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Audio CD ROT, Red Book CD-DA data recovery, Disc Rot, How to recover precious audio CDR from set of four that are rotting
post Jan 24 2012, 23:03
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Project: recover precious audio CDR from set of four that are rotting (using manual methods to maximize quality of recovered data)

In about 1996, my mother recorded herself singing 1940s folk songs with her brothers and sister. In about 2000, my cousin with a "recording studio" made a set of CDRs.

The CD copies worked when they were new. Now they are all badly rotted -- they look fine, but will not play. The CDs are greenish with full inkjet-printed labels. The ring is printed "119-H.002062194B09." (Does this tell who made the disc and when? It seems to be a serial number -- each disc has a different number, but the first part is always the same.)

This is my first experience with this type of bit rot. (If only I had known about the problem when it started!) Since this is precious material, I will spend endless hours trying to recover it as exactly as possible. I have not found any good tools or instructions for how to do this, so I will try to describe my plans and experiences here, in case they are helpful.

These CDs are almost impossible to read in an ordinary way. Occasionally it is possible to see the TOC. But EAC seems useless because even the lowest error-check setting seems to be too picky, and makes no progress reading the data.

I tried a pile of various CD/DVD drives in the hope that one would be magic, but no luck. The only drive that ever sees the TOC is a Rosewill ROD-EX001 which contains a Teac DV-W28S-R 1.0B. The drive only sees the TOC when it is cold, under 60-degF. Perhaps 40-50degF is best? Why does cold help?

It seems like the ideal way to recover data from severe disc rot would be to make micro-photo images and process the images. I don't have that technology, and have found no discussion of such serious forensics.

I found no tools for reading raw CD data, when the disc does will not "mount" (there seems to be some sort of auto-scan when the disc is inserted in the drive, and everything seems to hang until the drive is happy). Since EAC seemed unwilling to give me bad data, I discovered that fre:ac is more willing to give raw data. I could not use any included cdparanoia features, but was able to use jitter correction.

Out of the four discs, I was able to read one five times and one two times, so I have seven sets of 34 tracks. In general, they sound noisy, but the original sound is very audible. The rot seems worst at the beginning, and somewhat at the end.

The only relevant processing tool I have found so far is Audacity. There are various ways I could auto-process this data, but I want maximum control and understanding of the process. I am faced with an overwhelming quantity of data. To reconcile all these bits manually could take years!

I am starting with a short track in good shape. Audacity is usable (though inconvenient) for comparing tracks to see the differences. Because I am dealing with digital errors, I was expecting big, radical noise, that would be easy to spot. Wrong. The errors are quite subtle -- the hidden error correction is doing a very good job of guessing. This would be great if I were willing to just average the waveforms together or pick the best. But since I'd like to try to delete the errors, and use the "good parts" as much as possible, the fact that the errors are so minor will make finding them much harder!

It looks like the sound is not truly stereo - one track seems about one sample delayed and slightly different in amplitude compared to the other. I hope that the error correction did not know about this, and processed each channel independently. (Is this true?) My next step is to cross-correlate the channels to find the exact time-shift relationship, and figure out the amplitude relationship. Then when I am processing the data, and get to a point in dispute, I can use the level of discrepancy between the channels within a dataset to flag which data points are likely to be more unreliable. Well, that's my hope... I expect I'll have to write programs in BASIC to explore these approaches. Any better ideas? Do any programs exist for reconciling multiple sets of nearly-identical audio sound files? (Currently working on a not-powerful WindowsXP computer.)
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post Jan 25 2012, 05:47
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Cross-correlating the left and right channels of track 23 shows that the first (left) channel lags behind by almost two samples (actually somewhere around 1.7 samples, but I'd rather avoid interpolating samples for now).

The next step is to try to do a x-y scatter plot of the left-right channels, offset by this optimal 2-sample correlation peak, to get a feeling for how closely the channels match and whether it seems worth using the ratio of sample values as a measure of data validity.

This ugly little BASIC program was used to do the cross-correlation. It shows that for a simple LPCM .wav file it is easy to just skip over the header bytes and access the data area.

' CDrot.bas (SmallBASIC FLTK 0.10.6 Windows XP)
' CD disc rot processing
' kd 24 jan 2012

DIM LL(9),RR(9),LR(9)
? "THIS IS A TEST of CDrot.bas"

IF L>(2^15) THEN L=L-(2^16)
IF R>(2^15) THEN R=R-(2^16)

IF I<13 THEN 199


FOR T = 1 TO 9
LR(T) = LR(T) + LL(5)*RR(T)

IF (L=0) AND (R=0) THEN 199

SM = SM + L*R
' PRINT I;": ",L,R," ";ISM;" - ";L*R;" - ";INT(SM/ISM);" - ";SM,,ILR;LR


199 WEND
? "END TEST of CDrot.bas"

This post has been edited by db1989: Jan 25 2012, 21:04
Reason for edit: For large items, please use [codebox] rather than [code].
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post Jan 25 2012, 20:59
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Unfortunately, Audacity seems to have no function to automatically optimally align two waveforms (time-shift, amplitude, DC offset) for comparison purposes. And no way to visually superimpose two waveforms to see discrepancies.

Here is a manual proceedure for Audacity:
* Split stereo to mono
* Normalize each track. (If the amplitude match does not seem perfect, you could manually reduce the amplitude of one channel to improve the match.)
* Time-shift one channel as needed. To do this, zoom in on high-amplitude sections. Look for crisp-clear zero crossings that you can time-align. Keep zooming in until you clearly see individual samples that you can align exactly.
* After time-aligning, Invert one channel, then select both channels and apply "Mix and Render".
* Study and ponder this difference waveform. Can you think of any adjustments that would have yielded a smaller result? You can re-do the process with different amounts of time shifting, to see what is the best match, with the minimum difference.

So, I did not really need to write a BASIC program to do the cross-correlation -- this tedious manual process with Audacity would have been quicker.

And now that I see the resulting difference between the left and right channels, it looks like there is more stereo content than I had hoped. It does not look like I'll be able to identify bad data points very well by comparing the left and right channels. I'd still be curious to see an X-Y scatter plot of the channel values to study the degree of correlation, but that approach doesn't seem worth pursuing for now.

My plan is to proceed to write a BASIC program to find sets of data points that are different among the 7 copies, and see what ideas emerge from pondering them.
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Posts in this topic
- kethd   Audio CD ROT, Red Book CD-DA data recovery, Disc Rot   Jan 24 2012, 23:03
- - kethd   Cross-correlating the left and right channels of t...   Jan 25 2012, 05:47
|- - kethd   HOW TO USE AUDACITY TO COMPARE TWO CHANNEL TRACKS ...   Jan 25 2012, 20:59
|- - kethd   BASIC PROGRAM TO MEASURE CORRELATION OF LEFT AND R...   Jan 26 2012, 19:25
|- - kethd   BASIC PROGRAM UTILITY TOOL TO INSPECT BLOCKS OF DI...   Jan 27 2012, 20:53
|- - krabapple   Dude, seriously, you'll be better off just re-...   Jan 30 2012, 04:11
||- - kethd   QUOTE (krabapple @ Jan 29 2012, 22:11) Du...   Feb 3 2012, 06:23
|- - kethd   BASIC PROGRAM WAV-FILT-LIN TO RECONCILE MULTIPLE S...   Feb 3 2012, 06:33
|- - kethd   CD disc rot processing WAV-FILES-COPYR.bas COP...   Feb 4 2012, 04:12
- - hlloyge   Well, I am sorry, you're out of luck there - b...   Jan 25 2012, 08:16
|- - kethd   QUOTE (hlloyge @ Jan 25 2012, 02:16) You ...   Jan 25 2012, 20:16
- - 2Bdecided   To ask the bleeding obvious, what format were thes...   Jan 25 2012, 13:10
|- - kethd   QUOTE (2Bdecided @ Jan 25 2012, 07:10) To...   Jan 25 2012, 20:28
|- - 2Bdecided   QUOTE (kethd @ Jan 25 2012, 19:28) I have...   Jan 26 2012, 12:35
- - DVDdoug   QUOTE I'd still be curious to see an X-Y scatt...   Jan 25 2012, 21:24
- - GHammer   I have had luck in the past with ISOBuster. Haven...   Jan 26 2012, 02:29
|- - kethd   QUOTE (GHammer @ Jan 25 2012, 20:29) I ha...   Jan 26 2012, 19:08
- - gottogo99   I totally agree with 2Bdecided. Play the old cass...   Jan 26 2012, 13:55
|- - kethd   QUOTE (gottogo99 @ Jan 26 2012, 07:55) I ...   Jan 26 2012, 19:17
|- - gottogo99   QUOTE (kethd @ Jan 26 2012, 13:17) Unfort...   Jan 26 2012, 20:04
- - AndyH-ha   isobuster can be downloaded and installed and used...   Jan 26 2012, 21:41
- - kct_99   Have you tried out ExactAudioCopy yet ? http://exa...   Jan 27 2012, 03:15
|- - kethd   QUOTE (kct_99 @ Jan 26 2012, 21:15) Have ...   Jan 27 2012, 20:47
|- - greynol   QUOTE (kct_99 @ Jan 26 2012, 18:15) This ...   Jan 27 2012, 22:52
- - mjb2006   You might want to try the abandonware PerfectRip. ...   Jan 27 2012, 22:43
|- - kethd   QUOTE (mjb2006 @ Jan 27 2012, 16:43) You ...   Jan 29 2012, 23:48
- - knutinh   A topic that facinates me: Is it possible to get r...   Jan 28 2012, 09:58
- - LordWarlock   There are tools that allow low level reading for d...   Jan 28 2012, 16:59
- - kennedyb4   QUOTE (gottogo99 @ Jan 26 2012, 07:55) I ...   Jan 28 2012, 17:15

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