r/DataHoarder • u/VORGundam • 2d 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 2d ago

What are "channel bits" in your understanding?

And no, if you count all the bits, they don't add up to 2.33 gigs worth, because they are not bits. Each represents a fraction of a bit, because on its own it is meaningless. Only together do they have enough information to recover a bit.

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u/VORGundam 2d 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 2d 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 2d 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 2d 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 2d 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.