Audio Compression Manager
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4/8/2010
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 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 **