Thursday, 11 May 2017

Patch Tips #28 - Wavetable Clock Multiplier

Driving a wavetable oscillator to get reliable clock multiplication, regardless of changes in tempo. A follow-up to these posts (1, 2).

Clock division is easy and instantaneous. Getting faster pulse streams from a master clock is an imperfect science, even with dedicated solutions. Here is a multiplier patch that tracks changes in tempo and avoids the lags and glitches that can affect modules which try to average or predict incoming clocks.

In a sense, it’s an analogue rather than digital approach: instead of counting pulses or square waves from a master clock (tap tempo etc.), we use a sawtooth wave to voltage address a suitable waveform in a wavetable module.

If we scan a wave that contains four equally placed pulses (see image above) we can quadruple our ‘clock’. If the amplitude of our sawtooth remains constant - i.e., we read out the entire contents of the page - we will always get four pulses for each single sweep of the saw, regardless of how fast or slow that saw moves.

Here’s how it sounds: wavetable-clock-multiplier.mp3

For this patch, I used a sawtooth LFO to drive a Richter Megawave. I multed this saw and used one copy to trigger one envelope (master clock, left channel voice) and patched the other to the input of the Megawave. I patched its output to a second envelope and voice (multiplied clock, right channel). I used the sine waves in ’Miniwave’ (Bank 14, Wave 3) as they are more regular than the pulses in Bank 7. Fortunately, these sines were enough to trigger my envelopes. I kept the multiplication at x4 but modulated the frequency of the LFO (and the pitch of the audio VCOs) with a voltage offset and a clocked random voltage.

To get voltage control over the multiplier e.g., jump from quarter notes to 16-ths, we can do two things: either select a new wave via CV or manually select the wave with the most pulses and control the amplitude of the sawtooth with a VCA, as you would with a wave-folder. Much like the old Beat Folder patch, we are re-assigning voltages. But instead of using an analogue transfer function, the wavetable module allows us to look up a series of digitally stored arbitrary values.

If you want to read some more about the Megawave and its many uses, head here. The Megawave is full of maths, a ‘best-of’ transfer functions (see the Socket Rocket manual for ideas). This is my sort of module. It’s multifunctional - not because it has many modes but because its principle function can be re-imagined according to the desired use.

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