By John Moxey
High and Low Pass Filters
The most basic filter types, these filters progressively cut the amplitude of an input signal at a selected threshold frequency. In the case of a high pass filter frequencies below a selected frequency will be progressively cut. Similarly in the case of a low pass filter, the frequencies above a selected threshold frequency will be progressively cut. As such these filters are sometimes called Cut-off Filters.
High and Low Pass filters commonly have a gentle cut of -6 dB per octave, -12 dB per octave or a more aggressive -18 dB per octave.
It is worth noting that the circuitry used to implement filters is not ideal. This means that the gain does not suddenly change from 0 dB to -18 dB as the frequency crosses the threshold, the gain change is progressive. In fact the quoted cut off frequency is the frequency at which the gain reduction is already 3dB below the input amplitude. This point is also referred to as the 3dB Down Point.
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| High Pass Filter | Low Pass Filter |
Shelving Filters
This kind of filter applies an equal level of boost or cut to all frequencies beyond a user defined threshold frequency. Yet again the filter response is not 'ideal'
Band Pass Filters
This type of feature boosts frequencies, in a chosen bandwidth around a specified frequency. Frequencies outside this range are not allowed through the filter.
Notch Filters
Similar to a Band Pass Filter. This type of feature cuts the frequencies, in a chosen bandwidth around a specified frequency. Frequencies within the chosen band will not be allowed through the filter.
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About John Moxey
John Moxey started Songstuff in 2000 and is an active songwriter and musician as well as a regular Songstuff author of music and music technology related articles and member of the site crew.
John started playing his first instrument (piano) in 1971, and has been writing songs since 1979. He began playing his now main instrument, the guitar, in 1982. In 1986 he finished training as a studio engineer and in 1997 obtained an Honours Degree in Electronics and Music from the University of Glasgow.
John now plays several other instruments including mandolin, violin, bagpipes, didgeridoo, bodhran, and vocals.
John has a broad range of performance experience from choirs and orchestras to pipe bands. He has sung for, played in and written for various bands, and produced songs across a number of genres.
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