The specification covers two types of component, and the "AC-link" digital interface between them:
AC'97 defines a high-quality, 16- or 20-bit audio architecture with 5.1 surround sound support for the PC. AC'97 supports a 96 kHz sampling rate at 20-bit stereo resolution and a 48 kHz sampling rate at 20-bit stereo resolution for multichannel recording and playback.
In 2004, Intel released Intel High Definition Audio (HD Audio) which is a successor that is not backward compatible with AC'97. HD Audio has the capability to define up to 15 output channels, but in practice most motherboards provide no more than 8 channels (7.1 surround sound).
AC'97 has had several revisions:
The AC-Link is a digital link that connects the DC97 (the controller) with the audio "codecs." It is composed of five wires: the 12.288 MHz clock, a 48 kHz sync signal, a reset signal, and two data wires which carry the actual audio data: sdata_out and sdata_in. The first four are outputs from the controller, while sdata_in carries input from the codec. The link carries a bidirectional serial data stream at a fixed bitrate (12.288 Mbit/s) between the controller and one or more codecs.
Each 12.288 Mbit/s stream is divided into 256-bit frames (frame frequency is 48 kHz). This is therefore a time-division multiplexing (TDM) scheme.
Every frame is subdivided in 13 slots. The first (slot 0) is 16 bits long and contains validity flags for the remaining slots, while the remaining 240 bits are divided in twelve 20-bit slots (slots 1-12), used as data slots.
Slots 1, 2 and 12 are used for non-audio data, while slots 3-11 carry up to nine channels of raw pulse-code modulation (PCM) audio signals. Normally, six channels are used for 5.1 surround sound, and three channels are available for modem use. However, slots can be combined to provide a 96 kHz sampling rate for the L, R and C channels.
Lower sample rates (such as 44.1 kHz) are implemented using a handshake protocol between the controller and codec which skips data during certain frames. (This capability depends on the codec. Alternatively, sample rate conversion could be performed in the DC97 (controller) or in the software driver.)
Codec chips have an AC97 interface on one side and analog audio interface on the other. They are usually small square chips with 48 pins (48-pin QFP package). They are D/A and A/D or only D/A.
Computer motherboards often provide a connector to bring microphone and headphone signals to the computer's front panel with standard color jack. Intel provides a specification for that header; the signal assignments are different for AC'97 and Intel High Definition Audio headers.
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