Switched capacitor sine wave generator and keyer

DETAILED DESCRIPTION The present invention relates to a waveform synthesis system wherein both the waveform amplitude envelope and the waveform itself are generated by a switched capacitor technique.
This permits the entire circuit to be completely integrated with the pulse or DC level keying signal being connected to a control input and one or more clocking signals connected to the respective clocking inputs for driving the switched capacitor filters.
Rather than storing the waveform in a memory as in the prior art digital synthesis techniques, the waveform is produced by filtering a series of amplitude steps through a switched capacitor filter and controlling the clocking frequency of the filter to produce waveform segments having varying slopes.
Thus, either simple or very complex waveforms can be generated totally internally of the chip and without the necessity for a separate tone input.
The specific embodiment of the invention disclosed comprises a circuit for generating a damped sine wave signal characteristic of certain percussion instruments, such as drums and clave.
The control input comprises a DC level shift, such as a pulse, making a transition from one voltage level to another during attack and then making the opposite transition during decay.
This DC level change is converted to a pair of envelope waveforms having the appropriate exponential attack portions by means of a first pair of switched capacitor filters.
The filters are driven by an attack clock during the attack portion and by a decay clock during the sustain portion, and a positive going envelope and a negative going envelope are produced.
A chopping circuit alternately selects the positive going and negative going envelopes to produce a series of bipolar pulses having amplitudes which follow the amplitudes of the envelopes and which have a frequency equal to the chopping frequency.
This signal is then passed through a second switched capacitor filter having a variable frequency clocking input.
The amplitude of each pulse is incrementally transferred from the input to the output of the second switched capacitor filter to produce a plurality of discrete amplitude steps generally following the amplitudes of the individual pulses but having a shape dictated by the frequency at which the second filter is clocked