Dual Fader
  • While taking a look at the difference between the Epoch Modular TwinPeak filter and the one that Rob Hordijk uses in his own system, I got curious about his Dual Fader module. At first I thought that it might not be all that relevant in the Audulus context, but the closer I looked the more compelling I began to find it.

    The NOVARS Research Centre has forunately posted an extensive series of Hordijk tutorials (presented by the man himsel) and I found it very useful to take a look at two videos (#12 and #12a) on the Dual Fader:
    https://vimeo.com/channels/1311476
    #12 vimeo.com/236211771
    #12a vimeo.com/236208596

    My first take-away from the videos was the simple idea of using one half of a cross-fader as a VCA with an RMS curve:

    Here's a little Audulus VCA and a comparison patch with exponential and logarithmic curves. The shape control on the RMS VCA fades between linear and RMS.
    RM RMS VCA.audulus
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    RM VCA Shapes.audulus
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  • The http://hordijk-synths.info site still appearas to be down, but fortunately is still available via the Web Archive: https://web.archive.org/web/20170402070104/http://hordijk-synths.info:80/synth/2015/03/19/dual-fader.html

    There's also a schematic: https://web.archive.org/web/20170402070104/http://hordijk-synths.info/assets/DUAL_FADER_flowchart.pdf

    Hordijk makes clever use of normalized inputs and outputs to achieve a wide range of possibilities with only a few inputs and outputs:

    Crossfading: 2 input signals, 1 output signal
    Panning: 1 input signal, 2 output signals
    Ducking: 2 input signals, 2 output signals

    Since normalized inputs and outputs aren't (yet) a viable possibility in Audulus I've simply included an extra output for the crossfaded signals. Another feature of Hordijk's module is a 20db Gain control on two of the inputs to enable using external signals within the modular system. That isn't something that needs to be thought of within the Audulus context, but those controls can also be used to introduce clipping distortion, so I've kept that as a feature. (Soft clipping in my case, but that could be adjusted.)

    There's also a switch that changes between the crossfade output of channel A being routed on to channel B, that output only being fed to signal 1 of channel B, and the crossfade output of A being fed to B with signal 2 being inverted. That means that when fader B is set precisely to its center, the two signals cancel each other out resulting in silence. However if that fader is modulated at audio rates a type of Ring Modulation is heard, with a particular quality that is the result of the RMS curves.

    Here's a simple demo. My 3 way switch defaults to the upper position when opening the patch, so that need to be manually switched down to the lower position. (It also got me thinking of the similarities with @stschoen's FSK oscillator.)

    I'm impressed by Hordijk's take on a simple mixing module, and just how much can be gotten out of what is essentially two crossfaders, especially when modulation is introduced. (Hordijk's modulation inputs accept both audio and modulation signals – in my case the modulation inputs are intended for audio signals since it seems as if a lot of the interesting things that can be done involve the audio range. My thinking is that modulation signals can simply be connected directly to the fader knobs, although the modulation level knobs won't have any effect in that case.)
    RH-RM Dual Fader.audulus
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    RM Dual Fader Demo.audulus
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  • And here's a demo coupled with the TwinPeak filter:
    RH-RM DualFader TwinPeak Demo.audulus
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  • Here’s a mono synth played via MIDI input made from your DualFader TwinPeak demo. It can create some nice vowel formants, thank you.

    RH-RM DualFader TwinPeak Synth.audulus
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  • Here's an update to the Dual Fader with an added switch that makes it possible to select between unipolar modulation and bipolar audio signals for the modulation inputs. It also includes an important correction on how the fader settings and the external modulation signals were being combined.
    RH-RM Dual Fader v.1.1.audulus
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  • Always good too see a module get an update :)