How to Use FFmpeg aloop Filter to Repeat Audio

This article explains how to use the FFmpeg aloop filter to repeat a specific segment of an audio file. You will learn the key parameters of the filter, how to calculate the required sample values based on your audio’s sample rate, and how to structure the command-line syntax for seamless audio looping.

Understanding the aloop Filter

The aloop filter works by buffering a specific segment of audio and repeating it a set number of times. Unlike video looping, aloop operates using audio samples rather than timecodes (seconds).

To use the filter, you need to configure three primary parameters: * loop: The number of times to repeat the segment. Set this to -1 for infinite loops, or a specific integer (e.g., 3 for three loops). * size: The length of the audio segment to loop, measured in samples. * start: The starting position of the loop, measured in samples.

Step 1: Calculate Samples from Time

Because aloop requires sample counts, you must convert your target times (in seconds) into samples using the audio’s sample rate.

\[\text{Samples} = \text{Time in seconds} \times \text{Sample Rate (Hz)}\]

Common sample rates are 44,100 Hz (CD quality) or 48,000 Hz (video/professional audio). You can find your file’s sample rate by running:

ffprobe -v error -select_streams a:0 -show_entries stream=sample_rate -of default=noprint_wrappers=1 input.mp3

Calculation Example

Assume your audio file has a sample rate of 48,000 Hz, and you want to loop a 3-second segment that starts at the 5-second mark:

• Start Sample (start): 5 seconds × 48,000 = 240,000
• Loop Size (size): 3 seconds × 48,000 = 144,000

Step 2: Write the FFmpeg Command

Once you have calculated the values, insert them into the aloop filter graph.

Basic Syntax

ffmpeg -i input.mp3 -filter_complex "aloop=loop=3:size=144000:start=240000" output.mp3

In this command: * -i input.mp3: Specifies the input audio file. * -filter_complex: Invokes the filtergraph. * loop=3: Repeats the target segment 3 times. * size=144000: Loops a segment duration of 144,000 samples (3 seconds). * start=240000: Starts the loop at sample 240,000 (5-second mark).

Advanced Usage: Infinite Looping with a Limit

If you set loop=-1 for an infinite loop, the output generation will run indefinitely. To prevent this, you must specify a maximum output duration or limit the output frames using the -t (duration) or -to option.

For example, to loop the segment infinitely but cut the final output file off at 30 seconds:

ffmpeg -i input.mp3 -filter_complex "aloop=loop=-1:size=144000:start=240000" -t 30 output.mp3