There are three types of ACM drivers — codecs, converters, and filters.
A codec (the term is short for compressor/decompressor) converts from one format type to another. Typically, a codec converts between a compressed format, such as MS-ADPCM, and the uncompressed PCM format, which most hardware recognizes.
A converter converts between different formats belonging to the same format type, such as between 44 kHz PCM and 11 kHz PCM formats.
A filter modifies audio data without changing the format. For example, an echo filter might add an echo sound to a 44 kHz PCM waveform file.
For more information on the difference between codecs and converters, see Format Tags and Filter Tags.
ACM drivers provided by Microsoft include codecs that convert the MS-ADPCM, IMA ADPCM, GSM 6.10-compliant, and Truespeech compressed formats to PCM.
The ACM also provides a PCM converter that converts between 8-bit and 16-bit PCM, between mono and stereo, and among various PCM sampling frequencies. This converter is not implemented is a separate installable driver. Instead, it is contained within the ACM’s DLL file, msacm32.dll.