CRC match?

vbimport

#1

I used two drives to rip the same tracks, both of them were successful in EAC, but the CRCs were completely different with the different drives.

Why is this? I tried many discs and the results are all the same.


#2

I assume you are talking about audio CDs here. If so, then it is because all drive models have a different amount of skew. For CDs there is subcode and main channel. The main channel is the actual audio and the subcode is the track location information. Each drive model has different data at any given subcode point on the CD due to the electronics of the drive (and other things).

So, if you ask a drive to give you the audio at sector 00:02:00, you will get slightly different results between 2 drives if they are not the same model and firmware. The data on the CD doesn’t change it’s just that you get a different starting point for the data depending on the drive you use.

Another reason for CRC differences would be unreadable sectors, but you would probably know if that was happening.

Hope this helps.

RM


#3

[QUOTE=RichMan;2077021]

Hope this helps.

RM[/QUOTE]
Yes it does.

So is such thing as “consistent error” likely? Like if a CD has a scratch here or there, then each time the drive reads that part the light is going to be refracted in the same way. Just my wild non professional guess. Is this situation impossible?

Secondly, if the subcodes are different, that doesn’t affect the sound of the track at all right? Like the rip file is 100% bit for bit perfect and is exactly the same as the original when listened to?

Thanks.


#4

Any single drive can read an audio disc differently each time it reads it if it is poor quality. You may get unreadable errors in different locations but they should always be close to the same. Many factors come to play here. Even if you leave the CD in the drive and read it twice, you may get 2 slightly different results.

The subcodes and skew will not change the actual digital data of the audio tracks. The thing to remember is that you will get different starting and ending points of this digital data depending on the model of drive you use. The difference is almost always undetectable since it is so very very small.

As a reference, a CD is played at 75 sectors per second. The difference in skew between drives is usually much less than this. Almost never would you hear the difference between 2 drives’ skew.

RM


#5

[QUOTE=RichMan;2077030]Any single drive can read an audio disc differently each time it reads it if it is poor quality. You may get unreadable errors in different locations but they should always be close to the same. Many factors come to play here. Even if you leave the CD in the drive and read it twice, you may get 2 slightly different results.

The subcodes and skew will not change the actual digital data of the audio tracks. The thing to remember is that you will get different starting and ending points of this digital data depending on the model of drive you use. The difference is almost always undetectable since it is so very very small.

As a reference, a CD is played at 75 sectors per second. The difference in skew between drives is usually much less than this. Almost never would you hear the difference between 2 drives’ skew.

RM[/QUOTE]
So when the EAC CRCs match, then I can take that as a perfect duplicate of the audio?


#6

In my opinion, NO. You will never get an exact or perfect rip because each drive has a different skew. If you use the same drive, same CD and same software and get the same CRCs, then those 2 rips are the same but not exact. There is no way to get EXACTLY what is on the disc since there is no drive that I know of that has zero skew. You can only get what your drive will give you with it’s ‘built in’ skew. Even if you were able to read from point A to point B at your decision, you would still not know where the real A and B where on the original disc. Also, even the original CD itself can and does have some amount of skew.

With audio, there is never a perfect rip no matter what others may tell you. You simply can not control where your drive will start reading or where the true audio on the CD actually started. It’s just impossible with audio since the main channel and subcode are not locked together in any way at all.

The main thing to remember is that none of the this matters since the differences are so small and the human ear can not hear them. Only on a rare few cross-faded discs would you possibly be able to hear a very slight difference in how a song begins. But, very unlikely.

RM


#7

[QUOTE=RichMan;2077044]In my opinion, NO. You will never get an exact or perfect rip because each drive has a different skew. If you use the same drive, same CD and same software and get the same CRCs, then those 2 rips are the same but not exact. There is no way to get EXACTLY what is on the disc since there is no drive that I know of that has zero skew. You can only get what your drive will give you with it’s ‘built in’ skew. Even if you were able to read from point A to point B at your decision, you would still not know where the real A and B where on the original disc. Also, even the original CD itself can and does have some amount of skew.

With audio, there is never a perfect rip no matter what others may tell you. You simply can not control where your drive will start reading or where the true audio on the CD actually started. It’s just impossible with audio since the main channel and subcode are not locked together in any way at all.

The main thing to remember is that none of the this matters since the differences are so small and the human ear can not hear them. Only on a rare few cross-faded discs would you possibly be able to hear a very slight difference in how a song begins. But, very unlikely.

RM[/QUOTE]
So is it all about the beginning and end only? And not a distorted sound through the entire track? Like the “skew” only affects the beginning or end?

And what would the difference be like? Is it a slight longer silence or something?

Edit:In light of all this, is there any use for programs like EAC anymore?


#8

It is only about the start and end positions of what your drive gives you. If the TOC on the disc says that track 5 starts at 16:20:05 and you rip that track, your drive will go to subcode Q location 16:20:05 and start reading data. You will get the correct amount of data for the track but 2 different drives will give you slightly different start and end points. The data from both drives is the same except for where they started reading and stopped reading from the CD. So, a little longer silence at the start and a little less at the end. You should not be able to hear any difference 99% of the time since we are talking about miliseconds here.

If you’ve been happy with EAC then of course there is a use for it. No software can correct for the skew now matter what they may claim.

RM


#9

I would like to clarify my last statement about no software can correct for skew. Sure software can try to build up a list of all drive models and the skew they have. But, how do they know how much skew a drive has? The reason I say this is because even the CD itself has some sub to main skew. It would take special content for the CD and special equipment to make the glass master and special equipment to verify how much skew the CD truly has itself. Then we could read that CD in a drive and find the drives true skew.

RM


#10

[QUOTE=RichMan;2077225]It is only about the start and end positions of what your drive gives you. If the TOC on the disc says that track 5 starts at 16:20:05 and you rip that track, your drive will go to subcode Q location 16:20:05 and start reading data. You will get the correct amount of data for the track but 2 different drives will give you slightly different start and end points. The data from both drives is the same except for where they started reading and stopped reading from the CD. So, a little longer silence at the start and a little less at the end. You should not be able to hear any difference 99% of the time since we are talking about miliseconds here.

If you’ve been happy with EAC then of course there is a use for it. No software can correct for the skew now matter what they may claim.

RM[/QUOTE]

What I meant was, if Windows Media player and other more accessible programs all have error correction, and if EAC will also give you different CRCs for different drives, then how does EAC provide a more “accurate” rip?


#11

I think EAC uses a drive model skew database to eliminate as much skew as possible. If you use this feature and try 2 different drives, in theory I guess you should get exact same data from both drives. At least EAC tries while many other software just takes what the drive gives them and does not try to adjust.

I’m not sure of the above but I think that is what I have read. Maybe others will help answer. I think EACs method to ‘equalize’ the skew is a very good idea as long as your drive model is in the database. EAC probably has some other features that allow you to change the exact start and end points on your own if you wish. But again, I am not positive.

RM


#12
  1. Cofigure EAC to rip in Secure Mode. Leaving “drive caches audio data” and “drive is capable of retrieving C2 error information” UN-checked is a good, safe starting place.

  2. Enter the correct read offset correction value for your particular ripping drive in EAC’s Drive Options. In other words, if your drive has a -6 sample read offset, enter +6 under “use read sample offset correction.”

  3. Have fun. Run some CRCs on ripped tracks to see if you’re golden. Use AccurateRip if you’re truly anal (like me, I guess)–they also maintain a database of known EAC read offset correction values for most drives. Google AccurateRip if you’d like to check it out.