Filters and Filtering - what are filters and how do they work

Normally I would jump into an equalisation tutorial as filters are used in equalisers and the terminology is shared between the two which can, on occasion, complicate things when trying to deal with analogue filters which is what I am going to concentrate on in this tutorial.

For this reason alone and because the filter that I am using in this tutorial is based on analogue topologies it is best to stick with the traditional low pass and high pass filters.

A filter allows you to remove unwanted frequencies and also allows you to boost certain frequencies. Which frequencies are removed and which frequencies are left depends on the type of filter you use.

Cut-off frequency This is the point (frequency) at which the filter begins to filter (block or cut out). The filter will lower the volume of the frequencies above or below the cut-off frequency depending on the type of filter used.

This ‘lowering of the volume of the frequencies,’ is called Attenuation. In the case of a low pass filter, the frequencies above the cut off are attenuated. In the case of a high pass filter, the frequencies below the cut off are attenuated. Put simply: in the case of a low pass filter, we are trying to block the (higher) frequencies above a certain point and allow the lower frequencies through. In the case of a high pass filter, the opposite is true. We try to cut out or block frequencies below a certain point and allow the higher frequencies through.

On analogue synthesizers this cut-off was called the slope or gradient. The actual terminology was more accurately described as the RC (resistor/capacitor). Don’t worry about this for now.

Analogues use circuitry and for that reason alone, it takes time for the filter to attenuate frequencies, in proportion to the distance from the cut-off point. Today’s technology allows for instant cut-off as the filter attenuation is determined by algorithms as opposed to circuits. That is why the filters off an Arp or Oscar etc, are so much more expressive and warm as they rely completely on the resistors and capacitors to, first warm up, then to work but in a gradual mode(gradual meaning sloped or curved as opposed to instant). Depending on how well a filter attenuates or the way it attenuates gives us an idea of the type of sound we will achieve with an analogue filter.

The speed at which the filter attenuates is called the slope or gradient. Another point to raise now is that you will often see values on the filter knobs on analogue synthesizers that have 12dB or 24dB per octave. That basically means that each time the frequency doubles, the filter attenuates by 12dB or 24dB everything at that frequency. These are also known as 2 pole or 4 pole filters (fig 2), each pole represents 6dB of attenuation. This is how analogue circuits were built, the number of circuits being used by the filter to perform the task at hand.

Resonance Most synthesizer manufacturers, and in the case of most analogue synthesizers, the term resonance is used most commonly. Other manufacturers of synthesizers, or software synthesizers, might call it emphasis or Q.

Boosting the narrow band of frequencies at the cut-off point is called resonance. If you were to boost the resonance to the maximum, then the filter will begin to self oscillate. This means that it will generate an audible sine wave, more like whistling, even when receiving no input signal.

Filters and filtering is a huge subject as it pertains to both synth filters and equalisation. Let me simplify this subject for you and show you how to use filters creatively.