A filter is a process that removes frequencies from a signal.

1. In audio 🔗

1.1. Filter or equaliser? 🔗

An important thing to remember about filtering a signal is that once something is removed, it cannot be re-added. In recording, it is therefore often preferable to record without any filters cutting into the content of the audio source. Attenuating EQ is a milder choice that can be undone.

1.2. Envelope filter 🔗

A common effect especially used for synthesisers and pornographic film soundtracks is a resonant low-pass filter of which the frequency follows the envelope (the volume) of the input signal, meaning transients open the filter making for a more percussive sound, as the sustain of a tone is restricted to low frequencies. An excellent example of an envelope filter in metal is Dragonaut by Sleep 🔗 .

An auto-wah is a form of envelope filter that uses a band-pass filter instead of a low-pass filter. Other types of filters are less commonly used with envelopes.

2. Types 🔗

2.1. Low-pass filter 🔗

Frequencies below the cutoff point pass, frequencies above the cutoff point are removed.

A low pass filter may be used to remove AC, oftentimes with the purpose of removing noise from DC power.

2.2. High-pass filter 🔗

Frequencies above the cutoff point pass, frequencies below the cutoff point are removed.

A high pass filter may be used to remove DC, as it is effectively a 0Hz signal.

2.3. Band-pass filter 🔗

Frequencies within a certain band pass, frequencies outside the band are removed. A band pass filter can be constructed by combining a low-pass filter and a high-pass filter.

A common application for band-pass filters is the wah, which sweeps a resonant band-pass filter.

2.4. Band-reject filter 🔗

Frequencies outside a certain band pass, frequencies inside the band are removed.

2.5. All-pass filter 🔗

No frequencies are removed. However, phase is altered around a target frequency.

3. Implementations 🔗

3.1. Passive filter 🔗

Passive filters are most commonly constructed using capacitors and resistors. Inductors can be used too, though they are less commonly used due to cost and size.

3.2. Active filter 🔗

An active filter combines filters with active electronics, which can boost a signal. This lets one create EQ, shelves, etc.

3.3. Digital filter 🔗

Filters can be created digitally and can achieve things not easily done with electronic processing. They are also perfectly consistent, which cannot be said for electronic filters.

Linear-phase filters, i.e. filters that do not alter signal phase, can be made using digital signal processing. Their drawback is that they introduce a slight signal delay. A common application for linear-phase filters is in the use of crossovers.