AC'97

AC'97 (Audio Codec '97, also known as MC'97 for Modem Codec '97) is a standard for audio codecs that Intel Architecture Labs created alongside several codec manufacturers in 1997. It saw use in motherboards, modems, and sound cards. The specification defines two main component types and the AC-Link digital interface that connects them: a digital controller (DC97), usually integrated into the chipset's southbridge, and an audio or modem codec that handles the analog parts. An optional digital signal processor for audio effects is also part of the spec.

The standard describes a high-quality audio architecture supporting 16- or 24-bit resolution and 5.1 surround sound on PCs. It allows 96 kHz sampling at 24-bit stereo and 48 kHz sampling at 24-bit stereo for multichannel recording and playback. Integrated audio using an AC'97 codec could be placed directly on the motherboard, on a communications and networking riser card, or on an audio/modem riser card.

The first system to ship with AC'97 was the Cyrix MediaGX in 1997. Intel began shipping support in its I/O Controller Hub in 1999, but most PC OEMs did not make AC'97 audio the default until public criticism in 2000. In 2004, Intel introduced High Definition Audio (HD Audio) as a successor, which is not backward compatible with AC'97. HD Audio can define up to 15 output channels, though most motherboards in practice provide no more than 8 (7.1 surround sound).

AC'97 went through several revisions. AC'97 1.x compliant means a fixed 48 kHz sampling rate without extended features. AC'97 2.1 adds an optional extended audio feature set, including variable rate and multichannel support. AC'97 2.2 extends audio features, adds enhanced riser audio support, and optionally includes the Sony/Philips Digital Interface Format. AC'97 2.3 adds extended configuration information and optional jack sensing, enabling Plug and Play for the end user by letting the codec supply parametric data about its analog interface, similar to Intel High Definition Audio.

The AC-Link is a digital connection between the DC97 controller and the audio codecs. It uses five wires: a 12.288 MHz clock, a 48 kHz sync signal, a reset signal, and two data wires—one for output from the controller (SDATA_OUT) and one for input from the codec (SDATA_IN).