Audio Compression Manager (Windows Embedded CE 6.0)

1/6/2010

The Audio Compression Manager (ACM) adds system-level support for the following services:

• Transparent run-time audio compression and decompression
• Waveform Audio data format selection
• Waveform audio data filter selection
• Waveform audio data format conversion
• Waveform audio data filtering

The Audio Compression Manager Catalog item adds support for the ACM to your OS design.

The ACM uses existing driver interface hooks to override the default mapping algorithm for waveform audio devices. This allows the ACM to intercept device-open calls. After a call has been intercepted, the ACM can perform a variety of tasks to process the audio data, such as inserting an external compressor or decompressor into the sequence.

The ACM manages the following types of drivers:

• Compressor and decompressor (codec) drivers
• Format converter drivers
• Filter drivers

Compressors and decompressors change one format type to another. Format converters change the format, but not the data type. Filters do not change the data format at all, but they change the waveform audio data. A single ACM driver, or a filter tag or format tag within a driver, might also support combinations of these types of changes.

For waveform audio output, the ACM passes each buffer of data to the converter as it arrives. The converter decompresses the data and returns the decompressed data to the ACM in a "shadow" buffer. The ACM then passes the decompressed shadow buffer to the waveform audio driver. The ACM allocates the shadow buffers whenever it receives a prepare message.

For waveform audio input, the ACM passes empty shadow buffers to the driver. It uses a background task to receive a notification after the driver has filled the shadow buffer. The ACM then passes the buffers to the driver for compression. After compression is finished, the driver passes the data to the application.

The following ACM stream codecs are provided:

• GSM 6.10
• G.711
• **MS ADPCM
• IMA ADPCM
• **
• **

For more information, see Audio Compression Manager Drivers.

Implementation Considerations

The following table shows the Sysgen variables associated with the ACM.

Sysgen variable	Description
SYSGEN_AUDIO_ACM	Set this Sysgen to add the ACM to your OS design.
SYSGEN_ACM_GSM610	Adds GSM 6.10 codec support for the ACM to your OS design.
SYSGEN_ACM_MSFILTER	Adds a simple ACM filter to your OS design.

Modules and Components

The following table shows modules and components for the ACM.

Item	Module	Component
ACM support in the Waveform Audio API	wapiacm	None
Core ACM support	mmacm	None
User interface for ACM support in the Waveform Audio API	wapiacmui	None
User interface for core ACM support	mmacmui	None

Hardware Considerations

The ACM does not require special hardware support.

Dependencies

The following table lists the Catalog item dependencies that most directly relate to this item's functionality. There are additional dependencies as well. For more information on determining the complete list of dependencies for this Catalog item, see Catalog Item Dependencies.

Catalog Item	Sysgen variable
Waveform Audio	SYSGEN_AUDIO

See Also

Other Resources

Audio