You are absolutely correct.... it is extremely unlikely that a CD will ever play without some errors (probably many errors).
In fact, CD-R and DVD-R media is assumed to have errors, and the quality rating for it specifies the acceptable number of errors-per-second (BER) - among other things.
(Compared to CDs, DVDs have smaller pits, narrower tracks, higher allowable error rates, and even more robust error correction.)
HOWEVER, CDs also have a rather robust error
correction mechanism.
A significant amount of extra redundant information is stored on the disc (somewhere around 10% extra for a CD.)
Then, when the CD is played, the data is tested; and, if errors are detected, the extra information is used to correct the errors.
The first two levels of this error correction allow for
perfect correction of the data... and these are referred to as
correctable errors.
As long as the total amount and number of errors fall below this limit, once the data passes through the error correction algorithms, the errors no longer exist.
(There are no approximations involved... the data that is output is actually 100% perfect.)
(According to the standard, a single gap of up to 2 mm in a single track should be correctable... or a relatively large number of smaller errors... depending on their locations on the disc in relation to each other.)
This error correction is standard for normal Red Book CD readers - so all commercial CD transports, including low cost ones, are supposed to include it.
(Of course, it is possible that some custom designs might omit it, or fail to implement it correctly.)
Then, if and only if the amount of missing data, or the number of errors, exceed the amount which can be corrected, interpolation is used to simply "fill in the gap".
(At this point, we are replacing missing information with guesses, which may well be audible as ticks and pops, or may be audible in other ways as a degradation of the sound.)
For whatever reason, while it is easy to verify perfect data when using a computer player, audio CD players almost never report their error correction status....
(If this information was reported your CD player could include a "perfect data" LED to assure you that no uncorrectable errors had occurred or a "warning" LED to tell you that interpolation type error correction had been required.)
ASSUMING THE ERROR CORRECTION MECHANISM IS PRESENT AND WORKING PROPERLY, missing or misread pits don't matter unless they exceed the maximum number that can be successfully corrected.
Of course, there are several possible exceptions to this:
- The error correction mechanism may NOT be properly implemented on a given CD player.
- It's possible that enough vibration could cause so many data errors that they exceed the maximum number that can be corrected.
- It's possible that vibration could cause more errors to occur when reading a disc that is already dirty or damaged (thus rendering a "borderline" disc as "unreadable".).
- It's possible that, IN A CERTAIN SPECIFIC PLAYER, operation of the error correction mechanism itself could cause other issues.
(For example, it's
possible that, if correcting a large number of errors were to consume too much processing power, the EC processor could lose data, or deliver its output data with excessive jitter, or experience some other unforeseen problem.)
As I mentioned above, assuming everything is working as it should, then none of these issues should occur....
(But, then, that is probably not a safe assumption.)
(In the old days, many reviewers included a standard test, to confirm how well the error correction in a given transport actually worked.)
One thing that many people don't realize is that, while audio CDs can play with flaws and interpolated errors....
On a CD-R or DVD-R used for computer data - even a single bad bit which cannot be corrected perfectly will cause an unrecoverable error.
(So the maximum number of uncorrected errors that can be tolerated on a data disc is zero.)
Oops.... - somebody talking theory, without ever recording music ( and by that, I mean REAL music - musicians/room/microphones/some kind of storage ) in real life.
Nope, assuming optical disc playback to be perfect is NOT how it is in real life. Be nice and re-check the size of the CD pits - and then put any of your beloved CDs with hole on any of your fingers that fit into that hole. Flick the rim of the CD with the fingernail of the other arm - close to your ear. You WILL hear the CD vibrating.... - with amplitude exceeding the pit size. Forcing the laser serve to perpetually search for the focus.
Now repeat the same test - this time with the paper that comes in all/any CD-R media on top of the CD-R stack. You WILL hear much less of the CD resonance, both in amplitude and in duration - it is over almost instantly and does not ring like CD normally does. That paper of course can not compare to the effect of the mat - where probably the best is graphite, but so thin graphite disk is brittle to the max and most likely anything but durable. Next best thing - carbon fibre mat. While probably not as good as graphite, with reasonable care it will last in normal 1x speed transports such as CD and DVD player for a lifetime.
Of course, no one is flicking the rim of the CD while playing back - at least not with the magnitude of fingernail. Then again, re-check the size ( depth... ) of the information pits of the CD ... - and then, DVD. It is likely DVD benefits because of this even more than CD. Anything that rotates is bound to vibrate to some extent - no such thing as perfect bearings with zero free play and noise.
CD mat has another advantage - optical. CD-R ( and CD ) media is NOT entirely non permeable for light - just check some different samples looking trough them to a light bulb. Some are at least semi-transparent - and likely to be more or less so, depending on any label printing or graphics - meaning optical conditions are NOT uniform, but are changing according to whatever is on the label. Placing a black carbon fibre mat on top of the label will instantly make this translucence problem ( again, a possible source of forcing laser servo into constant focus correction ) obsolete. For each and every CD in your collection.
Unless you have "turntable type" CD transport - then no mat is required.