Rose and anyone else who wants background info about DAC's
This tries to answer "what is a DAC and why would anyone want one?"
DAC: Digital to Analog Converter - converts a digital signal to an analog signal.
Digital signal: music represented as 1's and 0's, similar to Morse code. It can be bits in a computer. hi and low voltage on a wire. The microscopic pits or lack of pits on the surface of a CD, which reflect or don't reflect a laser passing by. A digital signal is any information stored/transmitted in 2 states (binary), kind of similar to Morse code. I'll say "1's and 0's" from here on.
Analog signal: the air vibrations that hit your eardrums are analog pressure waves. you feel analog vibrations with your hands on the speaker or a nearby wall or window, or in your lungs when loud enough. These are the real musical vibrations as they came from the originating musical instruments. less frequent vibrations are bass notes. more frequent vibrations are treble notes, etc. An Analog electrical signal on a wire, if you could see it visually, would "vibrate" exactly the way you feel with your hand on the speaker. The electricity on the wire would oscillate exactly like the window does. I'll use "vibrations" from here on.
Here's picture showing gray analog wave and red digital samples
http://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Digital.signal.discret....the red digital signal has discrete values: 0,4,5,4,3,4,6,7,5 and so on. A sequence of 1's and 0's represents code, similar to Morse code. (ex: 0000 is 0, 0100 is 4, 0101 is 5, so the coded sequence is 000001000101 (0,4,5) Anyway, not necessary to remember this part. Just know that 1's and 0's represent the discrete red value's, which approximate the original vibration. the DAC tries to reconstruct the vibrations from the discrete values/pulses)
The analog signal is: the gray wave form or vibration.
The DAC coverts the red discrete values into a continuous wave (vibration). converts electrical pulses (codes) into a continuous electrical vibration
The quality of the DAC determines how accurately this gray vibration is produced.
A small digital mp3 player has music in 1's and 0's. but it has to go through a DAC before it goes out to the headphone jack(speakers), because speakers need the electrical vibration, not the 1's and 0's. (If you've heard the old sound a computer modem makes when it dials and connects to a service provider, that's the sound of pure 1's and 0's. Fax machine is doing the same thing. digital noise)
DAC quality differences (are multidimensional) and audible to us all. There's lots of DAC quality debate. The DAC in an mp3 player is cheap and relatively inaccurate. (it's just good enough)
There are many many separate DAC's on the market from $hundreds to many $thousands. and as many opinions of their quality and value per dollar.
A CD player take's 1's and 0's off a disc (the sequence of pits), and outputs small vibrations (thus an internal DAC had to have been involved). The quality of the DAC has a lot to do with the price of the CD player.
People who play music files straight from a computer...usually buy an external DAC of their choice. they feed 1's and 0's from their music files to the dac. The dac feeds small vibrations to an amplifier. the amplifier amplifies the vibrations to the needs of the speakers and then you hear those vibrations. (Decware amps do this will least alteration to those vibrations, so you hear what was originally there)
So, a DAC is a very fundamental device in modern music playback. most people don't realize they are there or what they do. People who care to control the quality of their music from source to ears, choose which dac they use. (or buy an excellent CD player, same thing).
to contrast: The only analog sources I can think of that don't need a DAC are cassette tape, reel to reel tape, vinyl records. The vinyl physically records the vibrations in the grooves. The magnetic tapes record the vibrations in the magnetic media instead of recording 1'0 and 0's into the magnetic media (There's is a device, a DAT (digital audio tape) that does exactly this, records 1's and 0's into the tape, and must use a DAC for playback))