Audio Codec '97 Revision 2.1

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AC ‘97 Component Specification Revision 2.1 1.3 Integrating AC ‘97 into the System Bus sources •Wave •DirectX •soft CD or DVD •soft AC-3 CPU PCI bus multi-function accelerator data pump, ... AC ’97 digital controller AC ’97 analog Digital (I 2 S or equiv) sources •cost reduced CD (or DVD) •HW digital synthesizer •HW AC-3 decode scalable digital audio sample rate conv, SB compat, synth, DirectX, AC-3, ... game port AC-link & control (Audio Codec ‘97 architecture supports migration of sources toward digital) P2P digital AC-link (2-way TDM 5-wires) volume up/down & mute System motherboard Analog mixer sources •CD: Redbook audio (or DVD) •VIDEO: TV tuner or video cap •AUX: int source or upgrade synth •LINE_IN: ext source high quality analog i/o (48Kss) Description: Full-duplex, 16-bit stereo, 90 dB SNR, 2-chip motherboard solution Digital/analog split for scaleable features/higher quality Digital: PCI stand-alone or multi-function combined audio/telephony bus I/F, sample rate conversion, AC-link & control, plus options vendor specific package: supports a scaleable audio family Analog: high quality, fixed 48K sample rate DAC’s and ADC’s industry standard package: standard pinout, mixer, interconnect, & i/o options: tone, 3D, 18 or 20 bits, addt’l channels, modem ADC & DAC ... 14 LINE_IN LINE_OUT HP_OUT PHONE MONO_OUT MIC1 (desktop) MIC2 (headset) (speakerphone) modem Rx* modem Hx* modem Tx* (to/from DAA) digital audio USB, 1394 Figure 2. AC ‘97 1.0 System Diagram headset PC riser modem Digital Speakers or Consumer Equipment Standard system audio i/o -and/or- L R -and/or- L C R S S headphones or headset w/ mic 2 speakers w/ desktop mic ProLogic stereo played thru Consumer Equipment via line out (or decoded by the speakers) Digital audio “outside the box” is an extension to system audio L C R S L LFE S R 2-6 speakers attached to PC or Consumer Equipment The System Diagram in Figure 2 shows the essential features of an AC ‘97 audio design. The AC ‘97 analog component performs fixed 48K or AC ’97 2.0 compliant variable sample rate DAC & ADC conversions, mixing, and analog processing (tone, 3D stereo enhancement, etc.). It always functions as a slave to an AC ‘97 Digital Controller which must be implemented in the digital portion of any AC ‘97 audio system. The AC ’97 Controller, primarily targeted for PCI, can be as simple as a stand-alone design which supports high quality sample rate conversions to/from 48kHz, Sound Blaster* compatibility (no longer recommended), FM (no longer recommended) and/or wavetable synthesis, with optional DirectSound* acceleration, AC-3 decode, etc. The AC ’97 Controller may also be embedded within a PCI multifunction accelerator, offering higher levels of integration by combining audio with telephony or graphics. However, nothing precludes ISA, USB, or 1394 designs based on the AC ’97 architecture. The digital link, "AC-link", connecting the AC `97 Controller to the AC `97 analog is a bi-directional, five-wire, serial TDM format interface, designed for dedicated point-to-point interconnect on a circuit board, and extended by AC ’97 2.0 Appendix C to support multi-point connections between Controller and up to 4 Codecs. The diagram shows the most common (high attach rate) connections, some digital and some analog. PC audio today requires that a number of analog sources be supported in the analog mixer. Over time, it will become attractive from both cost and functionality perspectives to move these sources toward dedicated digital connections or onto the bus 3 . 3. The support for dedicated digital connections requires frequency locking and sample rate conversion capabilities in the AC ’97 controller in order to reconcile independent time bases, the digital source and AC ‘97’s fixed 48kHz. May 22, 1998
AC ‘97 Component Specification Revision 2.1 The AC ‘97 architecture facilitates this migration. AC ’97 2.1 Appendix D describes new analog mixer cost reduction options. The AC ‘97 1.0 architecture is designed primarily to support stereo two-speaker PC audio. However, two multichannel extensions are shown in the system diagram, one utilizing the AC ‘97 architecture and one independent of it: • Multi-channel encoded stereo (such as Dolby* ProLogic*) can be played out through the 2-channel AC ’97 audio subsystem. This type of signal can be played on normal stereo speakers, decoded into 4-channel by the speakers, or sent to consumer equipment via a stereo analog line out connection. • True 2/4/6 channel digital audio output (such as 5.1 channel Dolby AC-3*) can bypass the 2-channel AC ‘97 audio subsystem and be transmitted via a digital link (such as USB or 1394) to digital speakers or digital ready consumer equipment which drives a multi-speaker arrangement such as the home theater 4 . The AC ’97 2.0 architecture, specifically Appendix A, defines support for up to 20-bit 6-channel audio at 96 kHz. 1.4 Software Driver support and AC ’97 Controller / AC ‘97 Interoperability Regardless of the bus and level of integration chosen, the driver written for the AC ‘97 Controller is responsible for exposing and managing the AC ‘97 analog features. Interoperability requires that every AC ‘97 Controller and AC ‘97 driver identify and support the basic AC ‘97 features. Every AC ’97 1.0 Controller, and every AC ’97 2.1 compliant system (either Controller, Codec, of software driver) must be capable of performing high quality (~90 dB SNR) sample rate conversions on a minimum of four simultaneous channels (stereo out + stereo in), between a variety of sample rates and 48kHz: • 8.0, 11.025, 16.0, 22.05, 32.0, 44.1kHz See Appendix D for AC ’97 2.1 recommendations and requirements for the digital SRC capability located in the AC ’97 2.1 compliant Controller, Codec, or software driver. AC ‘97 2.1 compliance requires • ≥85 dB SNR • ≥17.64 kHz - 3 dB frequency response • ≤-65 dB passband THD+N • ≤+/- 0.5 dB passband ripple Digital SRC implementation (originally published in the AC ’97 Technical FAQ, September 1996): A PCI based audio system needs to support multiple concurrent input and output sources at diverse sample rates, such as PCM, MIDI, CD digital audio streamed from memory, or decoded AC-3 streamed from memory. The choice is either implement multiple DACs and ADCs running at various sample rates followed by analog mixing, or perform digital sample rate conversion and mixing at one common rate. The digital solution is preferred based on quality, cost and flexibility of implementation. The following is one possible implementation of digital SRC and mixing which can efficiently support an arbitrary number of digital PCM output sources at any of the standard PC sample rates with a minimum of dedicated SRC hardware resource. 4 There are many PC audio sources that are still potentially bus independent, such as DOS games, HW accelerated Windows* 95 games, CD Redbook audio, and DVD-ROM subsystems with HW AC-3 decode. In order to hear ALL PC audio sources through one set of digitally connected speakers, backwards compatibility must be addressed. 15 May 22, 1998
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Audio Codec '97 Revision 2.1

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