Organ Sound Generator

Posted on Aug 30, 2012

Originally, the prime harmonics were generated with 74HC4046 PLLs which were spot-on accurate but would take some time to reach their final frequencies, especially when running at low frequencies. They also limited the useful frequency output range and I decided to dump them in favour of not quite spot-on divided-down frequencies. It occurred to me that with a limited number of gears and teeth on the Hammond tonewheel generator, this might be authentic anyway. You can tell that the harmonics are not quite locked by looking at the output signal on an oscilloscope. The principle of operation of a tonewheel organ is to have a fundamental output frequency for each note to which you can add upper and lower harmonics in varying amounts. This device uses a high frequency square wave oscillator which is divided down to provide the fundamental and even harmonics in square wave form. The fundamental is nominally 0.5kHz. The prime numbered harmonics are approximated by dividing down from the H.F. clock with programmable dividers.

Organ Sound Generator
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The frequencies are thus not exact but are very close. The remaining harmonics can be generated by using dividers from the prime numbered harmonics. Note that because of this, the "prime numbered" harmonics are not necessarily really prime, but often multiples of them to facilitate this dividing method. e.g. the tenth harmonic is generated from a divider, and the 5th harmonic is then obtained with a simple flip-flop divider from the 10th harmonic. These square-wave outputs need to be filtered to obtain sine waves. Switched capacitor low-pass filters are used to do this. The clock inputs of the filters are driven from one of the high-frequency divider outputs of the VCO. The particular divider output used is chosen based on getting the corner frequency close to the fundamental of the particular signal being filtered without causing too much attenuation. As the filter corner frequency changes with the clock inputs, the filters will track the input signal and maintain a constant output level and filtering relationship. The sine wave outputs generated can be added together as desired using front panel level controls, in a similar way to that possible using the drawbars on a tonewheel organ. The front panel controls also have voltage control inputs for each harmonic, allowing harmonic levels to be varied by an ADSR unit, or some other control voltage source. The vibrato unit in the original A-100 organ is based on a...

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