Pseudorandom-Sequential Ring-Modulated Tone Generator

Posted on Feb 5, 2014

The circuit illustrated produces a short duration of rapidly-sequencing tones, invoking the stereotypical sonic imagery of a sophisticated computer, such as would be scripted for the science-fiction genre. I designed this circuit for use in the composition `Be Quite Still, ` by the popular band, `The Cassettes. ` A similar riff had been provided by

Pseudorandom-Sequential Ring-Modulated Tone Generator
Click here to download the full size of the above Circuit.

a Moog Rogue analog synthesizer for the band`s studio recording, but dedicating the synthesizer to just this brief effect, alone, would have prevented its use for other parts of the composition in live peformances. The circuit is comprised of a 16-bit serial shift register (U3 and U4), clocked by multivibrator U1A-U1F, and employing data feedback via three exclusive-nor gates, U2B, C, and D. This implements a digital word generator that propagates pseudorandom data through the register, of which the first four bits are used to control the frequency of an audio oscillator, comprised of inverters U1B-U1C and quad analog switch U5. Similarly, bits 5 through 8 control a second audio oscillator, comprised of U1D-U1E and U6. The two squarewave oscillator outputs are fed to exclusive-nor gate U2A, which combines them in a manner similar to a ring-connected diode mixer, producing sum and difference frequencies of the two squarewaves, resulting in harmonically-complex tones at the AUDIO OUT jack. Flip-flops U7A and U7B, voltage regulator U8, and associated components form a power-control circuit that provides voltage to the tone generator for a duration of about two seconds. The duration is adjustable by varying the 270K resistor. U7B ensures that the circuit turns off after the last tone is completed, thus preventing the annoyance of a short interval at the end of the effect. To provide a large variety of tone pairs, the two...

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