A synthesizer is an electronic device that generates, alters, and combines a variety of waveforms to produce complex sounds. Often a piano-type keyboard allows the musician to select the notes to be constructed. Synthesizers may use electronic circuits to create the tones or use software that controls a circuit that converts a digital number to a voltage. Some synthesizers use a computer rather than a keyboard to select the notes. The computer can then control electronic circuits through the MIDI interface. The most common method of creating the synthesized sounds is to use a frequency modulated synthesis that creates higher harmonics that match those of the musical instrument being imitated, or create an entirely new musical sound. Instruments also have characteristic attacks, sustain times, and decays. Attack describes how fast the amplitude rises from zero to its full value. Sustain times describe how long the tone amplitude remains the same, and decay describes how the amplitude decreases at the end of the played note. Synthesizers can create hundreds of different sounds, typically called voices.