Ok. I thought I understood the issues but now I'm quite confused. Isn't the digital output produced by a computer/sound card a mathematically determined sequence of bits? In other words, once an MP3 is decoded in software, shouldn't the spdif output be exactly the same regardless of what sound card is used?

I ask those that really know what goes on, because I'm doing research to purchase a good audiophile quality sound card (I'm interested in the analog output), and reading the reviews, I keep seeing reviewers evaluate the quality of the digital (spdif) output. As in, the spdif output with card x sounds way better on my stereo system with digital input than the spdif output of card y. Does this issue of sampling rates and convertions apply to digital outputs? I am talking about straight sound with no effects.

Can someone shed some light on this?