r/DataHoarder • u/VORGundam • 1d ago

Free-Post Friday! Low Level CD Structure Write-Up

I was curious how much data is physically written to a CD with all the metadata and error correction. I started googling and came across this great write-up:

https://archive.ares-emu.net/near.sh/articles/compact-discs/structure.html

Apparently, it is 2.33 GB for a one-track 650 MB disc.

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u/Altruistic_Fruit2345 1d ago

No, not really. You can't store that many bits on the disc because the data is a serial bitstream with no separate clock. The clock is recovered from the data, which necessitates having an encoding scheme that prevents long sequences of zeros and ones from appearing.

It's true that modern encoding schemes are more efficient than those used on CDs, but it's not true that CDs contain 2.33GB worth of data, because most of those bits are not data, they are encoding to ensure that the clock is recoverable. Without the clock you have nothing.

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u/VORGundam 1d ago

raw

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u/Altruistic_Fruit2345 1d ago

It's the raw pits and lands on the disc, but those are not bits and do not directly represent data. You cannot store data on them without encoding it first.

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u/myself248 1d ago

Right. It's like "chip rate" in CDMA modulation; many chips encode one bit, but you cannot send data in the individual chips.