Circuit for producing musical tones

A circuit for producing musical tones comprising:

• keyboard means responsive to an external command for selecting musical tones to be generated by the circuit;
• tone generator means responsive to the keyboard means for generating a square wave signal only when the external command is present, the square wave signal having the frequency of the musical tone to be generated;
• a first capacitor;
• charging means responsive to the square wave signal for charging the first capacitor to a predetermined level of voltage when the square wave signal is terminated and for exponentially discharging the first capacitor when the square wave signal is initiated, including
• a first inverter having input and output terminals,
• means for connecting the input terminal of the first inverter to a high level DC bias source,
• means for AC coupling the input terminal of the first inverter to the square wave signal,
• a second capacitor,
• means for connecting the output terminal of the first inverter to the second capacitor whereby the second capacitor is charged when the square wave is initiated and the second capacitor is discharged when the square wave signal is terminated,
• a second inverter having input and output terminals,
• means for applying the voltage appearing across the second capacitor to the input terminal of the second inverter, and
• means for connecting the output terminal of the second inverter to the first capacitor whereby the first capacitor is charged to the predetermined level of voltage when the second capacitor is discharged, and the first capacitor is discharged when the second capacitor is charged;
• modulation means response to the square wave signal and the first capacitor voltage for amplitude modulating the square wave signal in proportion to the first capacitor voltage to produce an exponentially decaying signal;
• and
• speaker means responsive to the exponentially decaying signal for providing audible musical tones.