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 Audio tab of the Preferences dialog. You can perform any additional configuration of the Sound Mapper from the Windows Control Panel.

The Sound Mapper functions as follows. When faced with a sound file recorded at an unusual sample rate such as 22,257 Hz and a sound card that supports 22,050 Hz, the sound file normally cannot be played. The sample rate of the file must be changed to 22,050 Hz before it can be played back, but changing the sample rate without resampling causes a pitch shift. However, the Sound Mapper plays this file without resampling by mapping the sound to the best format possible and performing the resampling in real time.

In addition, the Sound Mapper plays compressed sound files, even on sound cards that do not support compression directly. A file compressed with Microsoft ADPCM or The DSP Group’s TrueSpeech plays on any sound card without first decompressing the file.

The Sound Mapper can, under the right circumstances, record compressed sound files. Compressing sound data can be computationally expensive, and the amount of time required is dependent upon the specific compression algorithm and how it is implemented. Decompressing sound data is typically faster than compressing the same sound data.

It should be noted, however, that Sound Forge software does not play and record compressed sound files directly. Rather, all compression and decompression is performed while opening and saving the files. This limitation is fairly insignificant, and the compressed sound files are saved using the best possible quality—something that cannot always be done in real time. Compressed sound files saved with the software typically sound better than those recorded with audio compression.

After you save uncompressed audio data to a compressed format, you should audition the file. Compression and decompression are performed during opening and saving; therefore, the compressed file is not accurately represented until it has been reopened.

338 APPENDIX B