Configure FFmpeg Soxr Chebyshev Bandwidth

This guide explains how to configure the Chebyshev phase response and transition bandwidth of the SoX Resampler (soxr) in FFmpeg. You will learn the specific command-line parameters required to enable Chebyshev phase routing, set the resampling precision, and adjust the filter cutoff frequency to customize your audio bandwidth.

Enabling and Configuring Chebyshev in FFmpeg

The SoX Resampler in FFmpeg allows you to use a Chebyshev phase response, which can be configured using the aresample audio filter. To use this feature, you must explicitly set the resampler to soxr, enable the chebyshev option, and adjust the cutoff and precision parameters.

The Command Syntax

Use the following basic template to configure these settings:

ffmpeg -i input.wav -af "aresample=resampler=soxr:precision=28:chebyshev=1:cutoff=0.95" output.wav

Parameter Breakdown

  • resampler=soxr: Directs FFmpeg to use the high-quality SoX Resampler library instead of the default software resampler (swr).
  • chebyshev=1: Enables the Chebyshev phase response. Note that this parameter only has an effect if the precision is set to a value greater than 20.
  • precision=28: Sets the resampling precision in bits. To utilize Chebyshev phase response, you must set this above 20 (typically 24, 28, or 32 for high-fidelity audio).
  • cutoff=0.95: Sets the filter cutoff frequency (bandwidth). This is represented as a fraction of the Nyquist frequency. For example, 0.95 configures the bandwidth to 95% of the target sample rate's Nyquist limit, allowing you to control the passband and roll-off steepness.

Example: High-Fidelity Downsampling

If you want to downsample a 96kHz audio file to 44.1kHz while preserving maximum high-frequency content using a steep Chebyshev filter, use the following command:

ffmpeg -i input_96k.wav -af "aresample=44100:resampler=soxr:precision=28:chebyshev=1:cutoff=0.98" output_44k.wav

In this setup, setting the cutoff to 0.98 ensures that the frequency response remains flat up to 98% of the 22.05kHz Nyquist limit (approximately 21.6kHz) before dropping off sharply.