Theoretical question about how CD's work

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 ?

IMO the reason many of the tweaks mentioned above by Geoff may be beneficial in some situations has nothing whatsoever to do with bit errors or error correction.

The main reason in most cases is likely to be related to electrical noise generated by the servo mechanisms and circuitry in the transport part of the player, as it tracks the disc, coupling into unrelated downstream circuitry in the player, causing jitter in the D/A conversion process, and/or intermodulation or other effects on the analog signal path. The degree to which that occurs will be dependent on the design of the particular player, of course, as well as on the condition of the disc.

From this Wikipedia writeup:

Reed-Solomon coding is a key component of the compact disc.... In the CD, two layers of Reed-Solomon coding separated by a 28-way convolutional interleaver yields a scheme called Cross-Interleaved Reed Solomon Coding (CIRC).... The result is a CIRC that can completely correct error bursts up to 4000 bits, or about 2.5 mm on the disc surface. This code is so strong that most CD playback errors are almost certainly caused by tracking errors that cause the laser to jump track, not by uncorrectable error bursts.

Note that the term "error correction," as properly defined in this context, refers to bit-perfect correction. "Error interpolation" is the term used to refer to the less than bit-perfect approximation that can occur (rarely) when bit-perfect correction fails.

And from a post by member Kirkus (who probably has more hands-on experience with the internal workings of CD players than the rest of us put together) in this Audiogon thread:

CD players, transports, and DACs are a menagerie of true mixed-signal design problems, and there are a lot of different noise sources living in close proximity with susceptible circuit nodes. One oft-overlooked source is crosstalk from the disc servomechanism into other parts of the machine . . . analog circuitry, S/PDIF transmitters, PLL clock, etc., which can be dependent on the condition of the disc.... One would be surprised at some of the nasty things that sometimes come up out of the noise floor when the focus and tracking servos suddenly have to work really hard to read the disc.

Apologies to the OP for the digression.

Regards,
-- Al