Convert 360 Video Spatial Audio to Binaural FFmpeg

Extracting spatial audio from 360-degree videos and converting it to binaural stereo allows listeners to experience immersive 3D audio on standard headphones. This guide provides a step-by-step walkthrough on how to use FFmpeg to extract first-order Ambisonics (typically 4-channel AmbiX format) from a 360-degree video file and render it into a binaural stereo track using virtual loudspeaker mapping.

Step 1: Verify the Spatial Audio Format

Before processing, verify that your input video contains a 4-channel Ambisonic audio track (usually track index 0:1). Run the following command to inspect the file:

ffprobe -i input_360.mp4

Look for an audio stream labeled with 4 channels (often tagged as ambisonics or containing layout details like 4 channels).

Step 2: Obtain an HRTF SOFA File

To render spatial audio into a realistic 3D headphone experience, FFmpeg requires a Head-Related Transfer Function (HRTF) dataset in .sofa format. This file simulates how human ears perceive sound from different directions.

You can download a standardized SOFA file (such as the MIT KEMAR or SADIE database datasets) from public repositories like the SofaSpatially Audio website. Save this file as subject.sofa.

Step 3: Extract and Convert Ambisonics to Binaural

To convert the 4-channel Ambisonic audio into 2-channel binaural stereo, you must decode the Ambisonics to a virtual multi-channel speaker layout (like 5.1 or 7.1) and then pass those virtual channels through FFmpeg’s headphone filter using your SOFA file.

Run the following command to perform the extraction and conversion:

ffmpeg -i input_360.mp4 -filter_complex \
"[0:a]ambidecod=inputs=4:outputs=5.1[sf]; \
 [sf]headphone=map=FL|FR|FC|LFE|SL|SR:hrir=/path/to/subject.sofa[out]" \
-map "[out]" output_binaural.wav

Command breakdown: * ambidecod=inputs=4:outputs=5.1: Decodes the 4-channel Ambisonic input into a virtual 5.1 surround sound layout. * headphone=map=...: Maps those virtual speaker positions (Front Left, Front Right, Front Center, Low-Frequency Effects, Surround Left, Surround Right) to the HRTF spatial coordinates defined in your .sofa file. * hrir=/path/to/subject.sofa: Specifies the path to your downloaded SOFA file. * output_binaural.wav: Outputs a standard 2-channel stereo file containing the 3D binaural effect.

Step 4: Merge the Binaural Audio Back into the Video

If you want to replace the original 4-channel spatial audio with the new binaural stereo track while keeping the original 360-degree video stream intact, use the following command:

ffmpeg -i input_360.mp4 -i output_binaural.wav \
-map 0:v -map 1:a -c:v copy -c:a aac -b:a 320k output_binaural_video.mp4

This copies the video codec directly without re-encoding (-c:v copy) and encodes the new binaural audio track to high-quality AAC at 320kbps, resulting in a shareable video optimized for standard headphones.