A synthesizer which is implemented as a software application or plug-in that runs on a personal computer, as opposed to being a keyboard or other physical object. A soft synth usually uses the processing resources of the host computer to do its signal generation and processing; polyphony is limited by the host computer and the number of other applications running at the same time. Soft synths provide the performer who already has a computer with an inexpensive way to acquire new synths; they cost less since there is no hardware or manufacturing cost involved. The compromise is that they are limited to the user interface provided by the computer, and don't have access to digital signal processor resources which allow hardware synths to execute more efficiently. Also, a soft synth's quality of audio output is determined by the audio interface attached to the computer.