What’s New for Core Audio APIs in Windows 7

The Core Audio APIs were introduced in WindowsВ Vista, which provided a new set of user-mode audio components that a client application can use to render or capture audio streams with improved audio capabilities. For a general overview of this API set, see About the Windows Core Audio APIs.

The Core Audio APIs have been improved in WindowsВ 7. The following table summarizes the new features and the improvements to the Core Audio APIs:

Feature	Description
Generic improvements	The following features have been improved in WindowsВ 7: In WindowsВ 7 share mode streams run in low-latency mode. The audio engine runs in pull mode with a significant reduction in latency. This is very useful for communication applications that require low audio stream latency for faster streaming. WindowsВ 7 provides better device role detection when a new device is added to the system. For more information, see Working with Device Roles. In WindowsВ 7 you can listen to music from your portable media player through your computer speakers. You can use this Capture Monitor feature by plugging a portable media player into your computer with an analog audio cable. In the past some OEMs have provided this functionality in the audio driver by using a hardware loopback. In WindowsВ 7, this functionality has been added to the operating system. Because this is in the system and not the driver, you can use this for any other device connected to the system, such as a USB headset. HDMI audio has been enhanced in WindowsВ 7, which provides support for high-bit-rate format. With this improvement, you can support multichannel audio and compressed audio formats over an HDMI connector to an audio receiver. In WindowsВ Vista, Windows Media Player plays music only through the default audio device, which cannot be changed by the user. For Windows Media Player to render audio to a particular device, the default device must be changed in the Sounds control panel. In WindowsВ 7, Windows Media Player provides APIs that enable an application to render to any device selected by the user and not just the default device. In WindowsВ Vista, if a computer that is playing audio switches to the power-save mode, the computer is locked, and if the user wants to interrupt the playback, the user must log on and press the stop key to stop the playback. In WindowsВ 7, if the computer is locked, you can still control the playback by using the HID control on the keyboard. WindowsВ 7 reduces power consumption for any application that uses DirectSound and DirectShow to render media. In addition, the Multimedia Class Scheduler Service enables glitch-resilient audio and uses less power while the audio samples are being generated.
Communication device (New)	In this release a new device type has been added to the Sounds control panel: Communications device. This device is used primarily for communications, that is, to place or receive phone calls on the computer. A communication application can use Core Audio components to get a reference to the endpoint of the default communication device and render audio streams for communication purposes. The operating system considers the stream opened on a communication device to be a communication stream. The WASAPI operations on a communication stream are similar to any other audio stream. For more information, see Working with Device Roles.
Stream attenuation or audio ducking (New)	Automatic ducking or Stream Attenuation is a new feature in WindowsВ 7 that is intended for VoIP and Unified Communication applications. By default, the operating system reduces the intensity of an audio stream when a communication stream, such as a phone call, is received on the communication device through the computer. The volume options are set by the user in the Sound control panel. New APIs have been added in the Windows SDK that enable applications to replace the default ducking behavior. For more information about implementing a custom ducking feature, see Providing a Custom Ducking Behavior.
Stream routing (New)	In WindowsВ 7, the Core Audio APIs have been improved to transfer an audio stream seamlessly from an existing device to a new default audio endpoint. High-level audio API sets that use Core Audio APIs, such as Media Foundation, DirectSound, and WAVE APIs, implement the stream routing feature. Media applications that use these API sets to play or capture a stream use the default implementation and do not have to modify the application. However, if your media application uses Core Audio APIs directly, the application needs to provide the stream routing implementation. To do so, the application must handle new events that have been added that notify a WASAPI client when the default device is connected or removed. For more information about this feature, see Stream Routing.
Protected User Mode Audio (PUMA) (Improved)	PUMA has been updated for WindowsВ 7 to provide the following features: Setting Serial Copying Management System (SCMS) bits on S/PDIF endpoints and High-bandwidth Digital Content Protection (HDCP) bits on High-Definition Multimedia Interface (HDMI) endpoints. Enabling SCMS and HDMI protection controls outside of a Protected Environment (PE). For more information about the improvements, see Protected User Mode Audio (PUMA).
The WAVEFORMATEXTENSIBLE structure has been extended to the WAVEFORMATEXTENSIBLE_IEC61937 structure (New)	In WindowsВ 7, a new structure has been added to support IEC 61937 transmissions. WAVEFORMATEXTENSIBLE_IEC61937 extends the WAVEFORMATEXTENSIBLE structure to store two sets of audio stream characteristics: the encoded audio format before transmission and characteristics of the audio stream after it has been decoded. The new structure explicitly specifies the effective number of channels, sample size, and data rate of a non-PCM format. With this information, an application can infer the quality level of the non-PCM stream after it is decompressed and played. For more information, see Representing Formats for IEC 61937 Transmissions.
IAudioClient::Initialize (Improved)	The IAudioClient::Initialize method has been improved to indicate specific errors that might occur while opening an audio stream. The new error codes are: AUDCLNT_E_BUFFER_SIZE_NOT_ALIGNED AUDCLNT_E_BUFFER_SIZE_ERROR AUDCLNT_E_INVALID_DEVICE_PERIOD For more information about these errors, see the Return Value section in IAudioClient::Initialize.
IAudioCaptureClient::GetBuffer and IAudioRenderClient::GetBuffer (Improved)	IAudioCaptureClient::GetBuffer and IAudioRenderClient::GetBuffer methods have been improved to return the AUDCLNT_E_BUFFER_ERROR error code that indicates that the endpoint buffer in the exclusive mode was not retrieved. For more information, see Remarks in IAudioCaptureClient::GetBuffer and IAudioRenderClient::GetBuffer.
Jack detection capability (Improved)	A new interface in WindowsВ 7, IKsJackDescription2, extends IKsJackDescription. By using the new interface, the audio stack or an application can get additional jack information. This includes the jack’s detection capability and whether the format of the device has changed dynamically.
Windows Samples (New)	New samples have been added to the Windows SDK that demonstrate the use of the Core Audio APIs. For more information, see SDK Samples That Use the Core Audio APIs.

Major New Interfaces

The following interfaces are new for WindowsВ 7:

Core Audio Interfaces

This programming reference for the Core Audio SDK includes the following interfaces:

MMDevice API

The Windows Multimedia Device (MMDevice) API enables audio clients to discover audio endpoint devices, determine their capabilities, and create driver instances for those devices.Header file Mmdeviceapi.h defines the interfaces in the MMDevice API. For more information, see About MMDevice API.

The following table lists the MMDevice interfaces available with the Core Audio SDK for Windows Vista.

Interface	Description
IMMDevice	Represents an audio device.
IMMDeviceCollection	Represents a collection of audio devices.
IMMDeviceEnumerator	Provides methods for enumerating audio devices.
IMMEndpoint	Represents an audio endpoint device.
IMMNotificationClient	Provides notifications when an audio endpoint device is added or removed, when the state or properties of a device change, or when there is a change in the default role assigned to a device.

WASAPI

The Windows Audio Session API (WASAPI) enables client applications to manage the flow of audio data between the application and an audio endpoint device. Header files Audioclient.h and Audiopolicy.h define the WASAPI interfaces. For more information, see About WASAPI.

The following table lists the WASAPI interfaces available with the Core Audio SDK for Windows Vista and later.

Interface	Description
IActivateAudioInterfaceAsyncOperation	Represents an asynchronous operation activating a WASAPI interface and provides a method to retrieve the results of the activation. Applies beginning with Windows 8.
IActivateAudioInterfaceCompletionHandler	Provides a callback to indicate that activation of a WASAPI interface is complete. Applies beginning with Windows 8.
IAudioCaptureClient	Enables a client to read input data from a capture endpoint buffer.
IAudioClient	Enables a client to create and initialize an audio stream between an audio application and the audio engine or the hardware buffer of an audio endpoint device.
IAudioClock	Enables a client to monitor a stream’s data rate and the current position in the stream.
IAudioClock2	Enables a client to get the current device position.
IAudioClockAdjustment	Enables a client to set the sample rate of a stream.
IAudioRenderClient	Enables a client to write output data to a rendering endpoint buffer.
IAudioSessionControl	Enables a client to configure the control parameters for an audio session and to monitor events in the session.
IAudioSessionControl2	Enables a client to get information about the audio session.
IAudioSessionManager	Enables a client to access the session controls and volume controls for both cross-process and process-specific audio sessions.
IAudioSessionManager2	Manages all submixes including enumeration and notification of submixes. It also provides support for ducking notifications.
IAudioSessionEnumerator	Enables a client to enumerate audio sessions.
IAudioStreamVolume	Enables a client to control and monitor the volume levels for all of the channels in an audio stream.
IChannelAudioVolume	Enables a client to control the volume levels for all of the channels in the audio session that the stream belongs to.
ISimpleAudioVolume	Enables a client to control the master volume level of an audio session.
IAudioSessionEvents	Provides notifications of session-related events such as changes in the volume level, display name, and session state.
IAudioSessionNotification	Sends notifications when session changes occur.
IAudioVolumeDuckNotification	Sends notifications about pending system ducking changes.

DeviceTopology API

The DeviceTopology API provides client applications with the ability to traverse the functional hardware topologies of audio rendering and capture devices. Header file Devicetopology.h defines the interfaces in the DeviceTopology API. For more information, see Device Topologies and DeviceTopology API.

The following table lists the DeviceTopology interfaces available with the Core Audio SDK for Windows Vista and later.

Interface	Description
IAudioAutoGainControl	Provides access to a hardware automatic gain control (AGC).
IAudioBass	Provides access to a hardware bass-level control.
IAudioChannelConfig	Provides access to a hardware channel-configuration control.
IAudioInputSelector	Provides access to a hardware multiplexer control (input selector).
IAudioLoudness	Provides access to a «loudness» compensation control.
IAudioMidrange	Provides access to a hardware midrange-level control.
IAudioMute	Provides access to a hardware mute control.
IAudioOutputSelector	Provides access to a hardware demultiplexer control (output selector).
IAudioPeakMeter	Provides access to a hardware peak-meter control.
IAudioTreble	Provides access to a hardware treble-level control.
IAudioVolumeLevel	Provides access to a hardware volume control.
IConnector	Represents a point of connection between components.
IControlInterface	Represents a control interface on a part (subunit or connector).
IDeviceSpecificProperty	Represents a device-specific property of a connector or subunit.
IDeviceTopology	Provides access to the topology of an audio device.
IKsFormatSupport	Provides information about the audio data formats that are supported by a software-configured I/O connection (typically a DMA channel) between the audio device and system memory.
IKsJackDescription	Provides information about the jacks or internal connectors that provide a physical connection between a device on an audio adapter and an external or internal endpoint device (for example, a microphone or CD player).
IKsJackDescription2	Provides convenient access to the KSPROPERTY_JACK_DESCRIPTION2 property of a connector to an endpoint device.
IKsJackSinkInformation	Provides information about the jack sink if the jack is supported by the hardware.
IPart	Represents a part (connector or subunit) of a device topology.
IPartsList	Represents a list of parts (connectors and subunits).
IPerChannelDbLevel	Represents a generic subunit control interface that provides per-channel control over the volume level, in decibels, of an audio stream or of a frequency band in an audio stream.
ISubunit	Represents a hardware subunit (for example, a volume-level control) that lies in the data path between a client and an audio endpoint device.
IControlChangeNotify	Provides notifications when the status of a part (connector or subunit) changes.

EndpointVolume API

The EndpointVolume API enables specialized clients to control and monitor the volume levels of audio endpoint devices. Header file Endpointvolume.h defines the interfaces in the EndpointVolume API. For more information, see EndpointVolume API .

The following table lists the EndpointVolume interfaces available with the Core Audio SDK for Windows Vista.

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