Theoretical question about how CD's work

marklings

I for one do not believe any error correction is needed. You copy a file you get a perfect copy unless someone can show me why a copy of a Data file from a CD should be different from a copy of an audio file ...

You need error correction because there are often errors when playing a CD in real time, which is quite different than simply making copies of a data file.

8th-note 6-27-2019
@almarg mentions EAC and FLAC. When I rip a CD to FLAC using db Poweramp what does the data on the FLAC file look like. Since it can be compared to a perfectly accurate copy (whatever that means) it shouldn’t need the error correction wizardry.

I made no mention of FLAC, which as stated above is a lossless format for storing audio data in compressed form. EAC ("Exact Audio Copy") is a software program that is widely used for "ripping" (copying) the contents of audio CDs onto computer hard drives. It provides the capability of re-reading data on a CD multiple times that depending mainly on the condition of the disc and the drive mechanism may not be captured accurately on the first pass (i.e., on the fly).

The "error correction" that I have been referring to is invisible to the user, is performed by circuitry associated with the drive mechanism, and makes use of error correcting codes that are on the CD and are an inherent part of the CD format. My understanding is that **for a CD and a drive mechanism that are in good condition** something like hundreds of bits or even more will typically be misread by the laser mechanism during a single pass, among the billions of bits that are on a CD, and all or very nearly all of them will be routinely corrected by that circuitry to bit-perfect accuracy, on the fly. Use of a program such as EAC, which can make multiple passes if necessary, provides additional assurance that will happen, and will flag an error if for some reason it does not happen.

When I rip a CD to FLAC using db Poweramp what does the data on the FLAC file look like. Since it can be compared to a perfectly accurate copy (whatever that means) it shouldn’t need the error correction wizardry.

I’m not familiar with db Poweramp, but even if it only rips using a single pass chances are that all or nearly all of your rips are bit perfect (assuming discs and drives are in good condition), with the necessary error correction having been performed by the hardware invisibly, ***prior to db Poweramp even seeing the data.*** EAC, as I said, just provides added assurance, especially if disc or drive condition may be marginal.

Regards,
-- Al

I suspect things are much worse than represented by almarg and his statement that perhaps 100s bits are in error. However, if that were actually true it would probably be inaudible. So, things must be worse than that. Since vibration isolation, CD disc beveling, disc coloring, CD liquid treatments, disc damping, disc demagnetization, disc static electric charge dissipation and other steps obviously improve the sound. Hell, just getting rid of the laser scattered light problem alone doubles performance! So, gentle readers, I think I’ll stick with my original evaluation that the whole CD playback system is FUBAR. I’ll leave it to others to speculate on how many errors there are on high end CD playback systems.

Responding to nonsense from kost_amojan, geoffkait writes:

Nurse! Thorazine! Man down! Costco hasn’t been paying attention. In the case of CDs the 1s and 0s are represented by transitions not by the ON or OFF. As I already explained it’s more complicated than you apparently think. The link I provided yesterday includes the allowable sets of data determined by both transitions AND the length of pits and lands.

Also, you are incorrect that - in the Case of CDs - the digital process is reliable. It is only *partially* reliable 🤭 within limits that are set by failure of the CD playback system to deal properly with scattered laser light and vibration, internal and external. Not to mention the obvious failure of Reed Solomon error codes and the laser servo feedback system to correct all errors. Wake up and smell the coffee! ☕️

Even CD players that are *isolated* have difficulty with laser reading the data on the disc because the *CD itself* is wobbling and fluttering. The CDs are often not perfectly round and the disc is often not absolutely level during play, which exacerbates this CD vibration problem. Yes, CD Compact Disc “works” well enough for those who don’t mind generic, thin, bass shy, brittle, honky, synthetic, congealed sound.

Right. Exactly. Could hardly have said it better myself.

About the only thing left unsaid is that yes indeed one could get a string of 1’s and 0’s from the output of a CD player. Just not the same one every time. And so no, the same sound cannot be replicated. Otherwise a CD would always sound the same. Which they don't. Because the CD is not digital.

OK, simple question
How do you realize one bit is wrong ?
How do you know this zero is a good zero but that one is a bda one ?