back when computer-based audio workstation were new,these unit were separate pieces,but today most DAW interfaces by manufacturers such as digidesign, M-audio, Universal Audio,and MOTU have ADCs and DACs that are built into the same box along with a few mic preamps. that said, it's still true that when you get into higher-endsystems,such as pro tools HD, the ADCs and DACs are separate higher-quality units capable of greater fidelity. You'll even find specialized super high-end outboard unit like thosefrom apogee or lavry capable of even higher fidelity than those used in DAW manufacturer's interface.