So, in reality on DVD the following scheme is applied
A block of data is written, at the end of which there is information for integrity checking and error recovery. An error occurring inside such a block is called PI. (Parity Inner) (The numbers in the picture mean nothing, they are just for beauty)
Data
Information for error correction within the block
{1 0 0 1 0 0 1 0} {0 1 1 0}
But what happens if there are so many errors in the block that they cannot be corrected using the redundant data at the end of the block itself? For this situation, after every 208 blocks, additional blocks are written with redundant data meant for correcting entire bad blocks that couldn't be fixed by the data at the end of the block. (I apologize for endlessly repeating the word Block, but then again I love him like the poet)
Data blocks
Information for error correction within the block
Error block correction area
{1 0 0 1 0 0 1 0} {0 1 1 0}
{1 0 1 0 0 0 1 0} {0 1 0 0}
{1 0 1 0 1 0 1 0} {0 1 0 1}
{1 0 1 0 0 1 1 1} {1 1 1 1} . . . . . etc. 208 times
{1 0 1 0 0 1 1 1} {1 1 1 1}
{1 1 1 0 1 1 1 1} {1 0 0 1}
{1 0 1 1 1 1 1 1} {1 0 0 0}
An uncorrectable error inside a block is called PIF (Parity Inner Failure) or PO (Parity Outer), and such a block is corrected entirely using the data marked in blue. However, if there are too many bad blocks, it's impossible to correct them, and the drive cannot read the disc, throwing a CRC Error message, which actually doesn't correspond to reality (for optical discs). It should be saying Unrecoverable PO errors. For us, the most interesting thing is that on Lite-On DVD drives you can check the error level using the Nero CD-DVD Speed or K-Probe programs. Drives from Sony and some other brands - are also Lite-On). In Nero CD-DVD Speed, which is almost always included in the NERO suite, you have to select from the menu "Disc Quality Test" and set the speed to 4x, which is the standard for checking quality.
So, up to a certain number of errors, the drive's microchip can correct them and continue reading. I won't give exact calculations and explanations here, I'll just say that for our purposes it's enough to know that the PI level displayed by the programs shouldn't stay above 280 for a long time, and PO (PIF) - above 32. Isolated peaks don't count - they might be read or write glitches. But more or less long stretches will make the disc unreadable, although in practice a lot depends on the reading drive. Naturally, a lot also depends on the recording device. Plextor and Pioneer used to be famous, now cheap and high-quality-writing NECs are popular. Unfortunately, the saying "the Chukchi isn't a reader, the Chukchi is a writer" works 100% here - it happens that NEC drives can't even read the discs they burned themselves (I mean the case of horribly low-quality discs). But blanks are crucial - even in the worst drive, TY will most likely burn perfectly and be preserved for a very long time (excluding cases of incompatibility between the drive and discs, which happens)

