how does a CD still play normally when it’s scratched?
In: Mathematics
CDs have extra data encoded on them. This allows the player to calculate what data it has not been able to read if it’s obscured by a scratch.
All the data on the disc is also re-arranged in a way that is best able to cope with intermittent read errors caused by scratches, dust, etc.
The system is called “Cross-interleaved Reed–Solomon coding” [wiki page](https://en.wikipedia.org/wiki/Cross-interleaved_Reed%E2%80%93Solomon_coding)
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