FIG. 1: Oscillators, filters, and polyphony are disabled when using Thor exclusively as a filter device.
One of the exciting features of Propellerhead Software Reason 4's new Thor Polysonic Synthesizer is its ability to route audio into the device and process signals with its variety of new filters. These include a classic lowpass filter with selectable poles, a state-variable filter with sweepable passband, a comb filter, and a formant filter.
Of Thor's three filter slots, Filter 1 and Filter 2 are monophonic and are not active until an incoming MIDI event instantiates a voice. Filter 3, in Thor's Global section, is stereo and always active, making it ideal for creating a filter-bank effect (see Web Clip 1).
Reason's browser does not distinguish between Thor instrument and effects patches, but it does identify Combinator patches as effects if a cable is connected to at least one of the Combinator's To Devices jacks. (Otherwise, Combinator patches are identified as instruments.) I've embedded the Thor patch described here in a Combinator effects patch for easy access (see Web Clip 2). This Combinator provides a framework for developing a flexible effects unit that's usable in a variety of applications.
Thor Modulation Bus Routing
Thor's programmable routing matrix offers a variety of signal-patching possibilities. To process audio with Thor, I've routed two of Thor's audio inputs to Filter 3 and set their amount parameters to 100 so as not to attenuate the signal (see Fig. 1). This should be the only signal path, and I've disabled the three oscillators and the other two filters. I've also set the polyphony to zero in the Keyboard Modes section to prevent the synthesizer section from instantiating a voice.
The Combinator's dedicated controls give you more flexibility than you have when using Thor alone. For example, a single knob or button can affect more than one target, and each routing's range and polarity is set independently. I've mapped Rotary 1 to the cutoff frequency, Rotary 2 to filter resonance, as well as Rotary 3 to the filter Drive control. Rotary 4 is routed to Filter 3's Type parameter, which lets you switch between the four different filter algorithms in real time without noticeable artifacts. The first three buttons are assigned to parameters that change aspects of the filter: passband mode (bandpass or highpass), self-oscillation, and comb-filter feedback polarity.
Button 4 is mapped to the delay-effect wet/dry balance to toggle delay at the output. Automating the wet/dry balance rather than the Delay On parameter prevents audio artifacts, making the control usable in performance. The delay is set to a tempo-synchronized duration of 3/16th notes, and the button toggles the wet/dry balance between 0 and 35.
Thor's step sequencer is programmed with a gate pattern using the step-duration and gate-length parameters. Rather than modulate the filter-cutoff frequency directly, the step sequencer gates the global envelope, which has a fixed modulation routing to Filter 3's cutoff. The sequencer rate is set to eighth notes, the mode is set to Repeat, and Thor's programmable Button 1 is routed to trigger the step sequencer. With Button 1 in the On position, the step sequencer continues to cycle regardless of the transport state.
The Combinator lets you assign Pitch Bend and Modulation Wheel as modulation sources, and I've used these to affect Thor's pattern modulation. The Modulation Wheel is assigned to Filter 3's Global Env Amount in the Combinator Programmer, so the Modulation Wheel determines the global envelope's effect on the Filter 3 cutoff.
The Pitch Bend wheel is mapped to select from three 1-step patterns on a Matrix Pattern Sequencer. The Matrix Curve CV output is routed to set Thor's step-sequencer rate. The three patterns set the rate to either quarter, eighth, or 16th notes.
Once you've set up the desired routings, you can automate the Combinator's rotaries and buttons in Reason's sequencer. In the example song, I've processed a Dr.REX drum loop and automated all the Combinator controls. For performance, you might want to automate some controls and MIDI map others to change them on the fly.
Kurt Kurasaki works on the Reason sound-design team.