A friend asked me information about good sound cards since he wants to build a computer for composing (actually he plans to build a recording room). He has a Korg PA600 keyboard and plans to use that keyboard with it. I don't really know anything about sound cards so I'm wondering: what suggestions do you have? (suggestions on the other parts of the computer are welcome too)

For audio recording work, I would tend to go to an external pro audio interface rather than any type of old style internal sound card. Needs vary, but something like a Focusrite Scarlett 6i6 would be an example. It has XLR, 1/4", and MIDI connections. Another possibility would be the Presonus Audiobox.

I don't know how much is his budget, but I guess he's willing to spend quite a bit (I mean, he's literally going to build a freaking room, walls and all). How do those external interfaces work? Or well, get connected (wanting to make sure so I can explain it to him, since I understand English but he doesn't) Also looking into the Scarlett 6i6 right now.

And yeah I pointed out the MIDI stuff too (obviously) but I imagine any good enough sound hardware will have at least decent MIDI support anyway, so that's probably superfluous to ask about, or am I wrong? (and yeah, of course the keyboard has both MIDI input and output)

Most are USB 2.0, there are a few Thunderbolt based devices if he prefers. I picked some consumer type ones but there are a LOT of options to go upmarket (and several down). Some examples of more expensive kit:

http://www.sweetwater.com/store/detail/8preUSB

http://www.sweetwater.com/store/detail/Scarlett18i20

But it's really just about needs. Does he need 8 simultaneous channels of microphones? That's up to him. Actually he might find a mixer/control interface to be a more useful type of system:

http://www.sweetwater.com/store/detail/R24

OK turns out the guy knows way more than I thought (which makes me wonder why he doesn't know much about sound cards). And yeah he's aware about interfaces, he's looking for whatever goes connected to the motherboard (i.e. not the external part). Apparently his previous sound card added hiss from the keyboard input and he needs one that, well, doesn't mess with the sound input (at least what's perceivable). He showed me this, he said it must be at least as good as this if not better.

Also he just told me to look for the best I can find (he's being very serious about this), so yeah, that's the guideline I suppose.

Basic: http://www.bhphotovideo.com/c/product/970957-REG/esi_juli_xte_juli_64_xte_4_in_4_out.html

The best: http://www.bhphotovideo.com/c/product/517138-REG/RME_HDSP9632_E_HDSPe_AIO_PCIe.html

If you need plain-jane PCI: http://www.bhphotovideo.com/c/product/358371-REG/RME_HDSP9632_HDSP_9632_PCI.html

But I do want to emphasize that this is a totally pointless way to spend money, compared to a USB interface. At least for a keyboard and maybe a couple mics.

On the basic level USB interfaces are working as Preamps, Line Inputs, and Capturing MIDI data from your keyboard. Your software is going to process that data and then your interface will spit it back out to your monitors (aka speakers). There isn't a ton of difference between these devices beyond the quality of the pre-amps in them. Digi, PreSonus, MOTU, Focusrite all make similar USB audio interfaces. These are all extremely cheap and you can get them in the $500 range. http://www.sweetwater.com/store/detail/UltraLiteAVB

With an actual hardware based audio card you'll have a dedicated external interface DA/AD (Digital to Analogue, Analogue to Digital) which picks up your line inputs and midi data. Sends that to your internal sound cards which handle the audio processing for the software (DSP). Then back to the DA/AD and out to your speakers. This is the kind of setup you'll see in most non-home studios. But it's really expensive in the $10,000 - $20,000 range, plus the pre-amps you'll have to buy on top of that. http://www.sweetwater.com/store/detail/PTHDX216ana

If you're doing a lot of MIDI but still want to record live instruments, a good Semi-Pro point is the HD Omni interface. You get a single non-DSP PCI-E interface card and a good DA/AD. It's fairly affordable at $3000 and is a good starting point for a home studio. http://www.sweetwater.com/store/detail/HDomni

Well, was talking to him some more, it seems the problem is not so much with the external interface (btw, apparently that Scarlett 6i6 isn't enough for him?), but rather with the internal hardware in the computer. Last time he was burned because the sound hardware was adding hiss to the audio output from the keyboard (bad ADC I suppose?) and he wants one that doesn't do stuff like that.

Stupid question just so things are more clear for me (to avoid even more confusion), how do interfaces work? I.e. how are they connected inside the computer? Because I imagine some part has to end up inside eventually.

The interface just talks to the software, it can be internal on a card, external on firewire/usb, or a combination of both. It will take whatever signal it's given, like an analogue microphone signal, and turn that into a digital signal the software can use. Then coming back it converts that digital signal into an analogue signal that goes to your monitors (speakers). There are many other things it does, but that's the basic over-view of it.

As for an added hiss it would either come from defective hardware, a grounding issue, or just really cheap hardware.

If you really want to get the right thing for you I would give SweetWater or B&H Photo a call. Tell them what you're trying to do, what your setup is, and how much you want to spend and they'll give you options.

I don't have a good sense of how much your friend understands, so take this as you will...

You don't solve noise/hiss problems by throwing more money at the problem and buying "higher end". That is a fool's errand. You solve noise/hiss by understanding exactly what the source(s) are, and isolating things appropriately to deal with the problem. Depending on your equipment this may require a ground lift control or in rare cases, dedicated isolation boxes. In the case of an interface, your computer's internals are no longer responsible for any audio tasks. Everything flows over a USB/Thunderbolt/Firewire cable. You can buy as big or as small as an interface as you like. 6i6 too small? Go for an 18i20 if you like. Or a Presonus 1818, MOTU 8pre, whatever. (Of course be aware that with this many preamps on a single box, individual quality will suffer compared to smaller dedicated units.)

Now it sounds like maybe what is happening is that running MIDI from the keyboard is actually adding hiss to the analog (or speaker?) output on the keyboard. If that's the case I would think about replacing the keyboard, personally. But if the keyboard has digital outs (S/PDIF) then you can run that either to an interface and monitors, or a power amp with digital in. That should also deal with the problem.