Equal Sums of Biquadrates

An echo effect in a polyphonic digital tone synthesizer in which a musical tone is repeated at a controlled repetitive rate but decreasing peak amplitude when a key is depressed. The tone is repeated until the peak amplitude decays to a predetermined level regardless of when the key is released. If the key remains depressed, the peak amplitude decay recycles so that an echo effect will be repeated. The echo effect is obtained on a plurality of overlapping notes that need not be keyed in unison. The echo control includes an echo envelope register which stores a control word for each key that is depressed. The control word includes bits coded to identify the current amplitude of the echo decay envelope. The value of the control word is decremented at a control rate to provide a decaying echo amplitude. An echo delay register stores a control word for each key depressed, the control word being incremented at a controlled rate. When a control word in the echo delay register reaches a predetermined value, it causes the tone to be repeated regardless of the current status of the associated key. At the same time the control word is reset to start another timing cycle. The associated tone generator is released at the end of an echo tone when the echo envelope control word is decremented to a predetermined value and the key is released.