อีกอันนึงที่ผมชอบมาก
SO:
a straight DtoA cannot generally touch mp3/WMA/etc
DtoD must be done for this happen, however that is accomplished (a DSP is involved, but there are specifics that do and don't matter depending on implementation); a soundcard's "processor" or your computer's general processor (either way, with the help of software) performs these calculations, to decompress/compute the stored data for a DAC to handle (basically theres a lot of "black boxes" between when you click play on the screen and an analog signal comes out)
a soundcard is *NOT* a DAC or an amplifier, xclusively, it has a DSP (Digital Signal Processor) which allows it to do many other things, in the case of something like X-Fi, it can actually do all of the positional audio from a game, decode movie soundtracks, perform SRC (Sample Rate Conversion) from more or less anything to anything (with better quality than anything I've seen with published #'s (Creative's #'s are -136 dB THD 20-20k for SRC, thats a lower noise floor than any speakers or amplifiers I know of)), and many other tasks; the reason I mention this is because many "audiophile" types (again, this is directed at nobody in this thread) will insist that a soundcard is "just a DAC", and that some sort of xternal device "offloads all processing", it does not, it simply moves the DtoA stage outside of the computer (sycraft xplained all of this in more detail); the general myth today holds that a S/PDIF DtoA or receiver somehow handles "all of the processing of audio" when connected, and therefore makes a soundcard worthless, neither of those devices (regardless of how much DSP horsepower they have) is actually handling the "rendering" of audio (I'll use my monitor analogy: if you have a DVI output graphics card, and a DVI input monitor, does the monitor therefore handle all of the graphics processing? no, you're simply sending the data out digitally, and just like with the monitor xample, DVI is not necessarily better than VGA)
amps aren't always required if your device (I use this word generically) can meet or approach IEC standards (which call for a 120 ohm headphone output), the vast majority of multimedia soundcards will handle/meet this just fine, the vast majority of mp3 players will not (for xample); again, don't buy the hype - plenty of people are happy to spend (waste?) your money for you (again, NOBODY in this thread has gone there)
for more directed comments at people in the thread:
audiochris gives a good xplanation and good links to information
archmage: ixbt still reviews cards, they have published reviews for X-Fi and a few other boards, for some reason they brand their site as both ixbt and Digit-Life though (so it makes finding some of their reviews a bit dodgy)
everyone thats said the DtoA on modern soundcards kick ass (I think the ntire thread has - xcept OP) I agree wholeheartedly, if anyone has a question about this, look at ither published manufacturer #'s (I don't want to hear whining about "well we can trust this vendor and not this vendor because I said so", that just defeats the ntire system and if you really wanna get that touchy theres nothing rational we can discuss) or published (and non-biased) review #'s (like ixbt) for quality multimedia/prosumer boards compared to some of these $300+ "audiophile" devices, in most cases they're qual or the difference is nearly immeasurable
however plenty of "audio types" will gladly tell you that one device sounds one way and one like another (I've started calling this kind of xplanation "this sounds like a strawberry while that sounds like a blueberry", for further understanding of why I object so much to this style of reviewing, read more James Randi)
USB devices are generally limited: they limit you to 16/48 stereo audio in the vast majority of cases, cost a considerable amount for the features they do provide, and if you're buying a USB to headphone amp output device, limit the hell out of you in terms of I/O (the only USB devices that go above/beyond this generalization, that aren't designed for professionals or project studios, are the USB Audigy/X-Fi hardware from Creative, and the very xpensive Bose Companion 5 speakers (they offer more features than 16/48 stereo, but cost A LOT and have no inputs; I am not suggesting them, simply providing them as an xample)
