They do the same thing. Just the one in the computer is usually really, really poor quality.
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They do the same thing. However, an external DAC that can be used with a computer can be much higher quality than any CD player. Soundcards or motherboard implementations of DACs are usually poor quality and suffer from the poor power quality in the computer. This is why it is best to get an external DAC that is independent of the computer.
Like the CD player, the most important part of this equation is the master clock. There is one or more in the CD player and there is one or more in the computer interface of the external DAC. This is actually more important than the DAC itself.
"audioengr, if you started from digital out in a computer, could it compete with the best "stand alone CD players"; if you used the best DAC?"
No way. No expensive DAC will fix this. In this case, the master clock is inside the computer. It's this clock that is critical. Need to get it out of the computer and into an external box with a good power source, like a USB converter or USB DAC. The clocks used in these also have lower jitter than anything used in a computer or soundcard.
Think of the master clock as the phonograph cartridge. If you dont have a good one, there is nothing you can do downstream for any amount of money that will repair the damage that it does to the signal. In the cartridge case, its an analog signal. In the digital case, its a digital signal.
Networked interfaces are another option, but these are mostly proprietary and have poor clocks IME (Sonos etc..). You can of course mod them to change to lower jitter clocks or put reclockers after them to improve the jitter performance.
Orpheus - There are no "good" master clocks in computers, or sound cards for that matter.
This is the point I'm trying to make. The master clock should be outside of the computer, in another chassis with a good power source etc.. This is the ONLY means to low jitter playback.
This is what is happening in Async USB interfaces and Squeezebox Touch. They have the master clock. The computer is the slave.
If you dont want to change the DAC, then the next step is to insure that you have a good ripper and good playback software. Good rippers are dbpoweramp for PC and XLD for Mac. Good playback software is Amarra 2.3.2 (4317-19), Pure Music or Jplay for PC.
These will not improve on the master clock jitter, but they will insure that the data is delivered intack.
Then, third-order effects for improvement include putting the OS on a SSD rather than spinning hard disk, improving the PC power supply and eliminating extraneous applications.