Let me try and simplify it a bit…(but it’s not a one paragraph dealio).
Optical media, by nature, is error prone. A spec of dust, finger prints, scratches, etc., all lead to the laser having issues reading the original data (video, file info, etc.). Burners too, have to fight bad dye, dust, finger prints, small substrate faults, etc.
Parity (PI/PO) is the built-in correction algorithm used to check and correct these errors. What you have is an array of data that is verified by parity data. Here’s an over-simplification of how this works:
Here we have 4 data bits in a 2x2 array. We also have parity bits that we can use to test if the data bits are correct.
If you read the data bits across the top row, you’ll see we have a ‘1’ and a ‘0’, and the PO bit is 1, meaning that one data bit should be high…and the first bit is. If you read across the second row, you’ll see we have 2 data bits set to ‘0’ and the PO is 1…but this is wrong…one of those bits should be a 1. Now, if we do the same reading the columns (up & down) and using the PI error checking, we’ll see that the second column is also wrong. By this array of testing, we can deduce that the data bit in row 2, column 2 is incorrect, and we know what it should be:
This is an oversimplified example, and true PI/PO correction is quite extravegant (complex arrays of self correcting parity). This just shows you how optical media readers can correct bad data.
There is a finite amount of parity errors before the data is lost forever. (See Jamo’s post above)
To answer your original question:
Media with low PI/PO error means that most data is written and read correctly. Media with high PI/PO error means that much of the data has not been written correctly and needs to depend on correction to figure out what it should be.
The lower your virgin-burn PI/PO correction error, the less the reader has to work to correct bad data. The media with the lower PI/PO correction will be more tolerant of scratches, dirt, dust, etc.
Nature itself, over time, will degrade the quality of media. High parity errors to start with, will have less tolerance for additional PI/PO errors. You might just exceed the correctable amount of error.
WHEW…good finger workout…hope this helps.