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APPENDIX

Microsoft Audio

B Compression

Manager

The Microsoft Audio Compression Manager (ACM) is a standard interface for audio compression in Windows. This interface allows applications such as Sound Forge to use compression algorithms provided by other companies.

Sound Forge fully supports audio compression through the ACM. This allows you to use any ACM- compatible compression. Sound Forge transparently opens compressed WAV files and provides all available compression formats for WAV files in the Save As dialog.

There are two major components to the ACM:

•Audio data compression and decompression

•Transparent playback and recording of non-hardware supported audio files

Audio data compression and decompression

The first component of the ACM allows audio data to be compressed and decompressed. Audio compression is used to decrease the amount of data required to represent a sound and results in smaller sound files. However, there are drawbacks to using audio compression on sound files:

•Most audio compression algorithms degrade sound quality. This is referred to as lossy compression because information contained in the sound is lost when it is compressed. The amount of sound degradation is dependent upon the algorithm.

•Compressed audio requires more processing time than uncompressed data. The amount of processing time is dependent on the algorithm as well as the system’s hardware. Typically, opening and saving compressed files takes longer than uncompressed files.

•Compressed files are not as portable as uncompressed files. To distribute WAV files in a compressed format, you must verify that the audience can use them. Also, not all audio software can use compressed WAV files, which may make using other applications with Sound Forge difficult.

In Sound Forge, any compressed WAV file can be opened if a compatible ACM driver is installed. If no compatible ACM driver is available for a compressed WAV file, Sound Forge alerts you to the problem.

Saving compressed WAV files is as simple as specifying the compression algorithm in the Format drop-down list of the Custom Settings dialog. Once a compressed file is saved, Sound Forge automatically saves updates to the file using the selected compression algorithm. The compression format can be changed later—or reverted to an uncompressed format—using the Save As dialog.

Transparent playback and recording of non-hardware supported audio files

The second component of the ACM is called the Sound Mapper. It allows playback and recording of audio data formats that are not directly supported by the sound card. You can select the Sound Mapper as the Playback and Record devices on the Wave tab of the Preferences dialog. You can perform any additional configuration of the Sound Mapper from the Windows Control Panel.

APPENDIX B

MICROSOFT AUDIO COMPRESSION MANAGER