Let's start with the basics. Sound is a pattern of pressure that we sense - usually thought of as sound waves in the air. The usual way to deal with sound waves in electronics is to create an electrical analogue by exposing the sound waves to a microphone. The signal from the microphone can be amplified and recorded - still in analogue form - on audio tape or on a vinyl disc. It can also pass through an Analogue-to-Digital Converter (ADC) to become a digital signal. One of the functions of a sound card is to provide an ADC to deliver a digital signal corresponding to an original pattern of pressure. Another function is to synthesize a digital signal from instructions in the form of a MIDI or similar file. On the other side, the sound card provides Digital-to-Analogue Conversion (DAC) which is then used to drive tape recorders, speakers or other analogue devices. The inputs used for the purpose may be WAV files, MIDI synthesis or other digital signals the card accepts. Finally, the sound card allows mixing of its various analogue signals on input and/or on output. So the card will allow you to mix the analogue signal provided from its CD input with a synthesized sound from a MIDI file to drive your speakers with the combination. And a "full-duplex:" card allows such an analogue signal to be digitized while being played.
There are two ways to get sound from your CD reader. First, there's a headphone output with a volume control. Run a cable from that output to line in on your sound card, and you're ready to go just as you would be if the source was a tape deck. Even better (especially on internal drives) is to connect a special, analogue cable from the reader's plug to one marked CD on your sound card. The sound is the same at the two outputs, but with the internal connection you don't have to remember to twiddle the headphone volume control and you don't have that cable hanging outside the case. Finally, most CD readers support Digital Audio Extraction - DAE - which gets a page of its own in this primer. Recording from the CD as analogue uses the DAC of the reader to generate the signal, then the ADC of the sound card to convert back to digital. Each of those processes is imperfect and can degrade the sound. However, if your DAE is not perfect, the analogue connection may sound better to you - so, use it! Only the purists will insist that you should put up with what you don't like because It's the Right Way. One more point: if you have two CD sources, such as a reader and a writer, do not try to connect them both to the same jack on your sound card. Either get a card with dual stereo inputs or run that cable from headphone out to line in. (Connecting them both to the same input may work or may appear to, but can have unfortunate consequences.)
Let's look more deeply into the question of recording from CD-DA. Since you are likely to have both options, should you use DAE or analogue recording? The answer already given is: use what works for you. But there is more to it than that. Assuming that you do have the choice (good DAE and an analogue connection), when should you use analogue? First, some recordings have an extra bit set for preemphasis. That is a shift in the frequency response away from flat. Under some circumstances, it can let the producer record a LOUDER signal. If it's set, then DAE from the disc will sound too bright and lacking in bass. If all you're going to do is write that extracted file back to a CD-R, you can live with preemphasis as long as your software allows you to set the bit when you record. (Adaptec's PC programs do not.) If you want to edit the WAV file, you will definitely want to correct the preemphasis first; for that, you will find it easier to record analogue instead of ripping the track with DAE.
Another factor is error correction, which in general is beyond the scope of this page. The important thing about it is that errors recorded onto a CD-R or read from a CD-DA track will get through DAE without correction. But when the reader sends the signal to one of its analogue outputs, it passes it through filtering circuits which clean up some of those errors. The more cleanup the circuits do, the more they alter the sound quality. One reason that a copy of a CD-DA track may differ from the original is that the filters have more to do (CD-R has more audio errors than a pressed disc) and therefore change the sound more. Should you use analogue or DAE on a CD-R? Whichever sounds better to you - but be aware that they will have some differences in sound.
There's a lot of good material at http://homepages.nildram.co.uk/~abcomp/lp-cdr.htm with details not covered in this low-level page; stop by for more when you're ready.
E-mail me at cdrecording@mrichter.com
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