Graphical Waveform Synthesiser


Posted on Aug 18, 2012

Originally I intended to have 32 faders. Once I tried fitting them on a veroboard, 32 seemed rather excessive so I have reduced to sixteen. This still allows generation of accurate eigth harmonic which compares well with the Hammond's ninth harmonic. A MAX038 oscillator plus breakpoint circuit as in the Hammondicon was going to be my HF oscillator solution. Good ol' Maxim seem to be incapable of producing any though, so I switched to a rather good transconductance triangle oscillator circuit which just about manages to get high enough in frequency with a clever compensation. Also, I originally used a resistor string and a switched system to interpolate between the fader voltages. This required the clock to run very fast and was abandoned after a new clockless linear interpolator was devised.


Graphical Waveform Synthesiser
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This sheet has the output connectors carrying the voltage from each fader to the second scanning board. The faders have each end of their travel connected to the +5V and -5V rails, so that each fader selects a voltage somewhere between these levels. 10uF capacitors on the wiper of each fader provide noise decoupling and a reservoir. Each fader is fed into the analogue switch bank. After reset, the switches both have "0" input values and thus select faders 0 and 1. The signal levels from faders 0 and 1 are fed through U7 and are then applied to each input of the linear interpolator. Q3 of U1 rising edge switches over the connections to each input of the interpolator and latches the current value of U1 onto U2 outputs. At this point, U2 would normally have registered the previous count, but we have just had a reset so the outputs remain at zero. This involves a method of clockless linear interpolation between the fader steps. The pairs of fader voltages are presented at the two switch outputs in the correct order i.e. 0-1 then 1-2 then 2-3 etc. The voltage difference between these outputs represents a voltage proportional to the slope between the two points. So if this voltage can be converted to a bipolar current and fed into a capacitor, the circuit will generate exactly such a straight line slope. The overall gain of the interpolator depends inversely on the capacitor value and the sample period, and directly...




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