How to Use FFmpeg afir for Real-Time Room Correction

This article provides a practical guide on how to perform real-time room correction on a live audio stream using the FFmpeg afir filter. You will learn how to set up the command-line interface, incorporate your room’s impulse response (IR) file, and optimize the filter settings to ensure low-latency, real-time audio playback.

Understanding the afir Filter

The afir (FIR Filter Relation) filter in FFmpeg applies a Finite Impulse Response (FIR) filter to an audio stream using a secondary input as the frequency response curve. For room correction, this secondary input is an Impulse Response (IR) WAV file, which represents the acoustic characteristics of your physical listening space. By convolving your live audio with the inverse of this IR, you correct room resonances and frequency imbalances.

Prerequisites

Before running the command, you need: 1. An Impulse Response (IR) File: A .wav file of your room’s impulse response, typically generated using measurement software like Room EQ Wizard (REW). 2. FFmpeg: Installed and configured with support for your system’s audio interface (e.g., PulseAudio, ALSA, or Jack on Linux; CoreAudio on macOS; DirectShow on Windows).

The Real-Time Command Structure

To process audio in real-time, you must capture the live system audio stream, pass it through the filter along with the IR file, and output it back to your physical playback device.

Below is the standard command structure using PulseAudio (Linux) as an example:

ffmpeg -f pulse -i default -i room_ir.wav -filter_complex "[0:a][1:a]afir=grms=1:maxp=1024[out]" -map "[out]" -f pulse default

Parameter Breakdown

• -f pulse -i default: Captures the real-time audio stream from your default system input or monitor source.
• -i room_ir.wav: Loads your pre-recorded room impulse response file as the second input stream ([1:a]).
• -filter_complex: Initiates the filtergraph to handle multiple inputs.
  • [0:a][1:a]: Feeds the live audio (input 0) and the IR file (input 1) into the filter.
  • afir: Calls the FIR filter.
  • grms=1: Automatically matches the output gain to the input level using RMS, preventing digital clipping.
  • maxp=1024: Sets the maximum partition size for the FFT calculation. Lower values (like 512 or 1024) reduce processing latency, which is critical for real-time audio stream alignment.
• -map "[out]": Directs the processed audio to the output.
• -f pulse default: Outputs the corrected audio stream in real-time to your system’s default playback device.

Optimizing for Latency and CPU

To achieve the lowest possible latency during live playback, consider the following adjustments:

• Truncate the IR File: Ensure your room_ir.wav file is as short as possible (typically under 100–200 milliseconds). Longer IR files require more FFT points, which introduces audible delay.
• Adjust Partition Sizes: Experiment with the minp and maxp parameters inside the afir filter. For example, afir=grms=1:minp=512:maxp=512 forces a small, fixed partition size to minimize buffer delay at the expense of slightly higher CPU usage.
• Match Sample Rates: Ensure your live audio stream, your IR file, and your output hardware are all configured to the same sample rate (e.g., 44100 Hz or 48000 Hz) to avoid real-time resampling overhead.