Posted December 21, 2011
I finally had some time to look behind the cryptic abbreviations in Nero Disc Speed. So, if anyone's interested in testing the quality and/or error-free-ness of DVDs, here's a walkthrough with some explanations. The following procedure assumes that you have a data DVD (not a CD, not a video or audio DVD) that you want to test, and a DVD burner to test it in.
1. Insert the DVD and start Nero Disc Speed
2. In the drop down menu at the top of the window, select the drive which holds the disc to test.
3. Click on the rightmost tab, which should read "ScanDisc". You apparently need a disc burner (not just a reader) for this tab to appear.
4. Look right for the "Settings" section. Here you have two choices, the first one is the speed at which the disc will be read. Enter "maximum" for normal test (the disc will be read at maximum speed, which means that some data might end uo being read as erroneous although it could be read at a slower speed), or a slower speed for a more lenient test.
5. The second choice is the type of test to perform. A "Read test" simply checks whether the data can be read correctly, whereas a "PI/PO test" gives a bit more information about the quality of the data (mainly whether the data read from the sector was correct, or whether it was not and had to be corrected by the DVD's error correction algorithm, see below for more details on that). Choose the test type you want, and click "Start".
6) If you're doing a "Read Test", then every block in the display should be green. If a block is not green, then you have unreadable data on the disc. That's all that a read test can determine. Note: Check the color pattern in the graphical display, not the percentages shown in the "Surface Scan" section. The percentages are rounded, and it's possible that DVDs with very few errors still show a value of 100% good sectors there.
7) For a "PI/PO test" (disregard the terms C1 and C2 that also appear there, they apply only to CDs, not DVDs), the matter is slightly more complicated. An explanation follows:
To understand the output from the PI/PO test, we need some technical information:
- DVDs are almost burned perfectly, each DVD has some mistakes in the data.
- To make DVDs a reliable means of storage nevertheless, the DVD standard implements a two-tiered system of error correction. This system ensures that a DVD with _some_ mistakes in the physical data can still be read perfectly.
- The error correction works through parity information (basically a form of checksums). You can imagine the data on a DVD as organized in rows and columns. When burning a DVD, checksums for each row and each column are automatically calculated and written to the disc too. If a block of data cannot be read correctly, then its correct value can be calculated with the help of the checksums. All this happens automatically in the background and you usually don't even notice it.
- The two-tiered error correction system of DVDs works like this:
First, the reader a row of data and compares it with the checksum for that row. If the data turns out to be correct, it gets sent to the data stream.
Otherwise, the reader tries to correct the data, using the row's checksum. This is the first tier of error correction, and the row checksums are called "inner code parity".
If this attempt fails, then the reader tries to correct the data using the _column's_ checksum. This is the second tier of error correction, and the column checksums are called "outer code parity".
As long as either tier of error correction works, the sent data will be correct in the end. However, it is possible that the amount of damaged data is so large that it can't be corrected any more. In that case, both tiers of error correction fail and the data can't be read.
- The PI/PO test makes this error information visible: It shows whether the data on the disc was correct and (if it isn't) whether and how it could be corrected. The test counts the occurrences of the following events:
PIE: Parity Inner Error. A block of data in the a row was incorrect, but could be corrected with the help of that row's checksums.
PIF: Parity Inner Fault. A block of data in the a row was incorrect and could NOT be corrected with the help of that row's checksums. This creates 1 or more POE:
POE: Parity Outer Error. A block of data in a column was incorrect, but could be corrected using that column's checksums
POF: Parity Outer Failure: A block of data in a column was incorrect and could NOT be corrected in any way. The data is wrong or lost.
The PI/PO test in Nero Disc Speed shows the location and numbers of these errors. In the graphical display, yellow squares denote blocks that were damaged but could be corrected, and red squares are (according to the help file) supposed to show sectors with errors that could not be corrected, though I strangely had files with such sectors that still weren't harmed. Perhaps there are different shades of red? In any case, red squares are not a good sign.
At the bottom of the window, the files on the disc will be listed. Damaged files will be shown in red. For each file you can see the number of PI errors and faults. Files that have many errors are shown in read. Note that the DVD specification defines less than 280 PI errors as okay, 280 is the (conservative) threshold from which onwards PI faults are to be expected. As far as I understand, the table is also supposed PO faults, but I haven't seen any yet. This may be due to my discs being intact - since a POF is an uncorrigible error, they are supposed to be missing.
If you haven't finalized a disc yet, you can use this list to read "endangered" files as long as that's possible, and burn them on the disc again. Of course, re-burning the whole data to a new disc would be better, but also costs a new disc.
Last note: Alternatively, you can also use the "Disc Quality" tab instead of the "ScanDisc" tab. The "Disc Quality" test shows PIE, PIF, and POF as well. If you just want to ensure that all the data is still there, you can run a "Disc Quality" test and just check whether any POF's pop up. However, the Disc Quality test won't show you which files are damaged.
Also (very last note) I'm not sure whether data on a disc that's currently in bad shape but still 100% readable (many PIEs and PIFs, but no POF) should be reburned. If DVDs degrade over time, then the answer would be yes, because there's already a lot of error correction going on when reading the disc, and further degradation might push it to the point where the error correction can't compensate any more. However, I do not know if and how fast (as in additional PIEs/PIFs per year) DVDs degrade. I do know that re-recordable DVDs and DVD-RAMs can show degradation effects within half a year or even less, but they are produced differently than DVD-Rs or DVD+Rs and shouldn't be used for long-term storage anyway.
Anyway, that's the extent of my current knowledge. I can't guarantee that all of it is corrected because I've just started researching it today. But perhaps someone finds it useful. :)
1. Insert the DVD and start Nero Disc Speed
2. In the drop down menu at the top of the window, select the drive which holds the disc to test.
3. Click on the rightmost tab, which should read "ScanDisc". You apparently need a disc burner (not just a reader) for this tab to appear.
4. Look right for the "Settings" section. Here you have two choices, the first one is the speed at which the disc will be read. Enter "maximum" for normal test (the disc will be read at maximum speed, which means that some data might end uo being read as erroneous although it could be read at a slower speed), or a slower speed for a more lenient test.
5. The second choice is the type of test to perform. A "Read test" simply checks whether the data can be read correctly, whereas a "PI/PO test" gives a bit more information about the quality of the data (mainly whether the data read from the sector was correct, or whether it was not and had to be corrected by the DVD's error correction algorithm, see below for more details on that). Choose the test type you want, and click "Start".
6) If you're doing a "Read Test", then every block in the display should be green. If a block is not green, then you have unreadable data on the disc. That's all that a read test can determine. Note: Check the color pattern in the graphical display, not the percentages shown in the "Surface Scan" section. The percentages are rounded, and it's possible that DVDs with very few errors still show a value of 100% good sectors there.
7) For a "PI/PO test" (disregard the terms C1 and C2 that also appear there, they apply only to CDs, not DVDs), the matter is slightly more complicated. An explanation follows:
To understand the output from the PI/PO test, we need some technical information:
- DVDs are almost burned perfectly, each DVD has some mistakes in the data.
- To make DVDs a reliable means of storage nevertheless, the DVD standard implements a two-tiered system of error correction. This system ensures that a DVD with _some_ mistakes in the physical data can still be read perfectly.
- The error correction works through parity information (basically a form of checksums). You can imagine the data on a DVD as organized in rows and columns. When burning a DVD, checksums for each row and each column are automatically calculated and written to the disc too. If a block of data cannot be read correctly, then its correct value can be calculated with the help of the checksums. All this happens automatically in the background and you usually don't even notice it.
- The two-tiered error correction system of DVDs works like this:
First, the reader a row of data and compares it with the checksum for that row. If the data turns out to be correct, it gets sent to the data stream.
Otherwise, the reader tries to correct the data, using the row's checksum. This is the first tier of error correction, and the row checksums are called "inner code parity".
If this attempt fails, then the reader tries to correct the data using the _column's_ checksum. This is the second tier of error correction, and the column checksums are called "outer code parity".
As long as either tier of error correction works, the sent data will be correct in the end. However, it is possible that the amount of damaged data is so large that it can't be corrected any more. In that case, both tiers of error correction fail and the data can't be read.
- The PI/PO test makes this error information visible: It shows whether the data on the disc was correct and (if it isn't) whether and how it could be corrected. The test counts the occurrences of the following events:
PIE: Parity Inner Error. A block of data in the a row was incorrect, but could be corrected with the help of that row's checksums.
PIF: Parity Inner Fault. A block of data in the a row was incorrect and could NOT be corrected with the help of that row's checksums. This creates 1 or more POE:
POE: Parity Outer Error. A block of data in a column was incorrect, but could be corrected using that column's checksums
POF: Parity Outer Failure: A block of data in a column was incorrect and could NOT be corrected in any way. The data is wrong or lost.
The PI/PO test in Nero Disc Speed shows the location and numbers of these errors. In the graphical display, yellow squares denote blocks that were damaged but could be corrected, and red squares are (according to the help file) supposed to show sectors with errors that could not be corrected, though I strangely had files with such sectors that still weren't harmed. Perhaps there are different shades of red? In any case, red squares are not a good sign.
At the bottom of the window, the files on the disc will be listed. Damaged files will be shown in red. For each file you can see the number of PI errors and faults. Files that have many errors are shown in read. Note that the DVD specification defines less than 280 PI errors as okay, 280 is the (conservative) threshold from which onwards PI faults are to be expected. As far as I understand, the table is also supposed PO faults, but I haven't seen any yet. This may be due to my discs being intact - since a POF is an uncorrigible error, they are supposed to be missing.
If you haven't finalized a disc yet, you can use this list to read "endangered" files as long as that's possible, and burn them on the disc again. Of course, re-burning the whole data to a new disc would be better, but also costs a new disc.
Last note: Alternatively, you can also use the "Disc Quality" tab instead of the "ScanDisc" tab. The "Disc Quality" test shows PIE, PIF, and POF as well. If you just want to ensure that all the data is still there, you can run a "Disc Quality" test and just check whether any POF's pop up. However, the Disc Quality test won't show you which files are damaged.
Also (very last note) I'm not sure whether data on a disc that's currently in bad shape but still 100% readable (many PIEs and PIFs, but no POF) should be reburned. If DVDs degrade over time, then the answer would be yes, because there's already a lot of error correction going on when reading the disc, and further degradation might push it to the point where the error correction can't compensate any more. However, I do not know if and how fast (as in additional PIEs/PIFs per year) DVDs degrade. I do know that re-recordable DVDs and DVD-RAMs can show degradation effects within half a year or even less, but they are produced differently than DVD-Rs or DVD+Rs and shouldn't be used for long-term storage anyway.
Anyway, that's the extent of my current knowledge. I can't guarantee that all of it is corrected because I've just started researching it today. But perhaps someone finds it useful. :)
