We look at Live's unusual Collision instrument and associated Corpus effect.
Collision is one of several Live instruments developed in collaboration with Canadian manufacturer Applied Acoustics Systems (AAS), whose hallmark is the use of acoustic modelling in the instruments they design. Corpus is an audio effect based on the instrument‑body modelling section of Collision. Both devices are included in Live Suite, but you can purchase the pair separately for €79$99 in other versions of Live. As its name implies, Collision is designed for modelling percussion instruments, both tuned and untuned, but that's only the beginning. It delivers a wide variety of sounds and sound effects that are neither percussive nor acoustic sounding. Likewise, Corpus can mangle, or let's say enhance, your favourite synthesized and sampled instruments and audio clips.
Collision is based on the AAS synth Chromaphone. Although the instruments are not identical, you'll get a much deeper knowledge of Collision's inner workings by reading both Dave Stewart's review of Chromaphone in the November 2012 issue of SOS and the Chromaphone manual (especially chapters 3 and 4), which is available as a free download on the Chromaphone page of the AAS web site (www.applied-acoustics.com/chromaphone/overview). My approach in this article will be more 'how to' than 'how it works', so while I'll cover the controls as I use them, I'll leave the acoustic‑modelling explanations to others.
Make A Sound
My usual approach to a new instrument or effect is to start with the presets and then fiddle with their settings, and with over 100 Collision presets in 11 categories, you can certainly take that approach. But, complicated as its multi‑tabbed control panel appears, I found it easier to get to grips with Collision by starting with the default preset you get when you insert the Collision instrument on a MIDI track.
The most important elements of Collision, the sound producing elements, are called 'excitators' and resonators, and you get two of each. In terms of real‑world percussion instruments, the excitators emulate piano hammers, guitar picks, drum sticks, marimba mallets and so on. Resonators emulate piano strings and soundboard, guitar strings and body, drum head and body, marimba beams and tubes, and the like. Observe the pattern: one excitator starts one of the resonators vibrating, after which the resonators stimulate each other to sustain and evolve the sound. If you need convincing that real‑world resonators interact, think of playing a note on the piano with the sustain pedal held down: the sympathetic vibration of the other strings is the result of excitation from the vibrating soundboard, which was itself set vibrating by 
1: The modifications indicated by red dots turn Collision's default Mallet instrument preset into a convincing emulation of a marimba. In the Mallet excitator section, the Mallet is made stiffer, velocity modulates volume and the noise component is removed. In the resonator section, a Marimba beam replaces the generic Beam in Resonator 1, and Resonator 2 is set to Tube mode and coupled to Resonator 1 (via the Structure 1>2 button). This emulates what happens when you strike a marimba beam with a fairly hard mallet and the beam's vibrations resonate in the tube below.the strings of the played note.
In Collision, the interaction between the resonators happens only when they are coupled, as indicated by the top Structure button ('1>2'). In '1+2' mode they work in parallel with no interaction. Change from coupled to parallel mode in the Marimba preset described in screen 1 to hear the difference. The pitched tone is from the mallet striking the Marimba beam, whereas the mallet striking the Tube resonator produces a short percussive hit because the tube is always hit on its end. (The Hit knob determines where all resonators except the Tube and Pipe are hit.) The Material slider, labelled Radius for the Pipe and Tube, also has a big influence on the tone. The Ratio setting, available for only the Membrane and Plate, determines their width versus length. The Brightness control is similar to a high‑pass filter in its positive range and a low‑pass filter in its negative range. The Inharmonics knob increases (right) or decreases (left) the spacing of the harmonics in the tone produced by the resonator, making the sound progressively less pitched. Finally, each of the resonators is stereo and the two Listening knobs determine the listening point (think mic placement) for each channel. Experimenting with the various resonator controls will have the greatest influence on the sound.
Make It Different
Conspicuously absent from this description is the second excitator, labelled 'Noise.' This is an enhanced version of the Noise and Color knobs in the Mallet excitator. In the default preset, crank the Mallet excitator's Noise knob all the way up and play with the Color setting. This adds noise to the Mallet output resulting in a high‑frequency component to the sound, and the effect is stronger at low Color settings because Color controls the cutoff of a high‑pass filter applied to the noise. Now turn off the Mallet excitator, turn on the Noise excitator and increase its volume to hear the result of noise excitation alone. What the Noise excitator adds is four resonant‑filter choices along with an ADSR envelope for modulating volume as well as the cutoff frequency of the filter (using the 'E' slider). Increasing the attack time and sustain level moves you out of the territory of percussive instruments and into sustained sounds and sound effects. Here are five tips for creating and modifying presets:
1. To increase the effect of Noise in the Mallet excitator, use Stiffness settings below 25 percent.
2. When using the Pipe resonator, manipulating the Opening setting in the low end of its range adds a subtle pitch‑bend. Map the mod wheel or one of Collision's LFOs to that.
3. For drum sounds use a stiff mallet. For Resonator 1 choose Membrane, set Key tracking for the Tune knob to zero percent and use a fast decay. For Resonator 2 choose Pipe. Edit the other resonator settings to taste.
4. To simulate a sustain pedal for any preset, map MIDI CC64 to Decay for both resonators and set the mapping Min and Max values to approximately one and three seconds, respectively.
5. With sounds that use coupled resonators, try swapping the resonator settings: take a screenshot of the Resonator 2 settings, click Resonator 1's 'Copy to 2' button and then set Resonator 1 from the screenshot.
Body Language
Despite the talk of acoustic modelling of physical instruments, the Collision excitators are not, of course, beating on the resonators with a stick. It's equally valid to think of the excitators as oscillators and of the resonators as audio effects. Corpus delivers a single Collision resonator as an audio effect, and you can use any source of
2: At the top, Corpus is used to enhance a MIDI‑played bass, with the incoming MIDI notes also mapped to tune Corpus. Because the bass is transposed down two octaves, the string resonance from Corpus is actually an octave above the bass note. The LFO adds a kind of vibrato with slightly different rates for each channel. At the bottom, Corpus acts as a resonator for an audio drum loop. A little goes a long way, so the Dry/Wet mix is controlled by the MIDI mod wheel. The very slow LFO causes the resonance pitch to slide when audible. audio (clips, instruments or live input) as the excitator. The output section includes a band‑pass filter and a Dry/Wet mix knob. The Bleed knob sets the mix of the resonator output and audio input processed by the filter.
One thing to keep in mind is that the resonators will emphasise some frequencies over others — a good reason to keep the level low when experimenting. Which frequencies are emphasised depends on the Tune and Fine knobs' settings as well as on the LFO. So, in order to use Corpus effectively with a pitched instrument as the input, you'll usually want to use the foldout side‑chain to map the same MIDI notes that play the instrument to Corpus's frequency. The Tune knob changes to Transpose when MIDI controls frequency; use that to dial in the resonance spectrum you want. Processing pitched instruments works best for monophonic parts (bass lines and leads, for example), but Corpus will let you choose last‑ or low‑note priority for processing polyphonic MIDI input.
Corpus is an excellent effect for colouring individual drums as well drum loops, but in both cases, a little goes a long way, so you may want to automate or map a MIDI controller to the Dry/Wet mix. The Tube and Pipe resonators usually work best with drums, and you might use the filter to minimise the effect in the very low pitch range.
