Thor Polysonic Synthesizer Part 2: Filters

The next set of components I would like to talk about are Thor’s filters.  Each filter has a unique function that can be used to cut harmonic content and shape a sound (called subtractive synthesis).

So lets take a look at the filters:

Low Pass Ladder Filter

This filter is just a “simple” low pass filter.  I say “simple” because it only has a single function: filtering out high-frequency content.  On the filter you have the capability to set the filter frequency and resonance level.  There are also controls that alter how the filter is effected by the filter envelope, velocity, and keyboard position.  You can also invert these functions as well as increase the amplitude with a linear slider labeled “drive”.  In addition, there is a selector switch that alters the slope (2x 24dB/18dB/12dB/6dB per octave).

The final selection switch turns on the ability for this filter to self-oscillate.  A self-oscillating filter can create a sine wave when the resonant frequency is high enough to oscillate itself. 

State Variable Filter

The state variable filter is more of jack-of-all-trades filter.  In addition to the common modulator functions, frequency/resonance selectors and “drive”, the State Variable filter allows you to change its function between Low Pass filter, High Pass filter, Band Pass filter, Notch filter or Peak filter.  You can also alter how close to high-pass/low-pass the peak and notch filters act.

Comb Filter

The comb filter is an interesting component that is normally used to add a chorus/flange-like effect to a sound.  This effect is achieved by taking a clone of the incoming signal, and delaying it slightly before summing the two signals back together.  This causes phase summation and cancellation at different intervals (when used with a pure sine wave).  When using more complex wave forms, more interesting “resonant regeneration” occurs.  There is a +/- selector that allows you to invert the resonant peaks caused by the phase modulation creating a slightly different sound.

Formant Filter

The formant filter is similar to the “wha” guitar effect pedal in that it can emulate the consonant “w” along with different vowel sounds (based on the spectral content of  the input signal).  The formant filter in thor allows you to replicate different consonants and vowel sound (albeit on a limited scale – nowhere near the ca[abilities of the human voice) in addition to changing the “gender” of the signal (most likely by altering the amplitudes of the lower frequencies).  The filter itself is controlled by a virtual X/Y pad, which can be manipulated (and even modulated via Thor or automation) to sweep through multiple consonants and vowels.  So how is this achieved?  Formant filters act like multiple band-pass filters acting on the same signal at the same time. 

This is just a brief look at Thor’s filters.  For more in-depth information, visit Propelloheads’ series called Thor Demystified, or this article from Reason Experts.

Next week I will take a closer look at modulation sources and the modulation bus matrix.  Thanks for reading and I hope you get a chance to play around with Thor!