There may be nothing about CD-Recordables more confusing than Digital Audio Extraction. So, let me try to explain what it's all about and why it is an issue. As usual, corrections are invited; I am not an expert in this field and can be accused of sharing my ignorance, not my knowledge.
In the beginning, the compact disc format was defined for digital audio - CDDA. The rules for CDDA are defined in a standard called The Red Book (not the Little Red Book - that's the one with Chairman Mao's wisdom). CDDA says that there's a small file (in the computer sense) on the disc which identifies the tracks that follow. Then there are the tracks with a precisely specified sequence of bits in which two channels of audio at 16 bits per channel and 44,100 samples per second are interleaved. Those tracks are not files - they are simply tracks with strings of bits. The only files on the disc are those little pointers which tell the laser where to go to get a track, how long it is, etc.; they could hold a bit more information, but since few CD players would use it, few publishers bother to put it in.
The difference between a track and a file is worth looking at. After all, a WAV file may be an interleaved pair of strings with the same properties as the track. But the WAV file contains information about its contents, for example, the fact that it's sampled at 44.1 KHz; it also has an associated CRC (cyclic redundancy check) which lets the retrieving software determine whether it has been read correctly. When stored on a CD-ROM, it has an extra layer of error correction; the track has a minimal level in the hardware that we'll discuss later. In short, the WAV file carries overhead, 'wasted' space, with information not in the CDDA track.
When a CD player recognizes an audio track, it routes the bit stream to its DAC - digital-to-analogue converter. There the minimal layer of error correction is applied to patch up some errors in a disc that's in good shape (it cannot do much more) and the analogue sound output is generated. Since an error is usually in a single sample and a sample lasts less than 25 microseconds, you're not supposed to notice it. However, we would like to capture that digital data stream to our system in order to record it onto a CD-R and perhaps even to edit it. That means we wish to take raw data extracted from the audio track and convert them into a file that can be stored, modified and written with a computer. To do that, the reader must feed the digital data to an output that the computer can read. If the drive will provide that information, it is giving you Digital Audio Extraction, DAE. Not all CD-ROM readers do that, so some simply cannot provide the signal that's needed. You can run any software you wish, if it won't provide the output, the output won't be there.
In some cases, the firmware of the reader can be changed to provide DAE; in others, it's a hardware problem - period. In addition, something must tell the drive that the computer wants the DAE output - that the signal should be diverted from the DAC to the digital output. It can also tell the reader how fast to try to read the CDDA stream. Another critical need when the computer is trying to convert the stream of bits into a file is that they come through on time and in sequence. Again, if there's a bit of inconsistency in the audio stream, we probably won't hear it; but the computer is not as forgiving as the human ear and it wants each sample to be meaningful, not just 90% or 99% or even 99.9%. So where a sample or two can be dropped every so often while listening to a CD, none can be lost if the data are to go into a file.
Thanks to all the error correction on digital data, a CD-ROM reader may run at 4 or 8 or 20 times the speed that a CDDA is supposed to play. But without that correction, the audio stream may not be able to run at full speed. In fact, on some supposedly high-speed CD-ROM drives, CDDA may not even extract at 1x - 150 KB/sec. In other cases, there are so many errors that the resulting file is unacceptable. Again, the problem is not the software but the reader's hardware and/or firmware. All that the software has done so far is to command that particular reader to read CDDA as digital data to the appropriate output port, receive the data stream, and slow down the reader if the data are not usable to build a file.
Finally, the software comes into play to do a bit more. It takes the received data stream of a track, converts it to a WAV file and writes it to the hard drive if that is being used as an intermediate. By writing to the HD, the DAE is allowed to be slower than would be needed to write directly to the CD-R. If you do not go to the HD, then noise or something else may slow the reader enough to underrun the buffer on the writer (it is not getting data fast enough to keep writing) and your CD-R is another coaster. Writing the data to the HD buffers the information so that varying read speed is acceptable. In addition, you can listen to the WAV file that the software records to confirm that it's good enough to write to your valuable blank. Of course, if you are sure that the reader and the CD-ROM are good, or if you are in more of a hurry to get a copy than you are in need of a good one, you are free to burn 'on the fly' - directly from the reader to the writer.
Most readers do not use even the little error correction available when performing DAE. Only the Plextor family consistently corrects the data stream on the way to the digital port. And few drives but the Plextors extract CDDA as quickly as they do digital data. Those using Plextors for DAE are not praising them because Plextor is paying them; they are simply sharing with you their best advice on saving money and time. You are free to ignore their (our) recommendations, but don't expect a whole lot of sympathy when you do. And I am sure that they/we will be happy to let you know when another vendor delivers quality signals from a quality product. Until then, we will have to restrain ourselves from saying: 'I told you so' when you find that you spent more for your 24x EIDE than you would have four a 4+ and cannot get decent DAE even at 1x.
E-mail me at cdrecording@mrichter.com
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