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# Filter

A Filter is used to amplify (boost), pass or attenuate (cut) defined parts of a frequency spectrum. Easy Effects uses the Filter from Linux Studio Plugins.

Type
- Low-pass - Low-pass filter with rejection of high frequencies.
- High-pass - High-pass filter with rejection of low frequencies.
- Low-shelf - Shelving filter with adjustment of low frequencies.
- High-shelf - Shelving filter with adjustment of high frequency range.
- Bell - Bell filter with smooth peak/recess.
- Bandpass - Bandpass filter.
- Notch - Notch filter with full rejection of selected frequency.
- Resonance - Resonance filter with sharp peak/recess.
- Ladder-pass - The filter that makes some ladder-passing in the spectrum domain.
- Ladder-rej - The filter that makes some ladder-rejection in the spectrum domain.
- Allpass - All-pass filter which only affects the phase of the audio signal at the specified frequency.

Filter Mode
- RLC - Very smooth filters based on similar cascades of RLC contours.
- BWC - Butterworth-Chebyshev-type-1 based filters. Does not affect Resonance and Notch filters.
- LRX - Linkwitz-Riley based filters. Does not affect Resonance and Notch filters.
- APO - Digital biquad filters derived from canonic analog biquad prototypes digitalized through Bilinear transform. These are textbook filters which are implemented as in the EqualizerAPO software.
- BT - Bilinear Z-transform is used for pole/zero mapping.
- MT - Matched Z-transform is used for pole/zero mapping.
- DR - Direct design is used to serve the digital filter coefficients directly in the digital domain, without performing transforms.

Equalizer Mode
- IIR - Infinite Impulse Response filters, nonlinear minimal phase. In most cases does not add noticeable latency to output signal.
- FIR - Finite Impulse Response filters with linear phase, finite approximation of equalizer's impulse response. Adds noticeable latency to output signal.
- FFT - Fast Fourier Transform approximation of the frequency chart, linear phase. Adds noticeable latency to output signal.
- SPM - Spectral Processor Mode of equalizer, equalizer transforms the magnitude of signal spectrum instead of applying impulse response to the signal.

Slope
The slope of the filter characteristics.

Frequency
The cutoff/resonance frequency of the filter or the middle frequency of the band.

Width
The width of the bandpass/ladder filters in octaves.

Gain
The gain of the filter. It is disabled for lo-pass/hi-pass/notch filters.

Quality
The quality factor of the filter.

Balance
The balance between left and right output channels.

## References

- Wikipedia Audio Filter
- Linux Studio Plugins Filter