Inside
There's nothing particularly spectacular-looking about the X-Fi PCI card itself; you could almost confuse it with an ordinary sound card. The X-Fi (CA20K1) circuit is a lot more compact than the one on the Audigy, for example. There are two models of PCI cards in the X-Fi line. The Elite Pro's has Cirrus Logic CS4398 D/A converters (192 kHz, 120 dB) and an AKM AK5394 A/D converter (192 kHz, 123 dB). The PCI card used on the other models has Cirrus Logic CS4382 D/A converters (192 kHz, 114 dB) and a Wolfson WM8775 circuit that includes a mixer for four analog sources and an A/D converter (96 kHz, 102 dB).
The Elite Pro's rack uses TI Burr-Brown PCM1804 A/D converters (192 kHz, 111 dB). For the headphone jack there's a Cirrus Logic CS4392 D/A converter (192 kHz, 114 dB). The conclusion you can draw from all of this is that the best input is the one on the PCI card of the Elite Pro. However, the inputs on its rack provide ample performance for its intended uses.
