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Topic: Help understanding EAC WAV Compare (Read 2140 times) previous topic - next topic
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Help understanding EAC WAV Compare

I have two copies of what are roughly supposed to be the same CDr. There is something wrong with these discs, making them very difficult to read, especially towards the second half. I'm trying to get the best rip that I can. They are from the early 90s, so they are quite old and from the early days of CDrs.

The CDr has two tracks, and I'm dealing with Track 1 at the moment. I've managed to securely copy Track 1 from both CDrs.

Unexpectedly, the TOCs were different.

EVIDENCE 1:

    Track |  Start  |  Length  | Start sector | End sector
    ---------------------------------------------------------
copy 1:
        1  |  0:00.00 | 45:44.08 |        0    |  205807 
        2  | 45:44.08 | 27:24.51 |    205808    |  329158 
copy 2:
        1  |  0:00.00 | 45:46.08 |        0    |  205957 
        2  | 45:46.08 | 27:24.52 |    205958    |  329309 


EVIDENCE 2:

I've compared the two files using the EAC WAV Compare feature, and here is what I get with copy 1 on the left, copy 2 on the right:



Here is an analogous comparison for Track 2, because I noticed that the first 2 lines are the same, and it may give us an additional clue:




EVIDENCE 3:

Looking directly at the samples at the beginning of copy 1 vs. copy 2 (either track), I see that: copy 2 is missing the first 588 samples in copy 1.


THOUGHTS:

- Given that copy 2 Track 1 is 2 seconds longer, perhaps copy 1 was ripped (with gap appended) and used as the source for writing copy 2?


QUESTIONS:

- What format are these "positions" in? Thousandths of a second? Is it possible to convert these accurately to precise samples?
- What does it mean that both tracks have  "588 repeated samples" at 0.6262?
- What does "repeated" samples mean? I don't see any frame that is equal to the previous frame until near the end of the tracks.
- What do the missing/repeated samples scattered throughout copy 2 of Track 1 mean?
- What is the significance of 324 samples?
- How many samples are in 0:45:44.0929 to 0:45:44.1066?
- Any idea why Track 2 of copy 2 would be 1 frame longer in the TOC?


Thanks for any help... I'm a bit confused right now.

Help understanding EAC WAV Compare

Reply #1 –
I don't understand all of the details...

Quote
I have two copies of what are roughly supposed to be the same CDr.
"Roughly" is generally useless with a digital compare...  i.e.  If you digitize a recording twice or if there is a slight time-shift, odds are that NONE of the samples will be the same.

If you are not hearing any defects, I wouldn't worry about it...    If you are hearing defects, choose the one with the least (audible) defects and try to fix it with an audio editor (fix the sound, not the data).  Or go find the original CD/recording.

Quote
- What format are these "positions" in? Thousandths of a second?
I assume  it's 10-thousandths since it's 4 digits to the right of a decimal point.

Quote
- How many samples are in 0:45:44.0929 to 0:45:44.1066?
If my assumption is correct, that's 0.1066-0.0929 = 0.0137 seconds (13.7 milliseconds).    With 44,100 samples per second, that's 0.0137 x 44,100 =  604 samples.

Quote
Is it possible to convert these accurately to precise samples?
Probably not, since you'd need at least 5 digits to represent the time between two samples (1/44,100th of a second).


Quote
What does it mean that both tracks have "588 repeated samples" at 0.6262?
Exactly what it says...  588 sample in a row that have the same value.    That's a flat line in the waveform for about 13 milliseconds.  I don't know if EAC would report clipping or silence as "repeated samples", but mathematically both of those conditions would be repeated samples.  If it's loud at that point, I'd assume clipping.  Clipping is not unusual in commercial releases (or homemade recordings).      You're not going to find that many repeated samples in a natural-undistorted waveform, but it could easily happen and could be perfectly normal with digitally-generated sounds.

Help understanding EAC WAV Compare

Reply #2 –
588 = 1/75th of 44100, which is called a CD sector. One second has 75 sectors, and CUE files indicate their positions in sectors.

1) explained on the other post..  time * 44100 = samples.

2/3) I don't know EAC enough to give you a good answer, but it most probably indicates a reading error which caused one full sector of bad data. What "repeated" might mean is either the same sample is there 588 times (which sounds like a brief silence), or that the sector is repeated and is the same than the previous sector (which sounds like... well.. as something that repeats  ).

4) Remember that you are comparing two files. What it seems to say is that on the second file, it had more difficulties reading and probably it filled some samples of repeated data while waiting for the new data, but later on it syncronized and so it skipped the samples that it didn't wrote yet. I think that a visual compare with an audio editor would definitely make it easier to spot (just remenber to zoom accordingly).

5) 324 samples are aproximately 7.35 millliseconds.  ( samples * 1000 / 44100 )
6) Answered on the other post
7) Two seconds is not one frame. In fact, the text you copied indicates the difference in number of sectors (the columns on the right). But since it is exactly two seconds, it sounds as if the burning program added the usual 2 second gap. (or it was somehow removed when ripping the copy 1)
 

Help understanding EAC WAV Compare

Reply #3 –
Thank you guys.

Quote from: [JAZ] on

588 = 1/75th of 44100, which is called a CD sector. One second has 75 sectors, and CUE files indicate their positions in sectors.

Where I said "frame" I meant "sector". (I'm pretty sure EAC themselves uses or used to use the term "frame" instead of "sector", which is why I used it.)

Quote from: DVDdoug on
"Roughly" is generally useless with a digital compare...  i.e.  If you digitize a recording twice or if there is a slight time-shift, odds are that NONE of the samples will be the same.

By "roughly" I meant that I would have expected the source files to be the same and the discs to be identical. But clearly there is a slight difference between them.

Given that the majority of the samples between the sources match, we can assume they're from a single original recording.

Quote from: DVDdoug on
Exactly what it says...  588 sample in a row that have the same value.    That's a flat line in the waveform for about 13 milliseconds.  I don't know if EAC would report clipping or silence as "repeated samples", but mathematically both of those conditions would be repeated samples.  If it's loud at that point, I'd assume clipping.  Clipping is not unusual in commercial releases (or homemade recordings).      You're not going to find that many repeated samples in a natural-undistorted waveform, but it could easily happen and could be perfectly normal with digitally-generated sounds.

Ah thank you, I hadn't thought of this. It's not clipping, so I'm going to assume it's a sector of bad data coming from 1 of the ripped discs but not from the other.


Quote from: [JAZ] on

2/3) I don't know EAC enough to give you a good answer, but it most probably indicates a reading error which caused one full sector of bad data. What "repeated" might mean is either the same sample is there 588 times (which sounds like a brief silence), or that the sector is repeated and is the same than the previous sector (which sounds like... well.. as something that repeats  ).

Thank you. This is a great theory. I will locate the repeated samples and investigate further.

Quote from: [JAZ] on

4) Remember that you are comparing two files. What it seems to say is that on the second file, it had more difficulties reading and probably it filled some samples of repeated data while waiting for the new data, but later on it syncronized and so it skipped the samples that it didn't wrote yet. I think that a visual compare with an audio editor would definitely make it easier to spot (just remenber to zoom accordingly).

Good theory. I will make a visual investigation as well, and follow up here if I find anything interesting.

Quote from: [JAZ] on

7) Two seconds is not one frame. In fact, the text you copied indicates the difference in number of sectors (the columns on the right). But since it is exactly two seconds, it sounds as if the burning program added the usual 2 second gap. (or it was somehow removed when ripping the copy 1)

Track 1 is 2 seconds longer, but track 2 is 1 sector longer. This seems a bit odd to me. I'm thinking maybe that if copy2 was sourced from a rip of copy1, that an extra sector could've been added as an error by, say, the ripping process. But maybe that's a dumb theory, I don't know.