The heart of a sound card are the digital to analog converters. Digital audio is stored within the computer not as wave forms, but as the plots, or maps, from which the digital to analog converters can re-draw and recreate the intended audio. As analog sounds are digitized, their wave forms are mapped out by an analog to digital convertor, which takes measurements of the audio's waveform many thousands of times a second. To play that audio back, the digital to analog converters re-build this information, making this the most important part of the sound card.
After the digital to analog converters have re-built the intended audio, various audio stages are required to amplify the signal to the proper volume, known as line level. These gain stages must be designed well as all of the sound card's output is colored by their amplification circuitry. Most integrated sound cards feature economy audio stages, which can erode the output sound quality. Dedicated, high quality sound cards feature custom designed audio stages featuring top quality components, providing a more clear and pure sound.
The digital to analog convertors map their output waveforms at a set frequency. The higher this frequency, the more times per second the waveform is plotted, resulting in a smoother, higher quality sound. Most computers operate at CD audio quality 44.1 kHz resolution frequency. Professional and higher quality sound cards can reproduce audio at rates up to 96 kHz, or higher, DVD quality.
The digital audio's information is stored within the computer at certain binary word lengths, known as bit depths. The higher the bit depth of the audio, the better sound quality is possible when recording or playing back. CD quality audio, and most sound cards, require these binary words to be 16 bits long, resulting in a depth of 16 bits. Higher quality sound cards can handle the DVD audio standard depth of 24 bits.