Synthesis 101 – Lesson 3 – Filters
In this lesson, we will look at Filters in more detail. What filters do, in general, is a tool to manipulate sound. It takes an input, and based on the type of filter (LP, HP, BP, Notch, etc.) and the frequency range, it either allows or amplifies that range and reduce everything else, in some case most or all frequencies outside of the range. It is a bell or curve shape and the shape/rate of the curve (transition band) varies based on the filter specs. This attenuation slope is based on a notation such as pole or dB/octave. The larger the pole or dB/octave, the more effective it is. A 24dB/oct filter is twice as effective as a 12dB/oct filter.
Let’s look at the types:
- Low Pass – all frequencies below the cutoff point to pass through
- High Pass – all frequencies above the cutoff point to pass through
- Band Pass – band of frequencies to pass through, and reject everything else
- Band Notch/Reject – reject a band of frequencies, and allow everything else to pass through
Let’s look at some common terms:
Cutoff – this is the frequency where the filter starts affecting the gain. Drops about half of the attenuation value at the cutoff point.
Pole – 1 pole = 6dB/oct, 2 pole = 12dB/oct, 3 pole = 18dB/oct, 4 pole = 24dB/oct. May also be referenced as -12dB/oct
dB/octave – every time the frequency doubles, it drops that many dBs. So a 1 pole drops 6dB per octave
Resonance/Q – The terms are not quite synonymous but many manufactures use them as such. The Resonance knob is where it boosts frequencies around the cutoff. Q is the width of the peak of Resonance. If you increase the resonance, you will start to hear a ring around the cutoff frequency, and may start to self oscillate. Some manufactures can control that ring using 1V/Oct and it somewhat can be another VCO to add to the sound.
Here’s how you can make some interesting movement in sound. You can in most cases use CV (control voltage) to affect one of those settings, like Cutoff, Resonance and/or Q. You can adjust the setting by modulating it using an LFO or a function generator. You can also adjust how the frequency cutoff moves based on an EG (envelope generator.) You can also move the Resonance or Q using EG, LFO, and/or function generators as well. You can even map key velocity, aftertouch, mod wheels, expression pedals, etc. to affect Cutoff and/or Resonance.
Take an Oscillator output and connect it to the Filter input (if not already connected in a semi modular or fixed architecture synth or software synth.) Take the Filter output and connect it to some sound output device or a VCA to some sound output device. If you have the ability to select a LP or Low Pass filter, select that and turn the Cutoff knob fully counterclockwise. Set the Q or Resonance to fully counterclockwise position. As you turn the Cutoff knob, you start to hear the sound. If you increase the Resonance or Q knob, you start to hear the Oscillator shape change. Set the Resonance/Q knob to like 60-70% and turn the Cutoff knob back and forth between 100% and 50%. Notice how that sound changes dramatically, and you may even hear that ring of the resonance bell. In some cases it is a high pitched ring, or a buzz, or a squeaky rubbery sound, or a wet juicy sound, depending on the oscillator type and the many factors that are involved in how a simple tool called the filter can have.
Now, here are some commonly known types of filters.
- Korg MS-20 (OTA based)
- Steiner Parker – Self oscillating filter without the loss in volume when resonance is increased
- Curtis
- Moog Ladder (diode ladder)
- Oberheim SEM (Synthesizer Expansion Module)
- OTA (Operational Transconductance Amplifier) Roland Jupiter 8 used an OTA filter
- SVF (State Variable Filter)
- Polivoks – Soviet IC based filter
This section may be updated later with the chipsets or IC designs used but that doesn’t make too much of a difference except maybe the output quality of the sound. The filters in general do just that, filter frequencies based on the setting selected, and modulate parameters like cutoff frequency and resonance and/or Q.
More to come as we now get into shaping the sound further.