What is bit perfect?

[janneman]

Originally posted by dhaen

Jan,

This just can't be so. If a dust particle shadows a "pit" on one read, and not on the next, then the data is different.[snip]

No, John, its not! The ERROR CORRECTION is different, but the data is the same each time. That's the crux of the matter.
There is a lot of redundant data on the disk, and if the dust particle is there, data from another frame is used to assemble the "dust" frame correctly. So, yes, the processing in each case is different, but you get the same data, each time. Without it, the CD could never been the succes it has been. It is a very robust medium.

Jan Didden

PS There is of course a limit to what can be corrected. If that limit is reached, error concealment is entered. But that is not what we talk about here, I assume.

[dhaen]

It doesn't mean perefct audio streams as some seem to be saying.

No, but it means the best result that can be obtained from that particular drive.

[Variac]

error concealment is entered. But that is not what we talk about here, I assume.

I think error concealment could be a big part of what we are talking about here. It might happen a lot.

[dhaen]

Jan,

I have some understanding of error correction systems and the maths. My dust particle was for example only. Yes, the CD is very robust, but there is sometimes a lot of damage.

Consider the situation where a scratch causes some diffraction, and the wobble of the disc is sufficiently chaotic that particular data is sometimes read, and sometimes not. In this case a CD player may sometimes correct or sometimes conceal. Perhaps it gets it right 60% of the time.
If an the data is read many times with statistical correction, the chances are that the result will include the correct value.

[dhaen]

Originally posted by Variac


I think error concealment could be a big part of what we are talking about here. It might happen a lot.

Yes it does - or at least did. I'll explain:
In the very early days of CD's I was privy to tests at a Japanese manufacturer of error correction tests on discs produced in Europe and Japan. We found error flags far more often on the Euro-discs. These were new discs. The errors were either manufacured-in or recorded-in. It meant some design changes to equipment to cope with this.
I don't know how things are now, but even in the last 5 years I've heard of dust problems in manufacuring plants.

[janneman]

Well, it gets a bit speculative now. But let me tell you, a CD that needs error concealment is unlistenable! You will very quickly get frustrated and throw it out, or at least skip that track. The fact that the vast majority of CDs off the rack play perfectly in most any player, scratches and dust and all, is a testimony to the robust error correction algorithms.

Those Philips/SONY engineers knew their trade!

I know a guy who worked at Philips, his job was to test third-party players to see if they conformed to the Redbook. Only then were they allowed to carry the well-known CD Audio logo. He told me that error free data reading is almost never a problem. Even the simplest, cheapest player delivered error free data. The problems were more with servo tracking under shocks, eccentric spindels etc.

Jan Didden

[yulquen]

questions:

if errors occur during playback and the CD player needs to
correct them, will the SPDIF output contain the corrected
data, or not (see question below)?

also, some "CD`s" with certain "copy protection" schemes are
made with known errors a redbook cdplayer are supposed
to correct without any loss of quality.how does this affect
the data integrity of the CD when it gets scratches, compared
to a real CD?

[janneman]

Originally posted by yulquen
questions:

if errors occur during playback and the CD player needs to
correct them, will the SPDIF output contain the corrected
data, or not (see question below)?[snip]

Of course the output will have the correct data. You are not even aware of the correction process (as long as it can cope).
You take a virgin CD, and play it. 100% correct data. Now you put a scratch on it, play it again, 100% correct data. You put on another scratch, play it again, 100% correct data. In all those cases, the raw data stream off the CD is different, but the data coming out of the digital output is identical.

Jan Didden

[gmarsh]

Originally posted by janneman
Of course the output will have the correct data. You are not even aware of the correction process (as long as it can cope).
You take a virgin CD, and play it. 100% correct data. Now you put a scratch on it, play it again, 100% correct data. You put on another scratch, play it again, 100% correct data. In all those cases, the raw data stream off the CD is different, but the data coming out of the digital output is identical.

Jan Didden

This is assuming that your scratches aren't too wide, or are spaced in such a way that the de-CIRC process corrects for it. But put a third deep scratch on your CD so that the CIRC algorthm fails, and anything could happen.

Most CD players will interpolate, producing a "non-bit-perfect" output. Some will snap/crackle/pop. Your CD player might even lose tracking and skip if it's a bad scratch.

I wouldn't mind seeing a professional CD player that will flash a LED whenever a CIRC failure occurs...

[yulquen]

Originally posted by janneman



Of course the output will have the correct data. You are not even aware of the correction process (as long as it can cope).
You take a virgin CD, and play it. 100% correct data. Now you put a scratch on it, play it again, 100% correct data. You put on another scratch, play it again, 100% correct data. In all those cases, the raw data stream off the CD is different, but the data coming out of the digital output is identical.

Jan Didden

so if I understand you correctly, it is not possible to create a
copy protection scheme for regular CD-players that
screws up the audio data at the SPDIF connector while
retaining flawless playback at the analogue outputs?
(not taking copy bit into account since it is easy to get around)