Derailer tutorial - Driver

In this post we will look at the driver section of our physical modelling instrument Derailer. The plugin uses mathematical models of three basic elements; stiff strings, metal bars, and nonlinear springs. Details can be found in this technical paper. The driver section uses the stiff string element which can be plucked or bowed, alongside effects units such as a low-pass filter. The signal chain is shown in the diagram above, with the output of the driver section being used as the forcing input to one of the metal bars of the resonator. The user interface of the plugin has 2 views, one for the Resonator and one for the Driver.

The Driver View

Firstly let’s look at the excitation section, which is the start of the driver signal chain. The control options are shown in the top-left quadrant of the interface.

1. Excitations

There are 5 different excitation options. The first is a basic sawtooth waveform with a variable slope to change the tone which is mapped to the Mod wheel. The other controls are standard attack, decay and release, as well as portamento and a mono/poly switch. In polyphonic mode there is a maximum of 6 voices. We included this sawtooth as it’s a useful tool for creating smooth pad sounds when driving bar 1 of the resonator. The physical modelling elements begin with the plucked string.

Plucked string

The plucked string uses a linear stiff string model (we will be prototyping a non-linear string instrument soon). Each note is actually a double string unit, like the lower-mid registers of a piano, which gives a fuller tone and allows natural chorus effects by varying the tuning offset. There is a maximum of 6-note polyphony. Being a physical model we can vary the pluck position (using the Mod wheel) and pickup position where the displacement is being read along the length of the string. These, along with parameters for sustain, tone, inharmonicity and chorus give a wide range of possible timbres.

We set the pitch-bend and vibrato to only affect the last Note-On. This allows you to bend intervals, as shown in the video above. Vibrato can be controlled by channel aftertouch or cc7.

Bowed string

Things get more interesting when we include a bowing mechanism on the stiff string. This is a continuous force excitation with friction, and the model uses 3 control streams. We have the force of the bow, the velocity, and also the position of the bow along the string. In order to make this playable we mapped the force and velocity together and assigned them to the Pitch-bend wheel. With pitch-bend down we have low force and velocity, moving to higher values as we move the wheel up. This leaves the Mod wheel to control the position along the string, which is picked up at each Note-On. By continuously varying the Pitch-bend and Mod wheels we can get a large range of bowing timbres. Bowing at the end of the string can give screetchs, with smoother tones as we move away from the edge. Note-On velocity can be used to modify the attack time of the bowing to get further harmonic variation.

Pitched bar

As we increase the inharmonicity of the stiff string we end up with bar-like sounds. These are still pitched, unlike the large metal bars that form the resonator. The controls for the pitched bars are the same as for the plucked string, giving a wide range of different tones.

Striker

The final excitation type is a striker. This is different from the other excitation types as we don’t hear it directly. It is an impulse that is used to strike one of the bars of the resonator, similar to hitting a bar with a mallet. The strikes are periodic, with the frequency mapped to the octave ranges of the keyboard. At C0 the strikes are at 1Hz, at C1 it is 2Hz, all the way up to C5 which is 32Hz. These audio rate strikes produce interesting tonal effects in the resonator, especially with nonlinearity in the spring connections. We use the Mod wheel to vary the force of the strike, and the Pitch-bend wheel can be used to vary the frequency.

2 - Effects

The output of the excitation section is then fed into a number of effects. First a filter unit, then an overdrive, and lastly either a delay or gate unit.

Filter

The filter section has 2 parts. A 3-band EQ followed by a low-pass resonant VCF, both of which can be switched on/off independently. The EQ is for making broad changes to the excitation tone, with low and high shelves and a mid range peak filter. The VCF is a model of the classic ladder filter design. This has a variable cutoff frequency and resonance, as well as an envelope that can vary the cutoff over time. You can assign the cutoff to the Mod wheel.

Overdrive

The overdrive is a model of a simple diode clipper. This gives smooth distortion to the output of the filter section. This works well with the VCF switched to Wah-wah, giving classic guitar tones.

Delay / Gate

The final effect unit is either a Delay or a Gate. The delay has variable feedback and level, and can be synced to the host at a multiple of the BPM. The Gate has two LFOs, which are either summed or multiplied together. This signal is then used to open and close the gate. With variable threshold and depth controls, you can create interesting rhythmic effects on sustained tones.

That concludes our look at the driver section of Derailer. In the second part of the tutorial we will examine the resonator network.