Configure Low-Shelf and High-Shelf in FFmpeg Anequalizer

This article provides a straightforward guide on how to configure low-shelf and high-shelf filters using the FFmpeg anequalizer audio filter. You will learn the specific syntax required to define these filter types, the parameters that control their behavior, and practical command-line examples for combining them to shape your audio’s frequency response.

Understanding the Anequalizer Filter Syntax

The anequalizer filter is a high-order parametric equalizer. To configure shelving filters, you must define individual bands using specific parameters. The general syntax for a single band is:

f=frequency w=width g=gain t=type

• f (Frequency): The center or cutoff frequency of the filter in Hz.
• w (Width): The band width in Hz.
• g (Gain): The gain in dB (positive values boost the signal, negative values cut it).
• t (Type): Specifies the filter type. This is the crucial setting for shelving filters:
  • 0: Peaking / bandpass (default)
  • 1: Low-shelf
  • 2: High-shelf

To apply a setting to a specific channel, prefix the band configuration with c0 (channel 0/left), c1 (channel 1/right), etc. If no channel is specified, the filter applies to all channels.

Configuring a Low-Shelf Filter (t=1)

A low-shelf filter boosts or cuts all frequencies below the specified cutoff frequency.

To configure a low-shelf filter that boosts frequencies below 200 Hz by 6 dB, use the following syntax:

anequalizer="f=200 w=100 g=6 t=1"

In this command: * f=200 sets the transition frequency to 200 Hz. * w=100 defines a transition width of 100 Hz. * g=6 applies a 6 dB boost. * t=1 designates the filter type as a low-shelf.

Configuring a High-Shelf Filter (t=2)

A high-shelf filter boosts or cuts all frequencies above the specified cutoff frequency.

To configure a high-shelf filter that cuts frequencies above 8,000 Hz (8 kHz) by 4 dB, use the following syntax:

anequalizer="f=8000 w=1000 g=-4 t=2"

In this command: * f=8000 sets the transition frequency to 8,000 Hz. * w=1000 defines a transition width of 1,000 Hz. * g=-4 applies a 4 dB cut. * t=2 designates the filter type as a high-shelf.

Combining Low-Shelf and High-Shelf Filters

You can combine multiple bands within a single anequalizer instance by separating them with the | (pipe) character.

The following example applies a 5 dB low-shelf boost below 150 Hz and a 3 dB high-shelf boost above 10,000 Hz across all audio channels:

ffmpeg -i input.mp3 -af "anequalizer=f=150 w=50 g=5 t=1|f=10000 w=2000 g=3 t=2" output.mp3

Channel-Specific Configuration

If you want to apply the low-shelf filter only to the left channel (c0) and the high-shelf filter only to the right channel (c1), structure your command like this:

ffmpeg -i input.wav -af "anequalizer=c0 f=150 w=50 g=5 t=1|c1 f=10000 w=2000 g=3 t=2" output.wav