1 don't worry about that, it's only if you get a dac without USB input.
2 when you plug the dac in your computer will recognise it straight away and just use it as a sound card ... There are things that can be done to improve the sound ... But you can come back to that when you get that far
3. The pc knows to use the dac because it is recognised as an external sound card ... Some computers require you to select the external sound card by going into the control panel, going to "sounds" and selecting it over the computers in built one. Some will just do it straight away without needing to be set .. If the problem arises and you cant fix it, pop back and ask the question and I'll tell you in more detail how to fix it...
4 the dac becomes the sound card for everything, although it is recommended that you turn of all the pc sounds - the little "ping" you get once in a while - apart from anything they get annoying when they come out the hifi
Also, the pc will be fine doing it's normal day to day tasks as the music players don't take up much power ... The only time my computer struggles with music is when I'm processing hundreds of photos ... Any other time it runs fine...
Don't worry to much if about the logistics of buying and setting this up, it only gets fiddly when trying to optimise the set up for the best quality... Until then it's just getting it plugged together correctly (very easy) and at worst having to change a fee very simple settings in windows ....
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