How is a severely scratched CD still readable sometimes?

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I’ve noticed this on DVDs and BluRays but also with games on any console. I’ve rented a movie and the disk is riddled with scratches and I’m sure it’s not going to read, but it somehow does. However, sometimes it hitches. But playing it back on a different DVD player presents no issues at all. What’s happening here?

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12 Answers

Anonymous 0 Comments

The data on a CD is actually on the top part where the graphic is. It’s a very thin film that holds the data. The rest of the CD is the plastic disc part that gives the data something rigid to hold onto. Most likely, it’s the under plastic part that gets scratched up and not the data itself. Disc readers use light to read the data through the plastic. Sometimes it can read the data through scratches, sometimes it can’t. That’s why when people perform CD repair, it’s usually some kind of liquid resin that just fills in the scratch gaps so that the plastic part can be transparent again.

Anonymous 0 Comments

CDs and DVDs have huge amounts of error correction built into the data stream. Each packet of data contains multiple backups, and then there are backups to the backups with different data in between.

The error correction on an audio CD is so strong, that you could drill a 2.5 mm diameter hole through a CD, and there should be no data loss at all. CD-ROMs have even stronger error correction. DVDs even stronger again.

The biggest problem with scratched discs is that the laser beam gets blurred out or distorted, and the laser is unable to see the line of data clearly. A series of lenses and sensors keep the laser perfectly aligned onto a thin spiral line of data. However, if the laser focus isn’t good enough, it may not be able to keep track, and it may jump to a different track, causing the sound to skip – or it may struggle to read a piece of data. Locking back onto a track can take long enough that the laser goes way past any backup copies of the data.

Anonymous 0 Comments

Not all of the data on a disc – audio or video – is pure content. There’s a decent amount of overhead devoted to providing error-correcting data for the player to read and fill in gaps as needed. Players can also do small amounts of interpolating (creating data to replace missing info).

[This video from the amazing Technology Connections channel](https://www.youtube.com/watch?v=sAbhPeTp51s&ab_channel=TechnologyConnections) gives a ton of info on the design of the Compact Disc in general, and discusses its error-correction design elements around the seven-minute mark. It’s worth watching the whole thing, though. For video the exact numbers will be different but the error-correcting principles are the same.

Anonymous 0 Comments

Audio CD, data CD, DVD, and Blue Ray all have several layers of error correction, data scattering, and redundancy built into the specifications. They are not like a vinyl LP, which has a single track of analog data, and no error correction. (Although, there exist modern playback systems which can digitally reduce hiss, pop, and scratch noise on the fly. None of that is in the LP spec.)

Different results in different DVD players can be the result of dirt on the internal optics, mechanical misalignment, or even the movement of a few particles of dust on the disc when moved between players.

Anonymous 0 Comments

The data on a CD is actually on the top part where the graphic is. It’s a very thin film that holds the data. The rest of the CD is the plastic disc part that gives the data something rigid to hold onto. Most likely, it’s the under plastic part that gets scratched up and not the data itself. Disc readers use light to read the data through the plastic. Sometimes it can read the data through scratches, sometimes it can’t. That’s why when people perform CD repair, it’s usually some kind of liquid resin that just fills in the scratch gaps so that the plastic part can be transparent again.

Anonymous 0 Comments

Not all of the data on a disc – audio or video – is pure content. There’s a decent amount of overhead devoted to providing error-correcting data for the player to read and fill in gaps as needed. Players can also do small amounts of interpolating (creating data to replace missing info).

[This video from the amazing Technology Connections channel](https://www.youtube.com/watch?v=sAbhPeTp51s&ab_channel=TechnologyConnections) gives a ton of info on the design of the Compact Disc in general, and discusses its error-correction design elements around the seven-minute mark. It’s worth watching the whole thing, though. For video the exact numbers will be different but the error-correcting principles are the same.

Anonymous 0 Comments

CDs and DVDs have huge amounts of error correction built into the data stream. Each packet of data contains multiple backups, and then there are backups to the backups with different data in between.

The error correction on an audio CD is so strong, that you could drill a 2.5 mm diameter hole through a CD, and there should be no data loss at all. CD-ROMs have even stronger error correction. DVDs even stronger again.

The biggest problem with scratched discs is that the laser beam gets blurred out or distorted, and the laser is unable to see the line of data clearly. A series of lenses and sensors keep the laser perfectly aligned onto a thin spiral line of data. However, if the laser focus isn’t good enough, it may not be able to keep track, and it may jump to a different track, causing the sound to skip – or it may struggle to read a piece of data. Locking back onto a track can take long enough that the laser goes way past any backup copies of the data.

Anonymous 0 Comments

Audio CD, data CD, DVD, and Blue Ray all have several layers of error correction, data scattering, and redundancy built into the specifications. They are not like a vinyl LP, which has a single track of analog data, and no error correction. (Although, there exist modern playback systems which can digitally reduce hiss, pop, and scratch noise on the fly. None of that is in the LP spec.)

Different results in different DVD players can be the result of dirt on the internal optics, mechanical misalignment, or even the movement of a few particles of dust on the disc when moved between players.

Anonymous 0 Comments

Adding to the above, I’m convinced that some of the cheaper readers are designed without enough processing grunt, or good enough firmware, to implement all of the layers of error correction. Thus a scratched disc won’t play on them but will on a higher quality device.

Anonymous 0 Comments

Adding to the above, I’m convinced that some of the cheaper readers are designed without enough processing grunt, or good enough firmware, to implement all of the layers of error correction. Thus a scratched disc won’t play on them but will on a higher quality device.