The errors we are scanning for are correctable errors. CDs and DVDs use a two-layered Cyclic Redundancy Check (CRC) method to correct bit errors on the media. This can be done because “extra” bits are written to the media in such a way that errors can usually be detected and corrected, or when the media is really damaged it can only be detected but not corrected.
CDs use C1 and C2 checksums. There will always be C1 errors on a disc. Uncorrectable C1 errors can usually be corrected by the C2 check. A good disc shouldn’t have any C2 (correctable) errors on it, but even with C2 errors it will usually still be readable in a decent drive. Uncorrectable C2 errors are known as CU, and you definitely shouldn’t have any CU errors on a disc. Even with CU errors the drive might be able to read the correct data by automatically re-reading the problematic portion of the disc. When drives encounter large number of errors they will normally slow down the spinning of the disc so that reading becomes easier for the laser.
DVDs use PI (Parity Inner) and PO (Parity Outer) checksums, which are generally considered superior to the C1/C2 error correction of CDs. There will always be PI errors (PIE) on a disc. Uncorrectable PI errors (known as PIF - PI Failed) can usually be corrected by the PO check. A good disc should have few PO (correctable) errors (POE) on it. Uncorrectable PO errors are known as POF (PO Failed), and you definitely shouldn’t have any POFs on a disc. Even with POF the drive might be able to read the correct data by automatically re-reading the problematic portion of the disc. When drives encounter large number of errors they will normally slow down the spinning of the disc so that reading becomes easier for the laser.
Interpreting the scans is not an exact science, and it depends on which scanning program and drive you are using. When scanning you are not only testing the burner but also the reading capabilities of the drive you use for scanning.
DVD standards (I don’t recall the name right now) specify acceptable limits for a DVD; PIE rate should be no more than 280 and POE rate should be no more than 4. In PlexTools these limits are shown as a horizontal line in the Sum8 and Sum1 tests.
When comparing scans made with different scanning programs, you should be careful not to compare apples with oranges; PIE and PIF are very different, but are often both labeled as just “PI”, and POE and POF are also very different but usually just labeled “PO”.
Even without a proper scanning program (or without a drive that supports scanning) you can get a reasonable idea of the disc quality by doing a read transfer speed graph in e.g. Nero CD-DVD Speed; if you get a smooth graph it means that the drive didn’t have to slow down due to an excessive (correctable) error rate.
You don’t normally have to worry that data written to optical media becomes corrupted (0110 in the source, and maybe an 0010 in the target disc). Many CD/DVD burning programs, including Nero Burning ROM, provides you with the option to verify the data after it has been written to disc - usually by checking an appropriate “Verify” checkbox somewhere.