The Virtual Studio Technology programming platform allows developers to create virtual instruments and audio effects that can run on multiple digital audio programs. Acoustica's Mixcraft digital audio workstation includes several built-in VST instruments and effects; to expand the program's capabilities, install additional VSTs into the program. VST instrument plug-ins are virtual synthesizers that generate sounds in Mixcraft, while VST effect plug-ins alter the sound character of either an audio clip or a VST instrument. Many VST plug-ins are freely available for download; other are available for purchase either in downloadable formats or on optical discs.