Vintage synthesizers such as the Minimoog are prized more than other synths because of the distinctive sound of their filters. In fact, the filter is one of the key elements in the sound of any synth, new or old. Filters have other uses, too: they can add character to recorded acoustic-instrument tracks, for instance.
A filter is a signal processor. When it's fed an input, which might come from the synthesizer's own oscillators, a recorded track, or a live mic, it changes the signal in some way. But this definition is too general to be useful. It could apply to any effects device. More specifically, a filter changes the frequency contour of a signal.
When a signal passes through a filter, the amplitudes of some of its component frequencies will be reduced, or attenuated. A filter can also increase the amplitudes of some frequencies and leave others unaffected.
FIG. 1: This figure shows the frequency response of a lowpass filter (left) and a highpass filter (right).
It's important to understand that a filter can't add components to a sound that wasn't there to begin with. The filter cuts or boosts only those frequencies that are already present. Some filter modules also have a distortion stage, which can be used to add new frequencies, but adding distortion is not the same as filtering.
The most common type of filter is called the lowpass filter. It allows frequencies at low frequencies to pass through, while attenuating the frequencies at higher frequencies. The opposite of a lowpass filter is a highpass filter. As the name suggests, a highpass filter allows high frequencies to pass through while reducing the amplitudes of low frequencies (see Web Clip 1).
The frequency response curves of two example lowpass and highpass filters are shown in Fig. 1. Wherever the curve is at 0 dB on the y-axis, the frequencies on the x-axis pass through without being changed. Where the curve drops below the 0 dB line, frequencies will be reduced in amplitude by the amount that the curve has dropped. For instance, where the curve shows -12 dB (on the y axis) at 800 Hz (on the x axis), a component whose frequency is 800 Hz will be reduced in level by 12 dB when passing through the filter.
Also common are bandpass and band-reject filters. A bandpass filter allows frequencies within a given frequency band to pass while attenuating both the higher and the lower frequencies around it. A band-reject filter does the opposite: it reduces the levels of components within its active band while allowing both higher and lower frequencies to pass through.
A multimode filter has a switch that allows it to operate in more than one of these modes. Most multimode filters offer lowpass, highpass, and bandpass modes. Some also have a band-reject mode. The mode switch may have many other settings as well, some of which are explained below. Some multimode filters have only one output. You choose the mode you want to use, and the signal coming from the output will be filtered accordingly. Other filters have separate outputs for each mode, so that one signal can be filtered in several different ways at the same time (see Web Clip 2).
On the Slopes
When a lowpass filter does its job, it allows the sound energy at low frequencies to pass through, but reduces the levels of frequencies at higher frequencies. However, the filter does not simply cut out everything above some point and leave everything below that point entirely unaffected. Rather, the filter gradually lowers the signal's strength until it reaches total attenuation. The point at which the signal's amplitude has been reduced to 0.707 of its original strength is called the cutoff frequency. With a lowpass filter, the portions of the signal above the cutoff frequency will be attenuated by an even greater amount, while the bulk of the signal below the cutoff will pass through without being attenuated. (There is an intermediate area called the transition band that lies between the cutoff frequency and the point at which the filter attenuates the signal completely.)
Just about every filter has a knob or slider labeled Cutoff Frequency, Cutoff, or (if panel space is tight) Cut. With a lowpass filter, if the cutoff frequency is set to 20 kHz, the entire incoming signal will pass through. The filter will do nothing. At the other extreme, if the cutoff frequency is set below the lowest frequencies in the signal, the entire signal will be filtered out, resulting in silence.
FIG. 2: Frequency response curves for lowpass filters with slopes of 6 dB, 12 dB, and 24 dB per octave.
To describe the amount of attenuation applied to the sound's spectrum at various frequencies, we use the term slope. Slope is usually discussed in terms of decibels per octave, and typical slopes for real-world filters are 6 dB, 12 dB, and 24 dB per octave, as shown in Fig. 2. With a slope of 6 dB per octave, frequencies one octave above the cutoff frequency are attenuated by 6 dB, those two octaves above the cutoff are attenuated by 12 dB, and so on. This is a relatively gentle type of filtering. With a 24-dB-per-octave slope, frequencies one octave above the cutoff will be attenuated by 24 dB, those two octaves above the cutoff by 48 dB, and so on.
Bear in mind that we've just been describing lowpass filtering. With a highpass filter, the frequencies below the cutoff frequency will be attenuated. Bandpass and band-reject filters are usually described in a different way, by referring to the width of their active band rather than their slope.
Because filters are so important, most synths give you several ways to change the cutoff frequency automatically while you're playing music. Also common though less frequently seen is a control on the filter's resonance. (Resonance, or Q, typically refers to the steepness of a filter and is the ratio of the filter's center frequency to its bandwidth. Resonant peaks can also occur in other types of filters.) Normally, all of a filter's control inputs can be used at the same time.
By moving the cutoff knob or slider in performance, you can do live filter sweeps. If the knob/slider can transmit MIDI data (most can), you can record your improvised filter sweeps into a sequencer and then edit them if needed.
Most synth filters have a dedicated envelope generator, whose output is used to sweep the cutoff up and down. The filter envelope can produce anything from a fast blip or click at the beginning of the note to a broad slow sweep (see Web Clip 3). Increasing the filter resonance will give the filter envelope a more pronounced sound (see Fig. 3 and Web Clip 4). There will also usually be a parameter with which you can adjust the amount of envelope modulation.
FIG. 3: The frequency response of a resonant lowpass filter. As the amount of resonance is turned up frequencies near the cutoff frequency are boosted more and more rather than being cut.
By controlling filter cutoff with MIDI Velocity, you can cause a lowpass filter to open up further on high-Velocity notes, letting more frequencies through. This type of modulation mimics the sound of many acoustic instruments, such as guitar and piano, whose tone is not only louder but also brighter when they're played harder.
With the filter's keyboard-tracking input (which might have a name such as Keytrack), you can cause the cutoff to move higher as you play higher notes on the keyboard and vice-versa. Key tracking helps make the synth's tone more uniform across a wide keyboard range. Without key tracking, low notes will tend to sound very loud, because lots of frequencies are passing through the filter, but high notes will sound very soft, because few or no frequencies make it through the filter without being attenuated.
Most synths allow you to route the output of an LFO (low-frequency oscillator) to the filter cutoff frequency (see Web Clip 5). This type of modulation can produce various types of tremolo, such as pulsing wah-wah sweeps. If the LFO can be synced to the tempo of your sequencer, you can produce rhythmic filtered effects.
Some filters have external audio inputs, with which you can pass any audio signal through the filter. By filtering a drumbeat, for instance, while triggering the filter envelope from a MIDI keyboard, you can add an extra rhythmic pattern to the beat.
As you get to know the filters on your synth, you'll discover other musical ideas. Go through some of the factory patches and analyze how the filter is being used. Adjust the cutoff frequency, the resonance, and the filter envelope parameters, and listen carefully to the results. The key to mastery of filters is experimentation.
Jim Aikin is the author of Power Tools for Synthesizer Programming, but his favorite instrument these days is his five-string Jensen electric cello.