3 ms·
I don't quite get why scrambling XOR is necessary. Isn't avoiding consecutive bits already achieved by EFM?
by garaetjjte 7y ago
I don't quite get why scrambling XOR is necessary. Isn't avoiding consecutive bits already achieved by EFM?
- tenebrisalietum 7y agoI think because of you can still get long sequences of 1's or 0's depending on which bytes are next to each other. http://www.reverse-engineering.info/CD/EFM_Table.html http://www.reverse-engineering.info/CD/EFM_Table.html
- byuu 7y agoCorrect. And since the scrambling XOR pattern is known, it's possible to burn a disc that ends up being all zeroes or ones post-scrambling anyway. I'm told such sectors tend to be unreadable, but I haven't tried it.
- garaetjjte 7y agoBut there are also 3 "merge bits" between each EFM code, so this shouldn't matter? edit: The problem solved by scrambler is that number of pits and lands must be equal on average, or else drive couldn't distinguish pit and lands as dark/light decision signal level wouldn't be longer average of output signal. There is excellent description here: https://archive.org/download/CDCrackingUncoveredProtectionAgainstUnsanctionedCDCopyingKrisKaspersky/CD%20Cracking%20Uncovered%20-%20Protection%20Against%20Unsanctioned%20CD%20Copying%20-%20Kris%20Kaspersky.pdf https://archive.org/download/CDCrackingUncoveredProtectionAg..., page 33 in PDF under "Sync Groups, Merging Bits, and DSV" header)