Stealing Thor’s Thunder – Part 2: Filters

Stealing Thor’s Thunder – Part 2: Filters

22 May, 2008 | 12.00PM

• Section: Music News Topics: Guide To Synthesis, Technology
• Written by: francispreve

In the previous tutorial on Propellerheads’ Thor, we explored the possibilities of its six different types of oscillators.

While these are a rich source of synthesis tools for tone generation, they are just the beginning.

Thor also includes four different filter types, each with their own unique properties.

This week, we’ll delve into Thor’s filtering capabilities.

If you’re not already up to speed with how filters work, check out Filters: The Wow Factor and Filters: Going Deeper from our guide to synthesis.

Common Elements

All of Thor’s filters share some features.

First up is oscillator routing.

The three red buttons next to each filter correlate to each oscillator.

If the corresponding oscillator is on, thus feeding the filter, it “glows” red.

In this image [right], oscillators 1 and 3 are being processed by the ladder filter.

The mix of the oscillators – their relative volumes – is controlled by the mixer section between the two modules.

Note that in this section [pictured below], there is also a pair of arrows below the shaper area.

These determine whether the filters operate in serial or parallel.

When the left arrow is lit, the filters are in serial mode.

That is, the output of the top filter feeds the input of the bottom filter, after which the audio is sent to the amp section.

When the right arrow is active, the filters are arranged in parallel mode, meaning that the outputs of both filters are sent directly to the amp – in parallel.

Each mode has its strengths and weaknesses.

Serial is useful for complex processing tasks, whereas parallel is handy for creating layered sounds.

Finally, every filter includes the following common parameters, most of which are explained in our filter tutorials.

Drive This adds a bit of distortion or overdrive to the filter, which can help fatten the sound and make it more aggressive.

Envelope This controls the amount of modulation by the filter envelope, which can be inverted via the aptly named INV button.

Velocity This allows key velocity to increase the amount of envelope modulation to the filter.

Keyboard Tracking. This allows the filter cutoff frequency to track the keyboard.

Now, on to the filter modules…

Low Pass Ladder Filter

More so than even its oscillators, the secret to the Moog sound lies in the use of circuitry called a ladder network.

This is what enabled the Moog filters to sound so rich, fat and creamy – a touch of distortion inherent in Dr. Moog’s implementation creates that warmth.

Thor’s low pass ladder filter emulates much of that character.

At least as much as can be simulated via software.

While all Moog filters operate as 24 db per octave (4-pole, if you’re a diehard tech head) mode, this implementation includes two versions of the classic mode, along with 18, 12, and 6 db/oct options.

A self-oscillation switch allows the filter to create a sine wave tone that follows the cutoff frequency when resonance is set to maximum.

State Variable Filter

The state variable filter is based on another vintage filter technology.

This time, the originator was a company called Oberheim.

Their SEM synth module incorporated a continuously variable filter that allowed musicians to smoothy adjust the behavior from highpass to bandpass to lowpass by turning a knob.

Unlike Moog filters, the SEM filters were 12 db per octave, which gave them a buzzier sound that some called “thin,” though it was really just a different texture.

Thor’s state variable filter offers multiple modes as well as fully sweepable peak and notch modes, for approximating the SEM’s flavor.

As with the ladder filter, the state variable filter can also self-oscillate in tune with the cutoff frequency.

Formant Filter

Formants are created by manipulating the resonant peaks of an acoustic sound.

For example, we change the formants of our voices by re-shaping our mouth cavity with our tongue and lips.

This creates various vowel sounds like “aaah” and “ooooh”.

Thor’s formant filter does much the same thing by allowing users to adjust its formant spectrum based on the settings of the x and y sliders – or by moving the circle around its Kaoss Pad type interface.

By using the mod wheel or a velocity-controlled envelope, you can coax the format filter into simulating the vocal sounds created by funk artists like Roger Troutman of Zapp (or Peter Frampton, if you’re a classic rock buff).

Note: The formant filter works best with an input signal that has lots of broad-spectrum harmonics, like a sawtooth, pulse wave or even white noise.

Comb Filter

Essentially, a comb filter is based on delay technology, which is why it sounds very similar to a flanger, which also relies on delay as its source.

Wikipedia has a technical explanation of comb filtering for the scientifically inclined.

In practice, it’s useful for giving a metallic edge to the oscillators – or, if modulated by an envelope or LFO, it can be used as a unique alternative to a standard flange effect.

Next time out, we’ll cover Thor’s extensive array of modulation options.

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