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Author Topic: CDR Technical Info (includes hints on "Ripping")  (Read 1870 times)

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Offline wbrisette

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CDR Technical Info (includes hints on "Ripping")
« on: September 04, 2003, 11:19:04 AM »
Not from me and I posted this under another question, however it contains come great info and I thought others might want to read this, so I posted it separately.

Wayne

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About Digital Audio Extraction
------------------------------
by Bob Starrett
http://www.cdpage.com

Accurately 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
Mics: Earthworks SR-77 (MP), QTC-1 (MP)

Editing: QSC RMX2450, MOTU 2408 MK3, Earthworks Sigma 6.2

 

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