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From: Mirko Parthey on 6 Jul 2010 07:10
On Mon, Jul 05, 2010 at 10:43:44PM +0200, Urs Thuermann wrote:
> For each CD I run
>
> cdrdao read-cd --datafile data.cdr --device /dev/sg0 toc
> and
> cdparanoia -d /dev/sg0 -B
>
> where /dev/sg0 refers to an Plextor Ultraplex 40max SCSI CDROM drive.
>
> [...]
>
> I can run cdrdao and cdparanoia
> repeatedly, say 10 times, and I get deterministic results, i.e. all
> runs of cdrdao give the same result and all runs of cdparanoia give
> the same result but the results of cdrdao and cdparanoia differ.
>
> Now my question is where these differences come from and which results
> are the correct (better) ones.

With a suitable CDROM drive such as yours and CDs in reasonably good
condition, there should be no need for cdparanoia's data correction
feature. In fact, I found that it can sometimes do more harm than
good, so I recommend disabling it (-Z), if only for testing purposes.

You could compare your results to Exact Audio Copy (Windows, free for
non-commercial use), which reportedly also runs on WINE.

Another option would bei morituri, a CD ripper for Linux modelled after
Exact Audio Copy.

Regards,
Mirko


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From: Mark on 6 Jul 2010 11:20
On Tue, Jul 6, 2010 at 4:04 AM, Mirko Parthey <
mirko.parthey(a)informatik.tu-chemnitz.de> wrote:

[...]


> You could compare your results to Exact Audio Copy (Windows, free for
> non-commercial use), which reportedly also runs on WINE.
>

+1. Also if you use EAC with AccurateRip enabled you can have quite good
confidence in your results.

Mark
From: Mark Allums on 6 Jul 2010 12:10
First, I would like to second the suggestion of trying Exact Audio Copy
(if a Windows environment is available) and comparing those results. +1

Second, I apologize for beating a dead horse, but in the event that a
few people read my earlier posts, I wanted to make a correction to what
I originally wrote. In particular, the statement that "there is no
error correction" on audio CDs is wrong. The following (taken from
ttp://www.cdrfaq.org/faq02.html) is rather long, but interesting:


Subject: [2-15] What are "jitter" and "jitter correction"?
(1998/04/06)

(below)

Subject: [2-17] Why don't audio CDs use error correction?
(2007/08/08)

(below)



On 7/6/2010 2:27 AM, Mark Allums wrote:
> On 7/5/2010 3:43 PM, Urs Thuermann wrote:
>> Now my question is where these [CD-rip] differences come from and
>> which results
>> are the correct (better) ones.

> Audio CDs live in a world where there is no guarantee that any two
> passes across a "sector" will ever give the same result. For one thing,
> there is no error correction. Give up the idea of getting a correct, or
> "perfect", rip; it's theoretically impossible.
>
> In fact, that's why cdparanoia exists. In the days of CD-ROM drives, the
> drives were not very good at ripping audio discs, and cdparanoia
> compensated for the problems. It was also unbelievably slow. Today, DVD
> drives have a much better disc transport, etc., and a lot of software is
> smarter than it used to be, so ripping is only a real problem with
> copy-protected CDs, and very damaged discs.



Subject: [2-15] What are "jitter" and "jitter correction"?
(1998/04/06)


The first thing to know is that there are two kinds of jitter that
relate to audio CDs. The usual meaning of "jitter" refers to a time-base
error when digital samples are converted back to an analog signal; see
the jitter article on http://www.digido.com/ for an explanation. The
other form of "jitter" is used in the context of digital audio
extraction from CDs. This kind of "jitter" causes extracted audio
samples to be doubled-up or skipped entirely. (Some people will
correctly point out that the latter usage is an abuse of the term
"jitter", but we seem to be stuck with it.)

"Jitter correction", in both senses of the word, is the process of
compensating for jitter and restoring the audio to its intended form.
This section is concerned with the (incorrect use of) "jitter" in the
context of digital audio extraction.

The problem occurs because the Philips CD specification doesn't require
block-accurate addressing. While the audio data is being fed into a
buffer (a FIFO whose high- and low-water marks control the spindle
speed), the address information for audio blocks is pulled out of the
subcode channel and fed into a different part of the controller. Because
the data and address information are disconnected, the CD player is
unable to identify the exact start of each block. The inaccuracy is
small, but if the system doing the extraction has to stop, write data to
disk, and then go back to where it left off, it won't be able to seek to
the exact same position. As a result, the extraction process will
restart a few samples early or late, resulting in doubled or omitted
samples. These glitches often sound like tiny repeating clicks during
playback.

On a CD-ROM, the blocks have a 12-byte sync pattern in the header, as
well as a copy of the block's address. It's possible to identify the
start of a block and get the block's address by watching the data FIFO
alone. This is why it's so much easier to pull single blocks off of a
CD-ROM.

With most CD-ROM drives that support digital audio extraction, you can
get jitter-free audio by using a program that extracts the entire track
all at once. The problem with this method is that if the hard drive
being written to can't keep up, some of the samples will be dropped.
(This is similar to a CD-R buffer underrun, but since the output buffer
used during DAE is much smaller than a CD-R's input buffer, the problem
is magnified.)

Most newer drives (as well as nearly every model Plextor ever made) are
based on an architecture that enables them to accurately detect the
start of a block.

An approach that has produced good results is to do jitter correction in
software. This involves performing overlapping reads, and then sliding
the data around to find overlaps at the edges. Most DAE programs will
perform jitter correction.


Subject: [2-17] Why don't audio CDs use error correction?
(2007/08/08)


Actually, they do. It is true that audio CDs use all 2352 bytes per
block for sound samples, while CD-ROMs use only 2048 bytes per block,
with most of the rest going to ECC (Error Correcting Code) data. The
error correction that keeps your CDs sounding the way they're supposed
to, even when scratched or dirty, is applied at a lower level. So while
there isn't as much protection on an audio CD as there is on a CD-ROM,
there's still enough to provide perfect or near-perfect sound quality
under adverse conditions.

All of the data written to a CD uses CIRC (Cross-Interleaved
Reed-Solomon Code) encoding. Every CD has two layers of error
correction, called C1 and C2. C1 corrects bit errors at the lowest
level, C2 applies to bytes in a frame (24 bytes per frame, 98 frames per
sector). In addition, the data is interleaved and spread over a large
arc. (This is why you should always clean CDs from the center out, not
in a circular motion. A circular scratch causes multiple errors within a
frame, while a radial scratch distributes the errors across multiple
frames.)

If there are too many errors, the CD player will interpolate samples to
get a reasonable value. This way you don't get nasty clicks and pops in
your music, even if the CD is dirty and the errors are uncorrectable.
Interpolating adjacent data bytes on a CD-ROM wouldn't work very well,
so the data is returned without the interpolation. The second level of
ECC and EDC (Error Detection Codes) works to make sure your CD-ROM stays
readable with even more errors.

It should be noted that not all CD players are created equal. There are
different strategies for decoding CIRC, some better than others.

Some CD-ROM drives can report the number of uncorrected C2 errors back
to the application. This allows an audio extraction application to
guarantee that the extracted audio matches the original.




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