Newer CD-RW, and CD-ROM drives are enabling people to create customized archives of their favorite prerecorded music. Roxio's Easy CD-Creator, for example, features the SoundStream utility to build music CDs and even removes pops, hiss, and other problems from old analog cassette tapes and vinyl LPs.

Digital audio extraction allows the digital tracks on commercial CDs to be transformed into WAV files by compatible mastering programs. Those WAV files, exactly like the ones created from older music sources via your sound card, can then create a CD-R, which can be played back in any popular CD stereo system.

Many users can take advantage of this type of software to burn greatest hits collections and holiday CDs from their purchased cassette and music CD collections.

This exciting technology is not intended to give you a way to create a free music library. Instead, use it to give the music recordings you've paid for an extra dimension of usefulness, and of course, to make legal backups of the discs you have purchased.

Digital Audio Extraction

All CD-ROM drives can play Red Book–formatted CD-DA discs, but not all CD-ROM drives can read CD-DA discs. The difference sounds subtle, but it is actually quite dramatic. If you enjoy music and want to use your PC to manage your music collection, the ability to read the audio data digitally is an important function for your CD (and DVD) drives because it enables you to much more easily and accurately store, manipulate, and eventually write back out audio tracks.

CD-ROM drives installed in PCs can play audio discs. The playing function is simple: Using a CD or Media player application (such as the one included with Windows), you can insert a CD-DA disc into a CD-ROM drive and play it just as you could with a standard audio CD player.

While playing, the analog sound waveform is sent over a thin stereo cable (usually referred to as the CD audio cable) connected between your CD-ROM drive and the sound card in your PC. The same analog waveform usually is also sent to the headphone jack on the front of the drive (or sound card).

Your sound card then amplifies the analog signal so you can hear it through the speakers plugged into your sound card or via headphones plugged into the front of the drive (or the sound card). That is just fine if all you want to do is play discs, but if you ever want to record one of the songs on your hard disk, you will run into some problems.

To transfer the song to your hard drive, you would have to play the song as you did normally and simultaneously use a sound recorder application, such as the Sound Recorder supplied with Windows 95 and later (similar recording software is also typically supplied with your sound card), to redigitize the audio waveform for storage as a .WAV file on the PC.

This means the sound goes from digital as originally stored on the disc to analog in the CD-ROM drive and back to digital in your sound card, with the resulting digital file being only an approximation of the original digital data. Another drawback is that this procedure runs only at 1x speed—hardly an ideal situation!

It would be much better if you could read the original digital data directly off the disc. That was not possible with older CD-ROM drives, but newer drives can do what is called digital audio extraction (DAE).

In this process they read the digital audio sectors directly and, rather than decode them into analog signals, pass each 2,352-byte sector of raw (error-corrected) digital audio data directly to the PC's processor via the (ATA, SCSI, USB, or FireWire) drive interface cable.

Therefore, no digital-to-analog conversion (and back) occurs, and you essentially get the audio data exactly as it was originally recorded on the CD (within the limits of the CD-DA error-correction standards). You would have essentially extracted the exact digital audio data from the disc onto your PC.

Another term for digital audio extraction is ripping, so named because you can "rip" the raw audio data from the drive at full drive read speed, rather than the normal 1x speed at which you listen to audio discs. Actually, most drives can't perform DAE at their full rated speeds.

Although some are faster (or slower) than others, most perform DAE at speeds from about one-quarter to about one-half of their rated read speed. So, you might be able to extract audio data at speeds only up to 20x on a 40x rated drive. However, that is still quite a bit better than at 1x as it would be on drives that can't do DAE (not to mention skipping the conversion to analog and back to digital with the resultant loss of information).

Virtually all newer CD/DVD drives can perform digital audio extraction on music discs. How fast or accurately they do this varies from model to model. One might think any extraction (digital copy) of a given track (song) should be the same because it is a digital copy of the original; however, that is not always the case.

The CD-DA format was designed to play music, not to transfer data with 100% accuracy. Errors beyond the capability of the CIRC in the CD-DA format cause the firmware in the drive to interpolate or approximate the data.

In addition, time-based problems due to clock inaccuracies can occur in the drive, causing it to get slightly out of step when reading the frames in the sector (this is referred to as jitter). Differences in the internal software (firmware) in the drive and differences in the drivers used are other problems that can occur.

Positioning can also be a problem because the CD-DA format was designed to stream (play continuously) and not to read individual sectors. CD-ROM sectors are 2,352 bytes long, and these bytes are further divided into 2,048 bytes of data plus 304 bytes of synchronization, header, and additional ECC information to control positioning and allow for error-free reads.

No such synchronization, header, or extra ECC information exists for audio sectors; instead, all 2,352 bytes are used for pure audio data. To address an audio sector, the Q subcode information is used instead.

Most audio players position to within only 75 sectors (1 second) using the Q subcode information. CD-ROM drives that can perform digital audio extraction are usually much more accurate than that, but because of how the subcode works (as well as the cross-interleaved way audio data is stored), designing a drive that can position every time to the precise audio sector that starts the track can be difficult.

All of this conspires to cause inaccuracies or slight differences in multiple extractions of the same track (song). Perfect extractions are possible, but making perfect extractions is difficult to achieve for many reasons. For example, even a slight amount of dirt or scratches on the disc has a great effect on the quality of your extractions, so be sure the discs are clean.

As a test of your drive's capability to perform DAE, try extracting the same track (song) multiple times from a new, clean, scratch-free disc, using a different filename for each extraction. Then, bring up a command prompt and use the FC (file compare) command to compare the different files to each other.

If they compare exactly, you have a combination of hardware and software that can do perfect or near-perfect extractions. If you intend to do a lot of extracting, you should ask around to see what hardware and software others are using for this purpose.

As a general rule, SCSI drives work better than ATA, but some ATA drives are just as good as the better SCSI drives. Plextor is well known for drives that are excellent when it comes to digital audio extraction, and I've always had good luck with Toshiba drives.

The bottom line is that DAE enables you to extract audio data tracks directly to your PC as .WAV files. Once on the PC, you can play the WAV files as is or convert them to other (usually more compressed) formats, such as MP3 (MPEG-1/2 Layer III) for use with the MP3 audio players on the market.

You can also use a CD-R/RW drive that can perform DAE to make copies of audio CDs (for backup purposes only) or to compile several songs into your own greatest hits collections that you can use to burn your own custom audio CDs.

"For Music Use Only" CD-R/RW Discs

According to the Audio Home Recording Act of 1992, consumer CD recordable drives and media sold specifically for recording music are required to have specific safeguards against copying discs, mainly SCMS. That means these recorders can make digital copies only from original prerecorded discs.

You can copy a copy, but in that case, the data being recorded goes from digital to analog and back to digital on the second copy, resulting in a generational loss of quality. The media for these recorders must be special as well. They work only with special discs labeled "For Music Use," "For Audio," or "For Consumer."

These carry the standard Compact Disk Digital Audio Recordable logo that most are familiar with, but below that, as part of the logo, is an added line that says "For Consumer." These discs feature a special track prerecorded onto the disc, which the consumer music recorders look for.

Built into the price of the AHRA-compliant media is a royalty for the music industry that this track protects. The media costs about 20%–30% more than what regular CD-R/RW media costs. If you try to use standard non-AHRA-compliant CD-R/RW discs in these drives, the drive refuses to recognize the disc. These music devices also refuse to copy CD-ROM or data discs.

Note that this does not apply to the CD-R/RW drive you have installed or attached to your PC. It does not have to be AHRA compliant, nor does it need to use AHRA-compliant "For Music Use" media, even if you are copying or recording music discs. Additionally, you can make digital copies of copies—the SCMS does not apply, either.

The bottom line is that you do not have to purchase AHRA-compliant discs for the CD-R/RW drives in your PC. If you do purchase such discs, despite the "For Music Use Only" designation, AHRA-compliant discs can be used in your CD-R/RW drives just as regular CD-R/RW discs and can be used for storing data. The extra information indicating AHRA compliance is simply ignored.