An electronic device which generates a periodic signal of a particular frequency, usually a sine wave, but other waveforms (square, sawtooth, triangle) are often used. Oscillators are common in audio devices such as synthesizers and test signal generators. Early synthesizers used oscillators as the basic component for all of the sounds of the machine. All of the filters and envelopes modified the sound created by the oscillator to produce the desired sound. Nowadays most keyboards produce sounds by playing back samples recorded on chips or by more modern synthesis techniques such as Physical Modeling (see WFTD archive Physical Modeling Synthesis), FM, LA, or any number of other methods that have been employed in the past 10 years.