From the ALSA wiki
AC97 (short for "Audio Codec 97") is a standard soundcard and network card interface Architecture developed by the Intel Corporation (back in 1997). Specialized hardware is available to peripheral manufacturers which implements the AC97 standard which has resulted in this standardized architecture being used in many motherboards, modems, and sound cards.
|Note: Intel's use of the word "codec" in "Audio Codec 97" refers to the signals being converted from analog to digital (enCOded) and from digital to analog (DECoded). This should not be confused with multimedia codecs which COmpress and DECompress video and audio data.|
In addition to AC97 defining control and data access functions (as well as a Plug and play setup interface), it also defines a couple standardized audio formats which are common to many desktop computers. This includes 16-bit and 20-bit sample size and 48 kilohertz and 96 kilohertz sample rates for multichannel recording and playback.
|Note: In 2004, AC'97 was replaced by the Intel's HD Audio standard.|
|Programming note: In addition to the audio interface, AC97 provides modem and network (Modem/LAN) interface functionality. If the Modem/LAN functions are to be used along with the audio functions, the audio device needs to be set as master and the Modem/LAN device set as slave. If the audio device is not used, the Modem/LAN should be set to be master device.|