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CD Recording FAQ
7. AudioCD
7.2. Digital Audio Extraction (DAE)
All cd readers come with an analog output. When you play back an audio cd, you can use this analog output to listen to music, either using earphones or your soundcard (if it is internally connected to your cd reader).
You can use this analog output to record music from any audio cd, but the final WAV file will not be a perfect copy. On the contrary, the result will be dependent on the ability of the DAC used by your sound card, while this process has a default speed of 1x.
The signal fed to the analog port of a cd reader is digital and is -theoretically- identical to the digital content of an audio cd.
Transferring this digital signal directly to your hard disk without any intermediate steps is called Digital Audio Extraction (DAE). With DAE, the cd reader reads the sectors of an audio cd and directs what is read through the interface which connects the cd reader to the computer (for example IDE or SCSI).
Though this step means that the signal is transferred directly without any quality loss, there are factors which influense the process. These are the optical capabilities of the source drive and its technical specifications, and the software used for the process.
7.2.1. The source drive
fA cd reader must be able to perform DAE. This has mostly to do with its chipset and firmware.
Apart from this basic demand, there are some other important points as well. Since the AudioCD format lacks an important layer of error correction, it is difficult to ensure that the stream retrieved is 100% perfect.
Accurate stream (SIA) support is a feature found in many modern drives, which ensures that the laser beam will be correctly guided without losing focus, thus eliminating jitter.
C2 errors ("soft errors") are errors which show that there is an error on the cd, but this can be corrected during the reading process. C2 errors are a measure of the condition of an audio cd (or the quality of the media used) and this feature is supported by the finest drives.
The interface also plays an important role. As the audio stream is digitally transferred from the cd reader, it is wise not to interrupt this flow. A SCSI drive supporting synchronous transfer and connected to a quality PCI SCSI controller certainly has an advantage when performing DAE at high speeds. This doesn't mean that IDE drives are not suitable for DAE though.
Whatever the case, it would be wise to avoid disturbing the DAE process by occupying the computer with other tasks at the same time.
7.2.2. Selecting the right speed for audio extraction
A cd reader may offer more than one speed selection for DAE, though many manufacturers prefer to offer only one speed, which is factory set to provide opimal performance.
Often manufacturers offer a utility to set DAE speed for their drives, or even third party utility may be used. Whatever the case, the speed selection available is directly defined by the drive's firmware. For example, if your software allows you to set DAE speed at 8x but your drive supports only 4x and 12x, setting speed at 8x will be illusional.
If your drive offers a selection of DAE speeds, not all of them ensure a perfect result. In general, we can define 3 speed limits, which are a result of a drive's limited optical capabilities:
- An optimal speed, which ensures 100% perfect results and usually is the default one. Many manufacturers will lock all other speeds to ensure that their products will always perform at optimal levels.
- A safe speed, which will give an audio stream with a few artifacts, which may not be even noticed by a casual listener. This speed will sound appealing for those who don't seek perfection.
- A highest speed, which will exhaust the mechanical and optical limits of the drive, but at a cost concerning audio quality which makes the result unacceptable.
To find whether a speed is good or not, you have to test it by extracting the same audio track a few times and then performing a binary comparison. If there are no differences, then this speed can be used to provide optimal results.
7.2.3. The software factor
In order to perform DAE, you need the appropriate software. However, not all DAE programs provide equal performance.
The main reason for this is that not all DAE programs check that the audio stream is properly transferred, while only a few programs provide support for C2 error retrieval.
Some programs go even further, by reading each sector multiple times and comparing the readings to ensure that their value is the right one.
To put it in another way, even if your drive can provide top quality
DAE performance, poorly designed software may give disappointing results.