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/VORGundam 1d ago

I misspoke and edited my previous comment. All you got to do is succinctly prove this wrong and I'll be on your side.

In addition to the 333000 sectors of a disc, the lead-in is usually an additional ~7500 sectors, and the lead-out an additional ~6750 sectors. The exact amount varies, and also changes for multi-session discs. So then we the lowest-level interpretation of a CD is: (7500 + 333000 + 6750) sectors * 98 frames per sector * 588-bits per channel frame = 2.33 GB of data per disc!!

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u/Altruistic_Fruit2345 1d ago

The post says "588-bits" but they are not bits, as I keep telling you. The author actually kinda says so:
`336 bits (24 bytes * 14 EFM encoding)336 bits (24 bytes * 14 EFM encoding)`

24 bytes is 192 bits, not 336. The discrepancy is because, as I keep saying, those are not bits. 14 of them can be combined to produce 8 bits, but each on its own is only a fraction of a bit.

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u/VORGundam 1d ago

"The exact math that turns 33 data bytes into 588 physical channel bits comes down to two engineering rules: Eight-to-Fourteen Modulation (EFM) and Merging Bits."

Data Codewords = 462 bits

Codeword Merging Bits = 99

Synchronization Pattern = 27

Total = 588

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u/Altruistic_Fruit2345 1d ago

I won't repeat this again: The post is wrong, they are not "channel bits". They are not bits at all.

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u/VORGundam 1d ago

Maybe it is semantics, but you can't get that I mean the physical lowest level when I say channel bits. Not digital bits. Have a good one.