monsieur:fatboyslimfast:pvanblarcum:USB or digital out, it just pushes digital signals to the DAC without any processing in the sound card, so if you ask me the soundcard is irrelevant Erm, nope. The soundcard may well do some mixing or resampling of it's own (as PJPro has mentioned, a lot of sound cards are 48KHz, whilst CD and MP3 are normally 44.1KHz), so there may be differences there.does mean the frequency range of the soundcard would be used over the range of the DAC then?
Forget frequency range. References to 44.1khz, 48khz, 96khz etc are the sampling rates (how many times per second the signal is sampled) which is an entirely different thing to the frequency of the music itself.
16 bits can support 65536 (or 2 raised to the power 16) integer values and this represents the potential 'resolution'. 24 bits can support well over 16,000,000 (2 raised to the power 24) integer values so the signal resolution is that much finer. (Imagine it as a bit like pixels on a camera sensor. 65536 pixels are not going to give as fine a 'resolution' or detail as 16,000,000 will.)
So just imagine an oscilloscope trace of one second's worth of musical signal diplayed on a screen. Now along the horizontal axis you have 44,100 or 48,000 or 96,000 time intervals withiin that one second of signal. Every time each of those intervals comes along the DAC will 'sample' (or measure) the signal along the vertical axis to an accuracy of 1/65526 (16 bits) or an accuracy of 1/16000000 (24 bits)
The bit rate and the sampling rate are fixed values. The frequency of the musical signal is obviously variable just as it is with analogue.