(compMus1) Basic Synthesis Controls – Filters

Filters

A general definition for a filter is anything that changes the gain of frequencies in a sound. This definition is so general it is useless, since by this definition all audio producing, recording, transmission, and listening devices (including rooms, ears, etc.) are filters.

Typically, we use the term filter to apply to devices that intentionally control the amount of gain in a prescribed frequency range of a sound. Filters are defined by the frequencies they let through (pass band) or frequencies they reject (stop band). The basic filter types for synthesis are low-pass, high-pass, band-pass, or band-reject (notch).

Low-pass

• allows frequencies below a cutoff frequency to pass through.
• cutoff frequency is defined as being at the -3 dB point on the downward slope of the filter curve.
• the slope is defined by its steepness. A typical filter slope is 6 dB per octave (note difference from text, which goes for 3 dB). Higher slopes produce more targeted filtering, but introduce other potential problems.

High-pass

• allows frequencies above a cutoff frequency to pass through.
• other parameters (cutoff and slope) are same as low-pass filter

Band-pass

• allows frequencies around a center frequency to pass through (or between a low cutoff and high cutoff).
• literally a combination of low-pass and high-pass filters.
• generally address center frequency, rather than cutoff frequencies.
• slopes are added, so a 6 dB slope below the center frequency and a 6 dB slope above the center frequency yield a 12 dB slope for the filter.

Band-reject (notch)

• rejects frequencies around a center frequency (or between a low cutoff and high cutoff), not allowing these frequencies to pass through.
• other parameters same as band-pass

Filter Resonance

A resonance control on a filter controls the amount of signal feedback to the filter. Feedback takes the output of a system, scales the amplitude (down), and mixes it with current input to a system. Feedback will boost frequencies around the cutoff or center frequencies.