http://developer.intel.com/pc-supp/platform/ac97/

You may also wish to read the Universal Serial Bus specifications. USB audio peripherals extend the built-in audio capabilities of mainstream Digital Ready PCs, and may even replace built-in audio with Digital Only² solutions for certain emerging market segments:

http://developer.intel.com/design/usb/

This section presents several highly visible and interrelated trends impacting PC audio:

• Increasing processor performance
• Integration of functionality
• External expansion busses

As processor performance increases, more functionality is accomplished in software. This is an industry-wide trend and can be observed across all platforms and CPUs. Hardware implementations face competition with software-only implementations and need to demonstrate a functionality, performance or quality advantage. There currently exist high quality software-only implementations of HRTF 3D, wavetable MIDI synthesis, and Dolby* AC-3* decode. By 2H98 or 1H99 there may be enough concurrency to perform more than one of these functions in software on mainstream platforms, but for high performance 3D computing and gaming platforms, hardware acceleration will continue to be desirable. It is becoming increasingly important that hardware acceleration be implemented in a scaleable manner (i.e. baseline software capability with a compatible hardware acceleration option).

The Intel audio is providing the industry with recommendations and supporting data on hardware vs. software partitioning (such as those which appear in Appendix A).

As the attach rate for a function goes up there is more incentive for integration onto the system motherboard or even into the SuperIO or chipset logic. This is also an observable industry trend. Full featured PCI-based SoundBlaster compatible and Digital Ready audio components will be available as standalone motherboard components in 2H97. The next potential points in this evolutionary trend could be to:

• combine the high performance audio controller with another motherboard device.
• incorporate a low cost baseline audio interface into the superIO (SIO) or similar device.

Many IHVs are proposing combination PCI devices for 1H98. This allows the IHV to amortize the PCI bus mastering controller support across multiple devices, saves pins, and potentially provides cost and layout size benefits.H98 Mobile platforms and add-in cards are immediate beneficiaries of these integration efforts. IHVs proposing combination devices face the additional complexity of needing to secure design wins for each subsystem device. OEMs and IHVs who choose to work together still face these issues relating to motherboard adoption of combination devices:

• It is difficult to project exact feature requirements 12-18 months in advance.
• There are diverse attach rates for the various audio, comm, graphics, and video functions.
• Scalability of hardware acceleration is often a requirement.
• Some subsystems face additional certification requirements (modem communications, for example).

A low-cost baseline audio interface (without hardware legacy compatibility or acceleration) might address the entry point of a fully scaleable architecture by 2H98. These interrelated issues must be considered:

• For which segments will baseline audio deliver adequate performance and compatibility?
• Is a fixed 48K or a dual (44.1/48K) rate Codec adequate support for Microsoft's WDM* digital audio?
• Should the implementation also support combined audio/comm or scaleable hardware acceleration?

External expansion buses offer PC OEMs system design and configuration flexibility, and PC customers user friendly upgrades. The gradual replacement of ISA add-in cards with USB is under way, and IEEE 1394 is also expected to gain momentum within the next couple of years. However, the transition to external digital audio is expected to be gradual because initial implementations will probably appear first at the mid- to high-end and cost more than highly-integrated motherboard audio solutions which make use of existing low cost analog peripherals. In addition, external solutions do not support specific capabilities currently required for many mainstream platforms, such as:

• hardware legacy compatibility (100% SoundBlaster* compatibility)
• internal analog connectivity (legacy CD-ROM audio, TV tuner, Video Capture audio in)
• some DirectSound* audio acceleration features (such as MIDI w/ DLS and multi-stream HRTF 3D)

For these reasons Intel recommends that in 2H98 external audio be viewed as an extension of the built-in Digital Ready audio capabilities, rather than as a complete system audio replacement.

However, on specific platforms, the PC OEM may choose to implement a Digital Only audio subsystem (entirely based on external audio components). For these cases Intel recommends that digital speaker and digital monitor implementations be selected so as to maintain maximum compatibility with existing (and emerging) PC applications which demand full-duplex audio capabilities by supporting both speaker output and mic input. External analog connectivity is also desireable, and can be met by providing line out, line in, and mic in connectors directly on the digital monitor or speakers.

This section briefly discusses the following developments:

1. Microsoft WDM digital audio infrastructure
1. SoundBlaster compatibility
1. Power management
1. DVD-ROM drives and PC to CE connections
1. Audio/communications integration
1. Digital "push audio" sources

The current release of Windows* 95 (OSR2.1) has limited support for USB devices, and does not put in place the necessary infrastructure to support a complete external audio solution. Enhanced audio support is targeted to be a future OS feature. Microsoft first described the WDM digital audio infrastructure in their September '96 Windows Hardware Newsletter. Hardware designs for 2H97 are nearing completion, but WDM audio potentially impacts hardware and device driver designs for 2H98 and beyond:

Key potnetial WDM audio features include

• low latency audio services
• ring 0 sample rate conversion and mixing
• streaming CD/DVD support
• SoundBlaster emulation (SoundBlaster Pro, MPU 401, software MIDI)

The low latency nature of the WDM audio services potentially enables Digital Ready audio implementations which employ PCI to USB (or IEEE 1394) re-direction using the multi-trip acceleration model, as well as Digital Only external USB (or IEEE 1394) based audio solutions.

SoundBlaster compatible DOS games write directly to 4 hardware devices (no APIs)

• SoundBlaster Pro or SoundBlaster 16 compatible registers
• OPL*-compatible FM music synthesizer
• MPU 401* compatible MIDI UART for General MIDI synthesis
• gameport (game pad or joystick)

The Microsoft DirectX* (Win9x) APIs have achieved significant momentum, but the reality is that SoundBlaster is still a significant standard for PC games, and includes well known CD-ROM titles such as Quake*, Doom*, Descent*, Duke Nukem 3D*, and Wing Commander*. A popular "Games on the Internet" web page shows (as of 3/7/97) that 907 out of the 1351 games catalogued (~67%) are DOS titles.

By Holiday 1997 DirectX games with 3D graphics & audio may outship DOS games. However, existing DOS games will not disappear overnight. Games have one life "on the (retail) shelf" and another one "in the (home) cabinet". PC buyers typically expect last year's best-selling CD-ROM titles to run on their new machines. Another example, those 10 CD-ROM game retail packs at the neighborhood superstore are often recycled DOS games. Last year's bestsellers, as well as shareware, freeware, and demoware games are also handed down within families, and/or traded via the internet and BBS.

No one disputes that SoundBlaster compatibility is a requirement for '98 systems. The only debate revolves around the viability of two alternate approaches to achieving compatibility:

• hardware SoundBlaster compatibility on ISA or PCI
• SoundBlaster compatible software emulation

Intel expects that both of these approaches will be evident in 1998, each with significant market share.

To date, OEMs have been adamant about requiring 100% hardware SoundBlaster compatibility for 2H97 systems — they don't want disappointed customers or to incur costly support calls. In terms of hardware cost, it is estimated that hardware SoundBlaster compatibility adds ~$2-3 to the total Codec cost in 2H97. With support calls costing an estimated $20 or more, it may be unrealistic to expect the majority of OEMs to commit to a software emulation capability until it has been successfully introduced into the marketplace, or been widely demonstrated and tested for compatibility and robustness by an independent software test house. Without this kind of data the level of exposure is unknown and the risks are not quantifiable. Many OEMs can be expected to choose the known quantity: 100% hardware compatibility. This means ISA- or PCI-based designs which implement ISA resources using various compatibility techniques.

SoundBlaster compatible single-chip ISA Codecs are currently mature and are inexpensive, but have limited features. Intel expects ISA-based audio to be phased out for add-in, and perhaps for motherboard components as well, by the end of 1998 for a number of reasons:

• generally lower audio and music synthesis quality (not all)
• limited multi-stream capabilities
• less configurability and manageability compared w/ PCI and USB
• obstacle to enabling Digital Ready audio
• growing momentum for ISA elimination

Intel has published detailed recommendations for one method of implementing 100% hardware SoundBlaster compatibility on PCI. The PC/PCI chipset implementation has been proven compatible by a generation of PCI-based mobile systems which support ISA SoundBlaster compatible cards in the dock, and is now available on desktop chipsets. "Implementing Legacy Audio on the PCI Bus" version 2.1 is available for download at:

http://developer.intel.com/pc-supp/platform/ac97

Intel expects PCI-based SoundBlaster compatible designs (including many first generation AC '97 designs) will be introduced in 2H97 and remain available until SoundBlaster emulation has been widely adopted (estimated by mid-1999).

WDM digital audio proposes to support software SoundBlaster emulation with the following features:

• SoundBlaster Pro register emulation
• MPU 401 UART emulation
• software wavetable MIDI synthesis (via emulated MPU 401)

OPL-compatible FM music synthesis is not proposed, but most current DOS games offer a choice of FM or General MIDI device (MPU 401) for music synthesis. The game port may be eliminated in systems without a SoundBlaster compatible Codec, but the analog joystick or gamepad can be emulated by a USB joystick.

The number of popular DOS games which cannot be supported via emulation is currently unknown, but expected to be small. No software emulator can deliver 100% SoundBlaster compatibility, because emulation is not expected to support the following:

• real mode operation
• DOS games which use proprietary memory extenders
• DOS games which require OPL-compatible FM for music synthesis
• DOS games which do not work w/ USB emulation of analog joystick or gamepad
• DOS games with hardware dependent features, timing, or performance
• legacy game controllers (analog joysticks and gamepads)

A viable SoundBlaster emulation option as an alternative to hardware would offer significant potential benefits to the PC audio architecture:

• enables cost reduced native PCI audio interfaces
• facilitates Digital Ready audio output via USB or IEEE 1394 by eliminating bus dependent SoundBlaster hardware

Intel expects standalone or combination PCI-based designs which utilize software emulation of SoundBlaster functionality will be introduced beginning in 1H98. These designs are still expected to provide high quality hardware wavetable MIDI w/ DownLoadable Samples (DLS), WDM-compatible DirectSound acceleration capabilities, and Digital Ready output. These designs are expected continue into 1999 and perhaps beyond.

There are two architecturally consistent specifications for power managing audio currently under development. These are the Advanced Configuration & Power Interface (ACPI), and the PCI Bus Power Management Interface Specification (PPMI). The choice of which specification to support is dependent upon the intended deployment of the audio subsystem. Any audio subsystem that is only targeting motherboard integrated implementations should ensure that their design can be implemented with an ACPI compliant motherboard/BIOS. On the other hand, if audio vendors wants to sell their audio solutions into the retail add-in market as well, the design must conform to revision 1.0 of the PCI Bus Power Management Interface Specification. The reader is encouraged to download one or both of these specs which are available at:

Advanced Configuration & Power Interface (ACPI)

http://www.teleport.com/~acpi

PCI Bus Power Management Interface Specification (PPMI)

http://www.pcisig.com

A more comprehensive discussion of the power management issues for audio appears in Appendix B of this paper.

DVD-ROM drives introduce new high-quality digital content into the PC environment. Potential digital audio formats include:

• DVD movies with high quality multi-channel audio formats: MPEG2*, Dolby Digital* AC-3*, etc.
• DVD audiophile music formats: 48 or 96 kHz sample rate, 20- or 24-bit audio, multi-channel
• DVD multimedia content specific to the PC: audio formats TBD

The PC and Consumer Electronics (CE) industries are working to enable the exchange of digital audio for audiophile quality movie and music playback, as well as high quality authoring. The Interactive PC Theater (aka Family Room PC), and eventually all PCs, will support digital, as well as analog interconnects to CE. But CE has a much longer average life cycle than PC equipment, and there is a significant installed base of recently purchased Dolby Surround* Pro Logic* (analog) home theater equipment. In the 1997-2000 time frame, universal backwards compatibility needs to be preserved via analog line out (stereo mini or RCA jacks) somewhere in the PC system. Higher performance and richer features can be enabled via digital audio output:

• S/P-DIF* or IEC958 support two-channel LPCM (up to 24-bits at 44.1 or 48KHz) or AC-3 encoded bitstream and are available on existing CE equipment
• USB or IEEE 1394 offer bi-directional exchange of multi-channel data and control and will be available on future CE equipment

The typical household is most likely to create a home theater in the Family Room. The home theater is a 10-foot social experience built around a large screen display and a high quality consumer A/V Receiver with six or more speakers. The typical Interactive PC Theater (Family Room PC) can be expected to deliver multi-channel audio output to the A/V Receiver in analog or digital form:

• Dolby Surround Pro Logic (or equivalent) encoded stereo, via stereo RCA jacks
• Dolby Digital AC-3 (or equivalent) encoded or decoded, via S/P-DIF, USB, or IEEE 1394

The home office or den is less likely to be used as a home theater. The office or den PC is a 2-foot personal experience more compatible with two speakers plus optional subwoofer (but nothing prevents the use of a larger display and/or more than two speakers). The typical baseline DVD enabled PC can be expected to deliver the following two-channel audio outputs:

• Dolby Surround Pro Logic (or equivalent) encoded stereo to a pair of analog or USB speakers
• HRTF 3D virtual multi-channel to a pair of analog or USB speakers

The current Audio Codec '97 specification offers no specific recommendations for the integration of audio and communications functionality. There has been substantial interest in developing a potential interoperable specification for integrating the audio and communications functionality. This would potentially enable a future generation of interoperable audio/communications designs which use AC-link (in addition to the proprietary AC-link designs which are currently being developed). Standalone audio only designs would not be affected. Shown below is one possible architecture that is under preliminary evaluation. Such an effort would require extensions to the baseline AC '97 specification, hence DC/AC'98 nomenclature. The Codec package may also have to be expanded to provide additional functionality that currently is not addressed in the AC '97 specification.

Desirable goals for audio/communications integration are:

• Optional extensions to existing AC '97 Codec specification (in the form of an appendix)
• Interoperable specification for portions of the Digital Controller (DC '98) functionality
• No changes to current AC-link timing and slot definitions
• No additions to baseline audio functionality, negligible impact on audio quality
• Support for software or hardware data pump via stuffing option (same DC '98 footprint)
• Definition includes support for one or two lines
• Support for low-cost "no telephony" stuffing options (easy upgrade from audio to audio/comm)

Direct Broadcast Satellite and similar technologies are expected to introduce new high quality "push audio" sources into the PC environment which potentially deliver digital audio data at a rate which differs (slightly) from the time base of the DACs built into the system. Several potential mechanisms need to be evaluated as solutions, but it is too early to specify or require one of these as a preferred solution:

• hardware A/V parsing and audio decode w/ "CD-like" analog interconnect to audio Codec
• hardware A/V parsing and audio decode w/ asynchronous I²S digital interconnect to audio controller
• software A/V parsing and audio decode w/ software drift detection and rate correction
• software A/V parsing and audio decode w/ hardware drift detection and correction done by the Codec

Figure 2. Digital Audio Migration

1. ISA - SoundBlaster compatible. These are mature, low cost single-chip implementations which deliver real mode and Win9x DOS box support for SoundBlaster registers and mixing, OPL-compatible FM synthesis, MPU 401 MIDI UART, and game port. They are fully Plug and Play compatible. The typical SoundBlaster compatible riser includes 3 or 4 stereo mini jacks (line and/or spkr out, mic in, and line in) and a DB15 for the game port.
1. PCI - SoundBlaster compatible. The majority of PCI implementations are new two-chip split digital/analog designs based on the AC '97 architecture. These motherboard designs support real mode and Win9x DOS box compatibility using PC/PCI, DDMA, or other similar techniques, and are fully Plug and Play compatible. They also support high quality wavetable MIDI synthesis and optional DirectX audio acceleration features. They use the same SoundBlaster compatible riser.
1. PCI - Digital Ready. The next generation of PCI-based designs potentially deliver WDM-compatible audio acceleration that can be part of a kernel mode streaming filter graph which targets either the built-in analog port or a USB port for output. These designs do not incorporate SoundBlaster hardware devices, instead they rely on WDM audio's SoundBlaster emulation capabilities. The Digital Ready audio riser potentially eliminates the analog game port but provides line and/or spkr out, mic in, and line in. The user can configure any mix of analog and/or digital peripherals.
1. USB (or IEEE 1394) - Digital Only. These systems eliminate all built-in audio resources and utilize external USB (or IEEE 1394) speakers and mic for audio I/O.

During this gradual analog to digital migration, Intel recommends that the built-in analog audio capability be viewed primarily as support for the existing 16-bit stereo analog standards, and that USB or IEEE 1394 be used to deliver new solutions for emerging high-end functionality which addresses authoring, audiophile rendering, and PC to CE connections for content which contains:

• greater than 16-bit data types
• more than 2-channels of decoded content
• higher than 48 kHz sample rate

Not all PC suppliers and customers will require or immediately make use of the PC's digital I/O capabilities. For 16-bit data types the quality of the built-in analog audio subsystem is likely to be comparable with audio rendered externally on USB. Stereo analog is a universal format supported by all existing PC and consumer audio equipment today, and the majority of PC to CE audio connections will still be analog in '98/99. Intel expects that for most systems:

• digital audio peripherals will complement analog peripherals
• support for existing Pro Logic CE will require an analog connection using stereo RCA jacks

Intel has worked to help establish USB as a mainstream platform capability. Establishing Digital Ready and Digital Only audio capabilities in 2H98 and beyond will help make the PC an attractive source of digital content. But for USB audio peripherals to become viable they will have to compete on features, quality, performance, and cost with built-in analog audio. It is Intel's position that market-driven technical and business requirements will dictate whether built-in or external, analog or digital audio peripherals be used in the various PC system implementations.

Audio has become a very important and highly visible part of today's PC experience. With the arrival of very high quality built-in audio components and external digital connectivity, the quality of the PC audio experience will rapidly become a function of the PC customer's budget for audio peripherals. The growing diversity of PC audio requirements, platform segments, and buses forces all industry players to acknowledge that there is more than one correct way to implement audio. What is needed is a scaleable (from low cost to high performance) architecture which supports audio on PCI, USB, and IEEE 1394, and comprehends built-in and/or external audio functionality.

Upcoming OS releases are expected to fully support external digital audio peripherals and emerging digital CE connections, increasing system flexibility and scalability on the high end. Until then a S/P-DIF output (built into the audio controller) offers the PC a high-end solution which can carry two-channel or AC-3 encoded audio to currently existing audiophile CE. By 1998, Intel expects digital extensions to the baseline system audio will emerge based on USB and IEEE 1394 specifications: USB for PC audio peripherals, and IEEE 1394 for connections to digital CE. Unlike S/P-DIF, these busses are capable of supporting supporting bi-directional exchange of multi-channel data and control between the PC and digital peripherals or CE. Baseline audio can support 100% digital expansion out USB or IEEE 1394 if all audio sources are available as digital streams and hardware audio acceleration is PCI-based and WDM-compatible (OS directs both source and destination).

AC '97 and USB (or IEEE 1394) should be viewed as overlapping yet complementary specifications that provide OEMs with more opportunities to address a wider range of platform implementations. Intel expects that the majority of PCs in 2H98 will support analog connectivity. But in the end, it is the PC OEM who is in the best position to determine whether a SoundBlaster compatible, Digital Ready, or Digital Only audio solution satisfies the customer's needs.

The volume PC delivers a high-quality baseline audio capability.

• CD quality output (~90 dB SNR), high quality mic and line in, full-duplex 16-bit stereo, 8 - 48 kHz
  (exception: mobile audio is driven by desktop equivalence in functionality, not necessarily quality)
• Analog stereo mini and/or RCA jacks for compatibility w/ existing peripherals and CE
• Speakerphone, headset, and POTS Video Conferencing support
• DirectX 3D games, DVD/CD-ROM, TV Tuner/VidCap, and multi-channel support
• Recommended: PCI-audio Controller + AC '97 Codec

Baseline PC audio is Digital Ready.

• Baseline audio supports 100% digital output via S/P-DIF, USB, and/or IEEE 1394
• All built-in audio sources re-directable as digital streams
• hardware audio acceleration PCI-based and Microsoft WDM-compatible

Baseline Digital Ready audio supports the following multi-channel output capabilities.

• Dolby Surround* encoded (Pro Logic) and virtual (HRTF 3D) multi-channel via stereo analog out
• True multi-channel (decoded Pro Logic, Dolby Digital AC-3, MPEG2, etc.) via USB or IEEE 1394

Recipe for advancing the audio platform and creating a scaleable architecture in 2H98.

• Target consumer quality audio, move audio hardware off ISA onto PCI
• Migrate the audio driver and PCI-based hardware acceleration to Microsoft's new WDM model
• Promote DirectX games and only DOS games which run under SoundBlaster emulation
• Support audiophile quality and true multi-channel via S/P-DIF, USB and/or IEEE 1394
• Partition hardware and software for scalability with respect to increasing processor performance
• Incorporate power management capabilities into components

Audio has become an essential element of every PC shipped today regardless of whether it is used in the office for its communications capabilities, or in the home for its entertainment as well as communications capabilities. While audio is integral to each and every PC that is shipped, it does not always appear on the motherboard, but rather is added in many cases by means of an expansion slot prior to shipment of the system by the OEM. There is also a flourishing retail upgrade market for PC audio to be considered as well.

Increasing awareness of the benefits delivered by operating system directed power management, from both the perspective of energy conservation as well as improvement of the user's experience, has fueled significant industry efforts targeting broad diffusion of intelligent, and interoperable power management features to all elements of the 1998 volume market segment PC.

There are two architecturally consistent specifications for power managing audio currently under development. These are the Advanced Configuration & Power Interface (ACPI), and the PCI Bus Power Management Interface Specification (PPMI). The choice of which specification to support is dependent upon the intended deployment of the audio subsystem. Any audio subsystem that is only targeting motherboard integrated implementations should ensure that their design can be implemented with an ACPI compliant motherboard/BIOS. If on the other hand a audio vendors wants to sell their audio solutions into the retail add-in market as well, the design must conform to the Revision 1.0 PCI Bus Power Management Interface Specification. The reader is encouraged to download one or both of these specs which are available at:

Advanced Configuration & Power Interface (ACPI)

http://www.teleport.com/~acpi

PCI Bus Power Management Interface Specification (PPMI)

http://www.pcisig.com

It is reasonable to assume that in most, if not all, cases audio vendors would not wish to limit their products' marketability to only motherboard design wins. As such it is highly recommended that all new PCI audio designs implement the PCI Power Management interfaces. Once an audio vendor has committed their product designs to the PCI power management interfaces these same products may be implemented as either motherboard or add-in solutions. And, in the case of motherboard integrated solutions, these PPMI compliant products would not impose a software burden on their OEM customers. ACPI compliant motherboard devices must be comprehended by the motherboard BIOS. PPMI compliant designs, on the other hand, rely solely on the device driver that ships with the audio hardware, as well as the power management aware PCI bus driver that ships with an OSPM-enabled operating system such as Window 95 or Windows NT* 5.0.

The remainder of these power management sections deal specifically with PCI power management interface design considerations.

The Audio Codec '97 specification defines 8 different power management states. These power management states, denoted PR0-PR7 power manage the following Codec subsections:

While fine granular power management states have been defined for the Codec, there are still a few issues to be resolved in order to apply them interoperably within a system that implements an operating system directed power management architecture.

The first issue is that these power management state definitions only address power management functionality for the Codec features and do not apply in any way to the AC '97 digital controller. The second issue is that the power management features of the Codec are only visible to the AC'97 device driver, and as such are not compatible with the WDM driver model until they can be made visible to its power management aware PCI bus driver, which completes the link between the device driver and the power management hardware interface.

The PCI Bus Power Management Interface Specification requires that a compliant PCI function, at a minimum supports the D0, D3_hot, and D3_cold states as defined below. The hardware infrastructure required to report, and manage the function's power management capabilities includes 2 DWORDs of PCI configuration space, as well as the linked list "Capabilities List" entry point hooks in the standard PCI function configuration space header. See the PCI Bus Power Management Interface Specification for detail.

When moving the PC into the family room, a change in the PC's character is needed that enables it to blend in with, and match the expectations users currently hold for the other consumer devices around it. Among these expectations:

• Always available
• Noiseless, and very low power when off

The IPCT (aka Family Room PC) should always be readily available when the user sits down to use it, much in the same way that televisions are expected to turn on almost immediately. Additionally, as the IPCT becomes the central processor for video/audio/data communications as well as entertainment, it should also be expected to always be connected to the phone line. Regardless of the operational state of the PC, it should always be capable of answering the phone, for example, to record an incoming call or to receive a fax.

Implementing the PCI power management interfaces for audio allows the operating system power management policy manager to be in control of turning audio on, and off. This architecture also enables a clean method for saving and restoring functional context which allows seamless resumption of suspended applications when the PC is ultimately brought out of a suspended state. And finally it specifies a wakeup protocol that is essential for AC '97 combo designs that integrate modem communications along with system audio.

The following picture illustrates a typical AC '97 combined audio/telephony solution suitable for the Interactive PC Theater (Family Room PC).

PCs shipped in the 1998 timeframe will incorporate a "dual mode" power supply that has on/off switchable V_CC as well as a persistent lower capacity auxiliary power source (V_AUX). In Figure 4 a combo AC '97 design is shown with certain portions of the subsystem powered by the main V_CC power supply, and other portions powered by an auxiliary power source (V_AUX).

The entire analog AC '97 Codec and AC-link are powered by V_AUX. The portion of the digital controller that is also powered by V_AUX are the areas of the modem data pump that must be kept alive in order to sense a ring indication from the line. The ring indication would be detected and used to initiate a wakeup event (PME#) to the system. Additionally, if Caller-ID is supported by the modem, the data pump logic must also be capable of parsing out and storing the Caller ID data as well.

In the lowest power "virtually connected" state, AC '97 Codec bits PR0-PR3, and PR6 would be set. This would internally power down all of the system audio subsections within the Codec while keeping the modem line Codec, and AC-link active. The digital controller would then be programmed to D3_hot, and at some point the V_CC power source would be switched off transitioning the digital controller to D3_cold, leaving only the previously identified "keep alive" logic active.

Note that in supporting power management events from D3_cold, all context involved in determining the need for, and generation of, a power management event must survive the transition from D3_cold to D0 (uninitialized). This includes the PME_Status, and PME_Enable bits of the PMCSR register, as well as any other PME context including perhaps a ring indication flag and Caller-ID data.