A filter works by allowing some frequencies through, while reducing or eliminating others. There are three different kinds of basic filters to work with in Pd: the Low Pass Filter, the High Pass Filter and the Band Pass Filter, as well as a Voltage Controlled Filter which can be controlled using some of the LFO techniques we have already looked at.
Low Pass Filter
A filter which allows only low frequencies to pass is called a Low Pass Filter. The object for this kind of filter in Pd is [lop~]. It has one inlet for audio and one inlet for a number which determines the frequency in Hertz where the filter starts to reduce the audio (the Cutoff Frequency). Frequencies above the Cutoff Frequency are reduced or eliminated.
High Pass Filter
While one which allows only high frequencies is called a High Pass Filter. The object for this kind of filter in Pd is [hip~]. It has one inlet for audio and one inlet for the the Cutoff Frequency. Frequencies below the Cutoff Frequency are reduced or eliminated.
Band Pass Filter
A filter which allows some range of frequencies between highest and lowest is called a Band Pass Filter. The object for this kind of filter in Pd is [bp~]. It has one inlet for audio, a second inlet for the center frequency that it will allow to pass and a third inlet for the Resonance, which determines the width of the range of frequencies it allows to pass (the Pass Band). The Center Frequency will pass unchanged, and frequencies higher or lower than that will be reduced or eliminated. How much they will be eliminated depends on the Resonance. Useful numbers for the Resonance tend to be between 0 and 10.
The three filters we've seen so far all take numbers to control their Cutoff or Center Frequencies as well as their Resonance (in the case of [bp~]. However, there are times when you might want to control the frequency of a filter with an audio signal. A typical situation is when a filter is swept by an LFO.
Voltage Controlled Filter
[vcf~] (Voltage Controlled Filter) is a filter whose Center Frequency and Resonance can be controlled by audio signals. The way this is done is quite similar to the tutorial on Frequency Modulation. A Slider sends a MIDI note to a MIDI to Frequency audio [mtof~] object to provide the Center Frequency to be swept, or modulated. Then we have an LFO [osc~] object, whose output is multiplied by the amount in Hertz which we want to sweep the filter frequency. This stream of numbers is added to the Center Frequency coming from the [mtof~] object and sent to the Frequency inlet of the [vcf~]