Cascading Filters.
So you decide you want to use a particular type of filter…lets say a State Variable, but it’s not quite right, you need a steeper filter roll off…what to do to fix this? Fortunately you can just add another SV filter in series with the first one, and this turns a 12dB octave low/high pass into a 24dB octave filter and the bandpass/notch into 12dB octave.
So far so good, but there’s one very important thing to watch out for…the resonance settings on the filters. If we have both filters with adjustable resonance then as soon as we start to approach any moderate level of resonance the second filter will be heavily overloaded at some frequencies. To avoid this we can use the structure below, where the resonance of the first filter is fixed at 0.
Be careful when cascading SV filters.
Cascading too many band-pass or high-pass outputs is not a
good idea. It can cause annoying high-frequency ringing, because state
variable filters boost high frequencies even at low resonance settings.
Another catch with filters cascaded in series is they do introduce phase shift, so beware if you are intending to mix them with the “dry” audio signal, you’ll quite probably get some unwanted peaks and notches occurring due to phase addition and cancellation effects. You can see below that there’s clearly a change in phase between oscillator and the output of the second filter that changes with frequency.
Parallel Filtering.
This is a good way of achieving a stereo effect, especially using two different models of filter, as shown below with a Moog filter for the left hand channel, and the SV filter for the right hand channel. By having two independently controlled filters (albeit with a common Keyboard Tracking CV (KB Track) you can get a really rich stereo effect, (especially when the Pitch is modulated by out of phase LFO’s, but that’s another topic for later)
Note: When constructing a layout like this inside a container (as is good practice) it’s a really good idea to follow these steps:
1) Set the filter Pitch plugs to 0V (they are pre-programmed at 5V)
2) Put both of the Pitch CV’s through their own Level Adj module with the Input 1 plug set to 0 volts (again they are pre-programmed to 5V), and the Input 2 Plug set to 10V.
This prevents our container inheriting any voltages from either the Filters, or the Level Adj and setting them as an unwanted voltage on the KB Track IO Module, leaving you with an odd 5V offset appearing on another module’s control voltage. I spent quite a time at one point tracking down why the controls on one of my synthesizer modules were not behaving as expected-due to this inherited voltage from inside a container!
State Variable Multifilter.
A multi-filter takes things a step further. I won’t show the frequency spectrum for any of the settings because you’ll get a far better idea of the sounds it will produce by building one, hooking up a white noise source and having a listen. What we are doing is taking separate outputs from each part of the filter, then mixing them to achieve a sound we like. Of course if you want to get really complex you could have four individual SV filters with individually controlled frequency and resonance settings… the world’s your oyster (or mollusk of choice)here, feel free to experiment.
The Oberheim Xpander multi-filter.
Oberheim took the concept of the multi-filter and ran with it.
The Voltage Controlled filter (VCF). The available modes on the filter are:
a) One-, two-, three- and four-pole low pass
b) One-, two and three-pole high pass
c) Two- and four-pole band pass
d) Two-pole notch
e) Three-pole phase shift
f) Two- and three-pole high pass plus one-pole low pass
g) Two-pole notch plus one-pole low pass
h) Three-pole phase shift plus one-pole low pass
The output from this filter could the be mixed and routed in a variety of ways.
TD Panda filter.
The nearest ready made SynthEdit module to this is the TD_Panda filter module
However all the mixing and levels are handled internally so while there’s a huge variety of modes for the filter you’re stuck with the internally set mixing.
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