FFmpeg Phaser Filter Delay Feedback and LFO Rate

The FFmpeg phaser audio filter allows you to create a sweeping, multi-phase delay effect commonly used in music production to add depth and motion to sound. This article provides a quick guide on how to configure the filter’s three essential parameters: delay, feedback (decay), and the LFO (Low-Frequency Oscillator) speed. By adjusting these specific settings, you can control the sweep range, intensity, and rate of the phasing effect to suit your audio processing needs.

To configure these parameters in FFmpeg, you apply the phaser audio filter using the -af flag. While you can pass arguments as a list of positional values, using named parameters is highly recommended for clarity and precision.

The three key parameters are configured as follows:

1. Delay (delay)

The delay parameter sets the delay in milliseconds for the phasing effect. This setting determines the frequency range where the phase cancellation (the “notches” in the sound) occurs. * Parameter Name: delay * Valid Range: 0.0 to 5.0 milliseconds * Default Value: 3.0 * Usage: Lower values produce a brighter, higher-frequency sweep, while higher values shift the effect toward lower, warmer frequencies.

2. Feedback (decay)

The decay parameter controls the amount of output signal fed back into the filter’s input, which dictates the sharpness and resonance of the phasing effect. In traditional synthesizer terms, this is the feedback control. * Parameter Name: decay * Valid Range: 0.0 to 0.99 * Default Value: 0.4 * Usage: A value close to 0.0 results in a very subtle, almost unnoticeable sweep. A value closer to 0.99 creates a highly resonant, metallic, and dramatic sweeping sound.

3. LFO Rate (speed)

The speed parameter defines the frequency of the modulation in Hertz (Hz). This determines how fast the phaser sweeps back and forth through the audio spectrum. * Parameter Name: speed * Valid Range: 0.1 to 2.0 Hz * Default Value: 0.5 * Usage: Lower values (e.g., 0.1 to 0.3) create a slow, spacey, evolving sweep. Higher values (e.g., 1.5 to 2.0) produce a rapid, warbling modulation.

Command Examples

Example 1: Subtle and Slow Phase Effect To create a gentle, slow-moving phaser effect, use a shorter delay, low decay, and a slow LFO speed:

ffmpeg -i input.mp3 -af "phaser=delay=2.0:decay=0.3:speed=0.2" output.mp3

Example 2: Intense and Rapid Warble To achieve a highly resonant, fast-sweeping dramatic effect, increase the delay, decay, and speed:

ffmpeg -i input.mp3 -af "phaser=delay=4.5:decay=0.85:speed=1.8" output.mp3

Example 3: Standard Setting with Sinusoidal LFO By default, the phaser uses a triangular wave shape. You can change the LFO wave type to sinusoidal (s) for a smoother sweep transition, alongside your custom delay, feedback, and speed:

ffmpeg -i input.mp3 -af "phaser=delay=3.5:decay=0.6:speed=0.8:type=s" output.mp3