What's happening is QuickTime is doing the conversion for you. This means you will at some point end up with files that are different because QuickTime isn't perfect (although it is a LOT better every version).
Now, here is some great details on what the hell goes on with a red book audio CD. This comes from Adaptec.
About Digital Audio Extraction
------------------------------
by Bob Starrett
http://www.cdpage.comAccurately copying a Red Book (audio) track from an audio compact disc
to hard disk or another CD is a continuing challenge, but it has
recently become less difficult due to advances in hardware and software
technology.
Audio tracks are not like regular computer data files; they are made up
made up of data that is meant to stream, and this stream contains more
than music. The stream itself is not simple; it is interleaved, meaning
that portions of a song that naturally follow each other when playing
the song do not follow each other in the physical layout of the disc
itself. This is part of the disc's error correction, used to ensure that
errors (caused by dust and scratches, for example) do not cause audible
errors when the disc is played.
(For more details on error correction, see our earlier article on
"Compact Disc Errors," linked from
http://listserv.adaptec.com/SCRIPTS/WA-ADAPTEC.EXE?A1=ind00&L=wincdr )
Digital Audio Extraction, or DAE, is sometimes, perhaps unfortunately,
called "ripping". Ripping involves moving the contents of an audio
track on a CD to a hard drive or other storage device, by reading the
track from the CD and creating a file that can then be manipulated in
various ways. A number of file formats can be used, including AIFF on
the Macintosh and the WAV format under Windows.
Why is it sometimes difficult to get good-quality audio extracted from a
disc? And why is the process so slow in many cases? This takes a little
understanding of how the data on an audio disc is organized.
An audio disc consists of frames, each of which contains 24 bytes of
user data, synchronization, error correction, and control and display
bits. The audio CD's data is not arranged on the disc in distinct
physical units. The data in one frame is interleaved with the data in
other frames. This prevents a scratch or other defect in or on the disc
from destroying a frame beyond the ability of the reader to correct the
data. A scratch will destroy a little bit of many frames, rather than a
whole frame or frames, so, using error correction technologies, the
missing data can be recovered and the disc can play normally without
discernible loss of content or quality.
Use these tips when ripping audio and your chances for success will
increase:
1. Make sure the disc is clean, free of dust, fingerprints and other
foreign matter. Discs can be cleaned with commercially available
cleaners and cleaning kits, but these are not necessary to ensure a
clean, readable disc. Simply hold the disc under warm, running water.
Lather one hand with hand soap (bar or liquid), and rub the soap gently
on both sides of the disc with your fingers. Rise your hands and the
disc well with warm water and pat the disc dry with a soft, lint-free
cloth or towel.
2. Make sure the disc does not suffer from any of the following
conditions: warping, deep scratches, or a nicked or peeling reflective
surface. These can cause the reading drive to seek excessively as it
tries to read damaged or unreadable errors, resulting in long ripping
times or corrupted files.
3. Use your best drive for ripping, even if it is not your fastest
drive. If you have more than one CD recorder or CD-ROM drive, try your
fastest drive first. If the results are not satisfactory (you can tell
by listening to the ripped file!, use a slower drive.
"Best drive" is, of course, a subjective judgment that you will need to
make for yourself after some experimentation. You can usually depend on
drives from well-known manufacturers to do a good job at audio
extraction. On the other hand, some models from major manufacturers have
been known to do extraction poorly or not at all. Many inexpensive,
non-branded drives rip audio just fine. Newer drives will perform better
than older drives, not just because they are newer, but because many of
them incorporate new technology that makes ripping faster and more
accurate. While many older CD-ROM drives will work for extracting audio,
they were not built or optimized for that task, and extraction software
will have to work longer and harder to get the audio track from the disc
into a clean file for recording to CD-R.
4. If possible, dedicate a hard disk drive to ripped files, perhaps an
older, smaller hard drive that you have lying around. This prevents hard
disk phenomena (such as cross-linked files and excessive fragmentation)
from causing problems when you re-record the files to CD. If you use a
separate drive, you should have to defragment it less frequently, as all
the files on it will be large files. An added bonus is that, instead of
defragmenting the drive, you can just format it after you have made your
CD and be assured of clean contiguous disc space for your next
extraction job. (Recall that full defragmentation of a large hard drive
takes quite a bit of time, and ties up your computer til it's done.)
5. Get a good CD-ROM drive for audio extraction. How do you know which
ones are good? The Adaptec CD-R discussion list is a good place to find
out the opinions of many other CD-R users; the question has been
discussed extensively in the list in the past, and is frequently
re-discussed as new models are released. To see what's been said most
recently, have a look at the list archives at
http://listserv.adaptec.com . You can also join the list yourself and
ask; see
http://www.cdrcentral.com/community/policies.html for more
information.
Why Ripping Can Be Such a Pain
------------------------------
To understand why audio ripping can be so unpredictable, we need to look
at the structure and function of audio discs as opposed to data discs.
Copying files from a data disc to hard disk is easy and reliable. This
is not always the case with audio tracks. An audio (Red Book) disc is
divided into three distinct areas: the Lead In, the Program Area, and
the Lead Out. The location, or address, of each audio track on a disc is
stored in the disc's Table of Contents (TOC) in the Lead In area of the
disc.
The TOC of an audio disc, much like a book's, is a good source for
finding out what is where on the disc, but it cannot always lead you to
the right place in the book. Let's say we have a chapter in a book that
is entitled "How to Record an Audio CD". If we want to learn about
ripping, the TOC will tell us that this chapter begins on page 123, but
it does not tell us where within the chapter the part about ripping
begins. The Table of Contents on an audio CD tells the CD-ROM drive
approximately where a song begins on the disc, but, unlike a data
CD-ROM, it doesn't tell the drive exactly where it starts.
Since audio discs were designed to be played sequentially in real time,
it was not thought necessary to have information on the disc that
pinpointed the exact location of the beginning of a track; it was good
enough to get close to the location. To have that extra data with an
exact starting address for every track would have taken up space on the
disc that could otherwise be used for music.
The sectors on a data CD, on the other hand, has only 2,048 bytes of
user data in each 2,352-byte CD-ROM sector. These sectors can be
accessed exactly because the header information (the remaining 304
bytes) in each sector holds the precise address of the data block.
An audio block also contains 2,352 bytes, but all of these bytes are
used for audio. There is no header, so there is no information within
the block to allow for the exact positioning of a drive's read head over
a particular block. To locate a specific audio block, a CD drive must
take advantage of the Q subcode, but this allows head positioning only
to within 1 second of the true block address. When seeking an audio
block, a CD-ROM drive only moves the read head to a position close to
the requested block, and then it compares the Q subcode to the block
address being sought. The Q subcode references the minute, second, and
frames relative to the start of the track and also the Absolute Time
(that is, the time in minutes, seconds, and frames relative to the whole
disc).
When a drive is asked to seek to an audio sector, it begins reading,
then compares the Q subcode information to the block address it is
looking for. Data transfer begins when the drive has located a Q subcode
address close to the requested block address. Many CD-ROM drives seek an
audio address within four Q subcode addresses of the address being
sought (4/75th of a second in playback time). In this scenario, a
request for a particular audio block could return any of nine blocks
close to the desired position. This is why extraction is not exact.
Clicks and pops that you sometimes hear in ripped files can be caused by
this inexact positioning.
About Accurate Streaming Technology
-----------------------------------
Recently, some advances in extraction technology have made ripping much
less troublesome, and completely error-free in many cases. The ATAPI
(SFF8020) specification includes the new MMC command set and is now used
by many drive manufacturers in current lines of CD-ROM drives. The
Multimedia Command Set (MMC) has this advantage: many of the commands
that were previously performed in software can now be executed by the
CD-ROM's controller chip. One of these functions is the real-time error
correction of Layer 3 Reed-Solomon Product-like Code (RSPC). Others are
error detection, real-time ECC correction of one byte per P-word and
Q-word, and repeated ECC passes. Repeated ECC passes increase the
reliability of the drive's read function. Controllers from Oak
Technology and Winbond, the most widely used CD-ROM drive controller
chips, have these functions built-in. Accordingly, recorders and drives
with these chips can extract audio more effectively and efficiently;
less complicated algorithms can be used by the ripping software. As
these controller chips position the read head more accurately than
before, existing synchronized read algorithms will also work faster.
This is because data comparisons will match sooner and the head can then
move to the next portion of data quickly. This new feature is called
"Accurate Streaming". Drives using Accurate Streaming can rip in a burst
mode. Thus, extraction speeds are faster and the extraction is much more
accurate.
So, follow the above tips and rip away! Extraction is getting easier and
more accurate all the time. You will still have bad days. But at the end
of the day, when you are sitting back enjoying your latest compilation,
the bad times will fade away into the music.
-------------------------
If a technical term is unfamiliar to you, you can look it up in the CD-R
Glossary:
coaster
http://www.adaptec.com/tools/glossary/cdrec.html
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