Forum Diagram

This circuit converts an input signal into one that is composed of a number of discrete steps but which re- mains otherwise identical to the input signal.
Because the steps are of equal height, the harmonic con- tent of the output signal will be dependent upon the amplitude of the input signal. This chararacteristic is extremely useful in the making of electronic music. The circuit uses quantitised pulse- width modulation for the adding of the step—shaped input signal. Pulse-width modulation is obtained by comparing a triangular voltage with the analogue input signal by means of a comparator; the quantitising, that is the adding of the steps, takes place by replacing the triangular voltage with a stepped l voltage.

The stepped-voltage generator con- l sists of three gates, N1 . . . N3 and transistor Tl. N1 operates as an astable multivibrator, that oscillates at a frequency depending on the value of Cl and Rl. Transistor stage T1 fu nctions as a charger circuit: each time the output of N1 is logic l, the transistor transfers the charge on C2 to capacitor C4. During the next half cycle C2 is recharged via Dl. ln this way the voltage across C4 increases in discrete steps, the height of the steps being deter- mined by the ratio C2:C4. When the voltage across C4 rises above a certain value, N2 switches transistor T2 on via gate N3 and discharges capacitor C4. When the capacitor is completely discharged, N2 switches off T2 and C4 continues to charge again in discrete steps The stepped voltage is set to the inverting input of lC2 which is connected as a comparator.

 Low- pass filter R4/C7 in the output of lC2 converts the pulse-width modu- lated signal back to an analogue one. The d.c. voltage level at the non- inverting input is set by potentiometer P2 to half the magnitude of the stepped voltage. The setting of P1 is dependent upon the input signal which must be attenuated such that the maximum value at the slider of P1 is always smaller than the maximum value ofthe stepped voltage. The number of steps can be selected by varying the value of C4. lt is possible to use a varicap in place of C4 with the varicap voltage being controlled by the music program or the input signal. Interesting and individual effects can be obtained in this way.