magnitude polynomial >> from error locator poly using (1 + S(x) )(error locator poly) modx^(2t>> +1)). >> >> I even calculated Forney , on paper, since I thought if errorlocations>> are correct, the error must be in the next step.I get an incorrectanswer>> even on paper. >> >> Any suggestions, on where to debug.Error locations are the roots ofthe>> error locator polynomial, so is it possible I get the first stepcorrect>> answer because Im using some multiple of error locator polynomial.Well, I was not accounting for all even powers in the derivative for Forney's equation. Silly mistake which i didn't catch because for less than 3 errors my code always worked. And that would work, because with 2 errors, the derivative of error locator polynomial would be just one value with power 0.> > >

Reply by ●December 7, 20072007-12-07

Hi, If you have some experience with C++, you can use my RS library to generate test vectors (parities, correction locations and magnitudes), also print out the values of each step trivially - might help in debugging your own implementation. The library is scalled Schifra and can be downloaded from: www.schifra.com Arash Partow __________________________________________________ Be one who knows what they don't know, Instead of being one who knows not what they don't know, Thinking they know everything about all things. http://www.partow.net