Stereo Rotary Cabinet FX (for VM Core)
Posted: Sat Jan 08, 2022 8:53 pm
I thought I'd have a try at creating a rotary cabinet (i.e. 'Leslie Speaker') simulation using Voltage Modular. I got very close to the sound I was looking for, although I'm sure there are a few aspects of the simulation which would horrify true Leslie nerds!
As ever, I used an article from Gordon Reid's Synth Secrets series in Sound on Sound as the basis for my experiments.
The incoming stereo audio is mixed down into a mono signal. This is then split into low and high frequencies by the low-pass and high-pass outputs of a 24 dB/oct filter with cutoff freq = 800 Hz. The low frequencies go to the 'Drum' rotor, whilst the high frequencies get sent to the 'Horn' rotor. Each 'rotor is made up of a sine wave LFO, which drives a VCA (for amplitude modulation or 'tremolo'), a chorus unit (for frequency modulation or 'vibrato') and a VCF (for tone modulation, to simulate dampening when the speaker is facing away from the mic). The LFO rate and filter settings are independent for the 'Horn' and 'Drum' outputs; generally speaking, the Horn rotates at a much higher speed than the Drum.
Both the Horn and the Drum have a duplicate set of LFO - VCA - Chorus - VCF. The two LFOs are synced in frequency, by the phase of LFO 2 can be shifted relative to LFO 1 so that it reaches a maximum earlier or later than LFO 1. This allows the simulation of having a second microphone picking up sound from a different angle than the other, leading to wide stereo effects. Knob 9 in the Perform rack shifts the phase angle of the duplicate processing chains for both Horn and Drum.
All four processing blocks are then fed into channels of a stereo mixer. If you want a mono effect, then simply mute channels 2 and 4, and pan channels 1 and 3 to the centre.
Usually, a Leslie Speaker will have three speed settings; Slow and Fast rotor and 'Brake' which halts the spinning motion completely. In this patch, a DC Source voltage is fed into two parallel Threshold modules. When the input voltage is less than 0.25V, a large negative voltage is sent to the LFOs to simulate the 'Brake' situation. It doesn't quite reach 0Hz, but it's still a lot slower than 'Slow'. For an input voltage between 0.25V and 3.62V, the 'Slow' positive voltage value is sent to the LFOs. Once the input voltage exceeds 3.62V, then an additional 'Fast' positive voltage value is sent to the LFOs. All LFO CVs are passed through a Glide module to simulate the inertia of the rotors speeding up and slowing down. For performance purposes, the DC Source knob should be mapped to a MIDI Continuous Controller. Alternatively, move the cables from the DC Source output up to the Mod Wheel socket on the Main Panel to shift between speed setting with the wheel. Slow and Fast rates and Inertia can be independently set for Horn and Drum rotors. 'Brightness' raises and lowers the cutoff frequencies of all VCFs. 'Normal' brightness is at 12 o'clock.
As ever, I used an article from Gordon Reid's Synth Secrets series in Sound on Sound as the basis for my experiments.
The incoming stereo audio is mixed down into a mono signal. This is then split into low and high frequencies by the low-pass and high-pass outputs of a 24 dB/oct filter with cutoff freq = 800 Hz. The low frequencies go to the 'Drum' rotor, whilst the high frequencies get sent to the 'Horn' rotor. Each 'rotor is made up of a sine wave LFO, which drives a VCA (for amplitude modulation or 'tremolo'), a chorus unit (for frequency modulation or 'vibrato') and a VCF (for tone modulation, to simulate dampening when the speaker is facing away from the mic). The LFO rate and filter settings are independent for the 'Horn' and 'Drum' outputs; generally speaking, the Horn rotates at a much higher speed than the Drum.
Both the Horn and the Drum have a duplicate set of LFO - VCA - Chorus - VCF. The two LFOs are synced in frequency, by the phase of LFO 2 can be shifted relative to LFO 1 so that it reaches a maximum earlier or later than LFO 1. This allows the simulation of having a second microphone picking up sound from a different angle than the other, leading to wide stereo effects. Knob 9 in the Perform rack shifts the phase angle of the duplicate processing chains for both Horn and Drum.
All four processing blocks are then fed into channels of a stereo mixer. If you want a mono effect, then simply mute channels 2 and 4, and pan channels 1 and 3 to the centre.
Usually, a Leslie Speaker will have three speed settings; Slow and Fast rotor and 'Brake' which halts the spinning motion completely. In this patch, a DC Source voltage is fed into two parallel Threshold modules. When the input voltage is less than 0.25V, a large negative voltage is sent to the LFOs to simulate the 'Brake' situation. It doesn't quite reach 0Hz, but it's still a lot slower than 'Slow'. For an input voltage between 0.25V and 3.62V, the 'Slow' positive voltage value is sent to the LFOs. Once the input voltage exceeds 3.62V, then an additional 'Fast' positive voltage value is sent to the LFOs. All LFO CVs are passed through a Glide module to simulate the inertia of the rotors speeding up and slowing down. For performance purposes, the DC Source knob should be mapped to a MIDI Continuous Controller. Alternatively, move the cables from the DC Source output up to the Mod Wheel socket on the Main Panel to shift between speed setting with the wheel. Slow and Fast rates and Inertia can be independently set for Horn and Drum rotors. 'Brightness' raises and lowers the cutoff frequencies of all VCFs. 'Normal' brightness is at 12 o'clock.
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