HD Audio Description

High Definition Audio is the new audio implementation that has been released by Intel to replace the older AC97 standard.
The HD Audio specification is not backwards compatible with the AC97 standard.
This is basically a protocol between a controller and a codec, including the electrical interface.

The HD Audio standard defines one controller and a number of codecs with 5 signals running between them; BCLK, SYNC, SDO, SDI, RST.
BCLK [bit clock] is a 24MHz clock signal generated by the controller.
SYNC [frame sync] is a 48kHz frame synchronizer generated by the controller.
SDO [serial data out] and SDI [serial data in] are serial output and input signals [referenced to the controller].
RST [reset] is a global reset signal. The ICs use 3.3 volt signaling.

The SDO line is bused to all codecs, while an individual SDI line runs to each codec.
S/PDIF Description

Intel controls the High Definition Audio specification
High Definition Audio Specification; Revision 1.0, 4/15/2004 [Initial release]
High Definition Audio Specification; Revision 1.0a, 6/17/2010
The specification may be down-loaded from Intel.

Description of other types of interface buses used in a personal computer;
Personal Computer Buses

Editor note; I only list the HD standard in case it comes up in conversation.
Otherwise it's really just a specification to interface to an Intel controller.
HD Audio is not an interface specification in its own right, in the general sense.
Also, no one is obligated to conform to the standard, unless interfacing to an IC that does.

Home
Distributors	Components	Equipment	Software	Standards	Buses	Design	Reference