Configure Loop Start Sample in FFmpeg Aloop

This article explains how to configure the loop start sample using the FFmpeg aloop filter. You will learn the correct syntax, how to calculate the start sample based on your audio’s sample rate, and how to apply these settings in a practical command-line example to control exactly where your audio loop begins.

The aloop filter in FFmpeg allows you to loop a segment of an audio stream. To control where the loop begins, you must configure the start parameter alongside the loop and size parameters.

The Aloop Syntax

The basic syntax for the aloop filter is:

aloop=loop=<count>:size=<samples>:start=<sample_number>

• loop: The number of times to loop the audio. Set to -1 for infinite looping, or 0 to disable looping.
• size: The maximum size of the loop in samples. This defines the duration of the audio segment that will be looped.
• start: The specific sample number where the loop should begin.

How to Calculate the Start Sample

Because FFmpeg measures the start parameter in audio samples rather than seconds, you must calculate the sample number using your audio’s sample rate. The formula is:

\[\text{Start Sample} = \text{Target Start Time (seconds)} \times \text{Audio Sample Rate (Hz)}\]

For example, if your audio has a sample rate of 44,100 Hz (44.1 kHz) and you want the loop to start at exactly 3 seconds into the audio:

\[\text{Start Sample} = 3 \times 44100 = 132300\]

Practical Example

To loop a segment of audio starting at 2 seconds, with a loop duration of 5 seconds (220,500 samples at 44.1 kHz), repeating 3 times, use the following command:

ffmpeg -i input.mp3 -af "aloop=loop=3:size=220500:start=88200" output.wav

In this command: * start=88200 tells FFmpeg to begin the loop at the 2-second mark (\(2 \times 44100\)). * size=220500 sets the loop duration to 5 seconds (\(5 \times 44100\)). * loop=3 repeats this 5-second segment 3 times before continuing with the rest of the audio.