Load HRIR Files into FFmpeg Headphone Filter

This guide explains how to load Headphone Relation Impulse Response (HRIR) files into the FFmpeg headphone filter to create a binaural 3D spatial audio effect. You will learn the exact command-line syntax required to map multi-channel audio sources and HRIR coefficient files, enabling you to convert surround sound formats like 5.1 or 7.1 into spatialized stereo for standard headphones.

Understanding the FFmpeg Headphone Filter

The headphone filter in FFmpeg uses Head-Related Transfer Functions (HRTF) represented by HRIR convolution streams to position audio channels in a virtual 3D space. To use this filter, you need two primary inputs: 1. The Audio Source: A multi-channel audio file (e.g., 5.1 or 7.1 surround sound). 2. The HRIR File: A multi-channel WAV file containing the impulse responses corresponding to each virtual speaker position.

Command-Line Syntax

The basic syntax for applying the headphone filter using a single multi-channel HRIR file is as follows:

ffmpeg -i input_surround.wav -i hrir_file.wav -filter_complex "[0:a][1:a]headphone=map=FL|FR|FC|LFE|SL|SR[out]" -map "[out]" output_binaural.wav

Explaining the Parameters

• -i input_surround.wav: The first input (0:a), which is your multi-channel audio track.
• -i hrir_file.wav: The second input (1:a), which is the HRIR coefficient file containing the spatial profiles.
• -filter_complex: This flag is used because the filter processes multiple input streams.
• [0:a][1:a]headphone: Feeds the audio track and the HRIR track into the headphone filter.
• map=FL|FR|FC|LFE|SL|SR: This is the crucial mapping parameter. It specifies the order of the virtual speaker positions as they appear in the channels of your HRIR input file. The channel abbreviations must be separated by a pipe (|) character.

Identifying the HRIR Channel Order

For the filter to work correctly, the map argument must precisely match the internal channel layout of your HRIR WAV file. Common channel layouts include:

• For 5.1 HRIRs: FL|FR|FC|LFE|SL|SR (Front Left, Front Right, Front Center, Low-Frequency Effects, Surround Left, Surround Right) or FL|FR|FC|LFE|BL|BR (using Back Left and Back Right).
• For 7.1 HRIRs: FL|FR|FC|LFE|BL|BR|SL|SR.

If the virtual speakers sound like they are in the wrong positions, verify the channel order of your HRIR file and adjust the map string accordingly.

Using Multiple Single-Channel HRIR Files

If your HRIR coefficients are stored in separate mono WAV files instead of a single multi-channel file, you can load them individually by passing them as sequential inputs and mapping them accordingly:

ffmpeg -i input_stereo.wav -i hrir_left.wav -i hrir_right.wav -filter_complex "[0:a][1:a][2:a]headphone=map=FL|FR[out]" -map "[out]" output_spatial.wav

In this scenario, each additional input index corresponds to the respective position defined in the map parameter sequence.