Monday, 9 April 2012

PotD - Swings & Roundabouts

How to patch a thru-zero frequency shifter with two quadrature VCOs and ring modulators.

I love my Cwejman FSH-1 for its wide range and smooth analogue tone. The one thing it lacks is thru-zero capability. At slow settings, frequency shifters yield beautiful spatial effects. Thru-zero is the icing on the cake as it allows the up & downshift channels to 'swap sides'. Heard in stereo, this can sound great - or disconcerting, depending on the amount of shift!

So, how to go about patching one?

James Clark's Nord Modular tutorial on spectrum shifting explains the workings of a frequency shifter but stumped me on the need for all pass filters/ Hilbert Transformers. A post by Matt Jones at the Synthedit Yahoo Group offered the necessary clue:

"Out = (Input) * (Sine oscillator) + (Input shifted by 90 degrees) * (Cosine oscillator)
Changing the + sign to a - switches the amount of the shift from up to down …"

Here's the proof of concept:

In the recording you can hear me manually sweeping a DC voltage from positive to negative, biasing the modulator and causing the shifts to move from left to right.

My patch (see below) has three caveats: one, you need a Zeroscillator or similar to provide the thru-zero shifts (!). Two, unless you have the means of generating a cosine from your complex input signal, you're limited to simple sines. Three, it's fiddly and not 100% precise. That said, the results sound great:

The patch details: the ZO is used as the modulator. Set its bias to zero and patch the 0 & 90 degree outputs to the CV inputs of two ring modulators (e.g. A-133). I used the Doepfer A-143-9 QLFO as my signal as it provides the necessary outs. The 0 & 90 degree outputs were patched to the respective ring mods. The Toppobrillo TWF can be used to generate a cosine from another source, but I wanted to keep the patch as simple as possible.

To generate the upshift, I mixed the result of the 0 and 90 degree multiplications. To get the downshift, I subtracted the 90 from the 0 degree via an external inverter. The Cwejman VCA-4MX was used as the output to my final mixer. It's perfect for this task as it allows you to independently tap the mix of channels 1&2 and 3&4, which you can then pan left and right.

The fiddly bits: to bias the ZO correctly in both directions, I had to tune two voltages (+ve & -ve, Maths or Fonik's mh-01) and send them to the linear FM input via a sequential switch. It would have been easier to use the pulse output of the clocking LFO, but this didn't give me equal positive and negative bias i.e. the side-swapping effect was compromised. You can hear this in the first recording where I apply the bias manually. The second detail you have to watch is the mix/ subtraction balance to ensure clean up & down shifts, but this isn't too tricky. Just use your ears for both.

The ZO makes this patch possible. With zero bias, the oscillator comes to a stand-still. Any voltage you apply to the linear input will cause it to jump into action. Reverse the bias voltage and the ZO changes direction causing the shifts to swap sides. You could use a second A-143-9 as modulator, but would then lose this ability. Still, two QLFOs, an A-133 and mixer/ inverter offer an inexpensive single-sideband-modulation patch.

Buy Swings & Roundabouts.

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