How to Use the FFmpeg SOFA Filter

This article provides a straightforward guide on how to use the sofa (Spatially Oriented Format for Acoustics) filter in FFmpeg to create virtual 3D surround sound over stereo headphones. You will learn what the filter does, how to obtain the necessary SOFA files, and the exact FFmpeg commands required to apply HRTF (Head-Related Transfer Function) filtering to your audio files.

What is the FFmpeg SOFA Filter?

The sofa filter in FFmpeg uses Head-Related Transfer Functions (HRTF) to simulate spatial audio. By applying a .sofa file (which contains measurements of how human ears receive sound from different points in space) to a multi-channel audio source (like 5.1 or 7.1 surround sound), the filter downmixes the audio into a 3D stereo output designed specifically for headphones.

Prerequisites

To use this filter, you need two things:

  1. An FFmpeg build compiled with libmysofa support. You can verify this by running ffmpeg -filters in your terminal and checking if sofa is listed in the output.
  2. A .sofa database file. These files contain the spatial acoustics data. You can download free, standardized SOFA files from the official SOFA spatially oriented format website or open-source repositories like the IRCAM Marie database.

Step-by-Step Usage

1. Basic Conversion (Multi-channel to 3D Stereo)

To convert a 5.1 channel audio file into a 3D spatial stereo file using a downloaded SOFA file, use the following command:

ffmpeg -i input_51.wav -af "sofa=sofa=/path/to/your/file.sofa" output_spatial.wav

In this command: * -i input_51.wav specifies your multi-channel input file. * -af applies an audio filter. * sofa=sofa=/path/to/your/file.sofa calls the filter and points it to the absolute path of your .sofa file.

2. Adjusting the Gain

Spatialization can sometimes reduce the perceived volume of the audio. You can adjust the output gain (in decibels) directly within the filter parameters:

ffmpeg -i input_51.wav -af "sofa=sofa=/path/to/your/file.sofa:gain=-3" output_spatial.wav

3. Customizing the Processing Type

The sofa filter allows you to choose between different processing types. The default is time-domain processing, but you can change it to frequency-domain processing for potential performance improvements on longer files:

ffmpeg -i input_51.wav -af "sofa=sofa=/path/to/your/file.sofa:type=freq" output_spatial.wav

The available options for type are: * time: Time-domain processing (default, more accurate but CPU intensive). * freq: Frequency-domain processing (faster for large impulse responses).

4. Applying to Video Files

If you are processing a video file with multi-channel audio and want to keep the video stream unchanged while spatializing the audio, use the following command:

ffmpeg -i input_video.mp4 -c:v copy -af "sofa=sofa=/path/to/your/file.sofa" output_video.mp4

This copies the video stream (-c:v copy) without re-encoding, saving time and preserving video quality, while applying the SOFA filter to the audio stream.