The first piece of the ACM allows you to compress and decompress audio data. Audio compression is used to decrease the amount of data required to represent a sound; this ultimately results in smaller sound files. However, there are drawbacks to using audio compression on your sound files:

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

• Audio compression requires more processing time than uncompressed data. The amount of processing time is dependent on the algorithm, as well as your hardware setup. As a result, opening and saving compressed files will usually take longer than uncompressed files.

• Compressed files are not as portable as uncompressed files. If you are distributing files in a compressed format, you must ensure the person receiving the files can use them. Also, not all audio software can use compressed .wav files, which could make using other programs with Sound Forge software inconvenient.

In Sound Forge software, any compressed .wav file can be opened as long as a compatible ACM driver is installed and enabled. If there is no compatible ACM driver available for a compressed .wav file, Sound Forge software will inform you of the problem when you try to open it.

Saving compressed .wav files is as simple as choosing the compression algorithm in the Format drop-down list of the Save As dialog. Once a file has been saved as compressed, Sound Forge software will always save changes to the file using the selected compression algorithm; you do not need to reselect the compression format each time you save. However, you can change the compression format or revert to an uncompressed format at any time with the Save As dialog.

The Microsoft Sound Mapper allows audio data formats that are not directly supported by your sound card to be played and recorded. Sound Forge software lets you use the Sound Mapper by selecting it for playback and recording on the Audio tab of the Preferences dialog.

Using the Sound Mapper with uncompressed files

The primary use of the Sound Mapper for uncompressed sound files is for your convenience. You don’t have to convert the sound to a supported format before you listen to it:

• If, for example, you have a sound file that is recorded at a nonstandard sample rate such as 22,257 Hz, and the closest sample rate that your sound card supports is 22,050 Hz, then the sound file normally cannot be played. You would have to change the sample rate of the file to 22,050 Hz before you could play it. However, changing the sample rate (without resampling) would cause the sound to play at a lower pitch. Using the Sound Mapper, you can play this sound file correctly without resampling the file first. The Sound Mapper will map the sound to the best format possible and perform the resampling in real time.

• The Sound Mapper will allow you to play 16- or 24-bit sounds on an 8-bit sound card, play stereo sounds on a mono sound card, and record stereo files on a mono-only sound card. However, when you use the Sound Mapper to record in stereo from a mono source, the mono input of your sound card is simply duplicated in both channels–the Sound Mapper cannot create something that is better than what the sound card can supply.

Using the Sound Mapper with compressed files

The Sound Mapper allows you to play (and sometimes record) compressed sound files—even on sound cards that do not directly support compression—so you can play a sound file that is compressed with Microsoft ADPCM or the DSP Group’s TrueSpeech without decompressing the file first.

The Sound Mapper cannot always record compressed sound files because compressing sound data can be very processor intensive: the amount of time required is dependent on the compression algorithm and how it is implemented. Decompressing sound data is almost always faster than compressing the same sound data.

Sound Forge software does not play and record compressed sound files directly. Rather, all compression and decompression is performed while opening and saving the sound files. Sound Forge software saves compressed sound files using the best quality possible—something that cannot always be done in real time. Saving compressed sound files with Sound Forge software will usually sound better than those recorded with audio compression.

Notes:

• The Open dialog allows you to preview compressed .wav files if you have an appropriate ACM driver is installed. However, you must have your Default playback device set to the Sound Mapper for this to work.

• When saving uncompressed audio data to a compressed format with the Save As dialog, it is a good idea to close the file and reopen it after saving. Since Sound Forge software performs the compression and decompression during saving and loading you will not be able to hear what the file sounds like with compression until after you’ve saved and reloaded the file.