how does a CD still play normally when it’s scratched?

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how does a CD still play normally when it’s scratched?

In: Mathematics

Basically, CDs are being read like reflections and when the scratches are small enough the reflection will still produce the proper sound.

Or I think the scratch can also sometimes be in a spot where there’s no music recorded.

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An old vinyl record is bumpy because the needle actually reeds the grooves directly. CD’s use a lazer. The lazer doesn’t read the actual surface of the CD, it can penetrate layers. So when you scratch a CD unless it’s deep you aren’t scratching the layer that holds the info.

For two reasons,

1. CD players have over the years become a lot better at dealing with scratches and other types of read errors. They are, simply put, decently good at overlooking optical issues.

2. The audio stream format on the disc, PCM, is deliberately designed to allow a misread here and there, and still produce sound that doesn’t get interrupted. It both has a massive amount of data that can be used to recreate data that went missing in the stream and, to be frank, an excess amount of data to begin with so that it’s not really the end of the world if a small amount goes missing.

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)