How to Use the FFmpeg firequalizer Filter

The firequalizer filter in FFmpeg is a powerful audio filter used to adjust the frequency response of an audio stream using a Finite Impulse Response (FIR) equalizer. This article provides a clear, step-by-step guide on how to configure and use the firequalizer filter, detailing its syntax, primary parameters, and practical command-line examples for boosting, cutting, or shaping specific audio frequencies.

Understanding the firequalizer Filter

Unlike standard parametric equalizers, firequalizer allows you to define custom frequency response curves using mathematical formulas or specific coordinate points (gain entries). It operates in the frequency domain, offering high precision and flexibility for audio mastering, noise reduction, and sound design.

The filter is configured primarily through two parameters: * gain: A mathematical expression that defines the gain (in dB) as a function of frequency f. * gain_entry: A list of specific frequency and gain pairs, which FFmpeg interpolates to create a smooth equalization curve.

Method 1: Using gain_entry (Point-by-Point Equalization)

The gain_entry parameter is the most intuitive way to shape audio. You define specific points using the format entry(frequency, gain). FFmpeg automatically connects these points with a smooth curve.

Example 1: Bass Boost

To boost frequencies below 100 Hz by 6 dB and leave frequencies above 200 Hz untouched (0 dB):

ffmpeg -i input.mp3 -af "firequalizer=gain_entry='entry(0, 6); entry(100, 6); entry(200, 0)'" output.mp3

Example 2: High-Pass Filter (Low-Cut)

To completely cut rumble and low-end noise below 80 Hz:

ffmpeg -i input.mp3 -af "firequalizer=gain_entry='entry(0, -inf); entry(80, -inf); entry(120, 0)'" output.mp3

Example 3: Multi-Band Equalizer

You can chain multiple entries together to create complex curves, such as a “smiley face” curve that boosts bass and treble while scooping the mids:

ffmpeg -i input.mp3 -af "firequalizer=gain_entry='entry(0, 6); entry(100, 4); entry(1000, -4); entry(10000, 4); entry(20000, 6)'" output.mp3

Method 2: Using gain (Mathematical Expressions)

The gain parameter allows you to use mathematical expressions to define the equalization curve. The variable f represents the frequency.

Example 1: Simple Low-Pass Filter

To apply a low-pass filter where frequencies above 1000 Hz are attenuated by 20 dB:

ffmpeg -i input.mp3 -af "firequalizer=gain='if(gte(f,1000), -20, 0)'" output.mp3

Example 2: Linear Gain Reduction

To gradually reduce gain as the frequency increases (creating a warmer sound):

ffmpeg -i input.mp3 -af "firequalizer=gain='-0.001*f'" output.mp3

Advanced Parameters

To fine-tune how the firequalizer processes audio, you can append additional parameters:

scale: Sets the scale used for frequency interpolation. Options include lin (linear), log (logarithmic), and sinh (hyperbolic sine). Logarithmic is recommended for audio as it matches human hearing.
delay: Sets the filter delay in seconds (default is 0.05). Higher values provide more accurate frequency resolution, especially at low frequencies, but increase latency.
accuracy: Defines the FFT accuracy. Default is 0.005.

Example: Logarithmic Multi-Band Equalizer with Custom Delay

This configuration uses a logarithmic scale for smoother natural crossovers and a longer delay for better low-frequency accuracy:

ffmpeg -i input.mp3 -af "firequalizer=delay=0.1:scale=log:gain_entry='entry(0, 5); entry(200, 0); entry(4000, -3); entry(20000, 5)'" output.mp3