A method and apparatus for producing a tone (e.g. for music) without use of a waveform memory and using a feedback loop. The feedback loop includes a waveform generator which calculates, in real time, a parabolic approximation to a sine wave. The feedback loop includes a delay phase ...http://www.google.com.tw/patents/US5834672?utm_source=gb-gplus-share專利 US5834672 - Non-linear tone generator
