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Editing A Wave With A Program..., How would I do it?
post Nov 4 2002, 20:43
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I have some wave files from a set of cd's that I'm trying to fix. They have what I'd call "digital screw-up" on them (these are purchased cd's, yet have artifacts like a bad cd-r)..

It appears that groups of 5 samples in the right channel are offset by varying amounts, in patches throughout the tracks. It sounds incredibly annoying, and I have tried lots of different ways to fix them, filtering, click removal, interlacing other sections of the song over them. They definately improve the sound quality, but naturally, I want what they *originally* sounded like!

What I dawned on recently though was that the affected samples, and their offsets seem to follow a pattern. You get regularly spaced groups of 5, with regular spacing between each bad sample. And the amount it is offset by, from what I can tell so far, it appears to be by the difference between it's actual value and the value before it.. which would be hard to work out mathematically, because how are you meant to know what it "should" be?

Anyway, the best way I thought to fix this problem would be to get a friend to write a program that manipulated the samples in mathematical form. I can do maths, so could probably find a reasonable way of correcting the samples, then he could write a program that performed the algorithm on the correct samples in a wave.

My question is, could this be done by converting a wave file into a file that contains the sample values as integers? Or is this what a wave file is, meaning it could be acted on directly by a program that could search through the values and alter certain ones?

Thanks! smile.gif

(i can post samples for any kind person who wants to help figure out how to fix them!)

Ok, I've looked at the files some more, and noticed something quite obvious. The problem samples all take the same value as their preceding sample. I found that taking the difference between the corresponding left channel's sample and it's preceeding sample was a good approxomation (it moved the bad samples in the right direction and close to the right amount), and that would be why.

Is there a way of predicting how the samples should be changed to follow their natrual curve (as would be hinted at by the preceeding and following samples?) It would need to be specifically for when one sample is of unknown value, but the others are correct.. I'm not sure if standard click-restoration in Cool-Ed can deal with this.. just single bad samples, rather than a click in a vinyl which spans dozens of samples...

This post has been edited by Mac: Nov 4 2002, 21:08

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post Nov 9 2002, 13:12
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You can use what I would call polynomial interpolation :

1st degree : from samples x1, x2, x3,
That's the way cd players correct errors. Maybe your samples already match this formula.

2nd degree x1,x2,x3,x4 :
If the origin of the coordinates is X1, and the interval between the samples is 1, and the parabol formula is
y=(b*x+a)^2 + c

solving the system


Knowing X1, X2 and X4 will give you a, b, c, and X3.

Besides, I would be very interested in the exact pattern your samples are coming in.
In the DAE quality analysis I'm running, I found some groups of 60 samples, showing mostly groups of 3 (bad-good-bad-good-bad) separated by 24 ones.
After a lot of research in the rare and wrong documents about CIRC on the web, and some help from BobHere, in EAC forum, I found that it was the exact pattern of a 17 frames burst error passed through a CIRC chipset correcting 4 wrong symbols at the C2 stage.
I'd be curious to see if yours match a 2 or 3 errors chipset.
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