How to Adjust Virtual Speaker Positions in FFmpeg

This article provides a quick guide on how to configure and adjust virtual speaker positions using FFmpeg's binaural audio filters. You will learn how to use the headphone and sofaspatial filters to map multi-channel audio (such as 5.1 or 7.1 surround sound) into a 3D spatialized stereo output for headphones by defining custom coordinates for each virtual speaker.

Understanding Virtual Speaker Coordinates

FFmpeg defines virtual speaker positions using two coordinates:

  • Azimuth: The horizontal angle in degrees, ranging from -180 to 180. 0 is directly in front, -90 is directly to the left, 90 is directly to the right, and 180 (or -180) is directly behind.
  • Elevation: The vertical angle in degrees, ranging from -90 (directly below) to 90 (directly above). 0 is eye-level.

Method 1: Using the headphone Filter

The headphone filter is the standard built-in filter to create a binaural mix from multi-channel input. You can explicitly map input channels to specific spatial coordinates using the positions parameter.

Here is the command structure to adjust speaker positions for a 5.1 audio source:

ffmpeg -i input_51.wav -af "headphone=map=FL|FR|FC|LFE|SL|SR:positions=FL -30 0|FR 30 0|FC 0 0|LFE 0 -10|SL -110 0|SR 110 0" output_binaural.wav

How to configure the parameters:

  • map=FL|FR|FC|LFE|SL|SR: Specifies the order of the input channels (Front Left, Front Right, Front Center, Low-Frequency Effects, Surround Left, Surround Right).
  • positions=...: Assigns coordinates (Azimuth and Elevation) to each mapped channel, separated by spaces. Each speaker mapping is separated by a pipe (|).
    • FL -30 0 places the Front Left speaker 30 degrees to the left at eye level.
    • SL -110 0 places the Surround Left speaker 110 degrees to the left (slightly behind the listener) at eye level.

Method 2: Using the sofaspatial Filter

For high-fidelity spatialization, FFmpeg supports the Spatially Oriented Format for Acoustics (SOFA). The sofaspatial filter allows you to use a custom HRTF (Head-Related Transfer Function) file while manually adjusting the virtual speaker layout.

To use this filter, download a compatible .sofa file and run:

ffmpeg -i input_51.wav -af "sofaspatial=sofa=path_to_hrtf.sofa:positions=FL -30 0|FR 30 0|FC 0 0|LFE 0 0|SL -110 0|SR 110 0" output_binaural.wav

How to configure the parameters:

  • sofa: Specifies the path to your downloaded SOFA file.
  • positions: Uses the exact same coordinate syntax as the headphone filter, allowing you to fine-tune the virtual speaker angles to match the acoustics of the chosen HRTF profile.