From the ALSA wiki
Realtek produce a series of 2,6 and 8 channel audio chips which support the Intel Audio Codec 1997 (AC'97) and the more recent Intel High Definition Audio standard for converting between analogue and digital audio data streams. The current (July 2007) set of audio chips include:
2-Channel: ALC101, ALC202, ALC203, ALC250 6-Channel: ALC650, ALC655, ALC658 8-Channel: ALC850
High Definition Audio
2-Channel: ALC260, ALC262, ALC268 6-Channel: ALC662 8-Channel: ALC861, ALC880, ALC882, ALC883, ALC888, ALC885
Higher numbers may indicate better quality or more options.
For use with a computer, the chips need to be connected to one of the computer buses. This may be done via a dedicated soundcard or, if the chip is on the motherboard, by direct connection to the motherboard southbridge. See Sound Cards: Introduction
Thus if the motherboard is based on the Nvidia Nforce4 chipset, an ALC650 may be connected to the Nvidia CK804 chip. It can then be driven using the intel8x0 Alsa kernel module which supports the Nvidia interface. If the motherboard is based on a Via chipset, the ALC650 may be connected to a Via8233 southbridge chip. It can the be driven using the via82xx Alsa kernel module. Note that in such cases the system will appear to contain a Nvidia or Via soundcard.
As input this accepts digital PCM (pulse coded modulation) data from the computer and analogue signals from a CD, microphone, telephone and other audio equipment. PCM data is first converted to an analogue signal. The different analogue signals can then be amplified and combined to produce the main stero line-out signal. Output to the additional four analogue output channels can come either from their own PCM input channels or from the line-out signal.
In a separate analogue path through the chip, the input analogue signals (plus the line-out signal) can be combined and amplified to produce a digital output signal for input to the computer. The chip can also send and receive data from other audio devices using the Sony/Phillips Digital Interface Format (S/P DIF).
http://www.realtek.com.tw/ Realtek Semiconductor Corporation