XAPO Overview

The XAPO API allows the creation of cross-platform audio processing objects (XAPO) for use in XAudio2 on both Windows and Xbox 360. An XAPO is an object that takes incoming audio data, and performs some operation on the data before passing it on. You can use an XAPO to perform a variety of tasks, including adding reverb to an audio stream and monitoring peak volume levels.

Creating New XAPOs

The XAPO API provides the IXAPO interface and the CXAPOBase class for building new XAPO types. The IXAPO interface contains all of the methods that need to be implemented to create a new XAPO. The CXAPOBase class provides a basic implementation of the IXAPO interface. CXAPOBase implements all of the IXAPO interface methods except the IXAPO::Process method, which is unique to each XAPO.

For an example of creating a new XAPO, see How to: Create an XAPO.

For an example of creating a XAPO that accepts run-time parameters, see How to: Add Run-time Parameter Support to an XAPO.


XAPOs implement the IUnknown interface. The IXAPO and IXAPOParameters interfaces include the three IUnknown methods: QueryInterface, AddRef, and Release. CXAPOBase provides implementations of all three of the IUnknown methods. A new instance of CXAPOBase will have a reference count of 1. It will be destroyed when its reference count becomes 0. Implementations of IXAPO and IXAPOParameters should follow the same pattern to allow for their proper management when used with XAudio2.

XAPO instances are passed to XAudio2 as IUnknown interfaces. XAudio2 uses QueryInterface to acquire an IXAPO interface and to detect whether the XAPO implements the IXAPOParameters interface. Implementations of IXAPO must accept requests for __uuidof(IXAPO). If IXAPOParameters is implemented, it must also accept requests for __uuidof(IXAPOParameters).

Using an XAPO in XAudio2

XAPOs are used in XAudio2 by attaching them to voices. Each XAudio2 voice has an effect chain containing zero or more audio effects. Audio data sent to a voice is passed through each effect in the chain before it is sent to the voice's output targets. Data is passed from the voice to each effect using the pInputProcessParameters parameter of the IXAPO::Process method. Then it is returned to the voice using the pOutputProcessParameters parameter. The voice takes the output of each effect, and feeds it into the next effect in the chain until no effects are left in the chain.

For more information about XAudio2 effect chains, see XAudio2 Audio Effects.

For an example of using an XAPO in XAudio2, see How to: Use an XAPO in XAudio2.

Effect Libraries

The XAPO effect library contains several XAPOs, and a common method of instantiating them. See XAPOFX Overview for information about XAPOFX. Also, XAudio2 has built-in reverb and volume meter effects. See XAudio2 Audio Effects for more information about the built-in XAudio2 effects.

Audio Effects

XAudio2 Audio Effects