Loop Audio Segments with FFmpeg aloop Filter
This article explains how to use the aloop audio filter
in FFmpeg to loop a specific sub-segment of an audio stream. You will
learn the exact syntax of the filter, how to calculate the required
parameters based on your audio’s sample rate, and how to execute the
command to repeat a precise section of an audio file.
Understanding the
aloop Filter Parameters
The aloop filter loops a segment of audio by storing it
in a buffer. To use it, you must define the loop count, the duration of
the loop, and the starting point. Because the filter operates on audio
samples rather than time (seconds), you must calculate these values
based on your audio’s sample rate (usually 44,100 Hz or 48,000 Hz).
The three primary parameters for aloop are: *
loop: The number of times to loop the
segment. Set this to -1 for infinite loops, 0
to disable looping, or any positive integer (e.g., 3 to
repeat the segment three times). * size:
The length of the audio segment to loop, measured in
samples. * start: The
starting position of the segment you want to loop, measured in
samples.
Step-by-Step Calculation
To target a specific time range, use this formula to convert seconds into samples: \[\text{Samples} = \text{Time in seconds} \times \text{Sample Rate}\]
For example, if your audio file has a sample rate of 44,100 Hz, and you want to loop a 3-second segment that starts at the 2-second mark:
- Calculate the Start (
start): 2 seconds × 44,100 samples/sec = 88,200 samples - Calculate the Segment Length (
size): 3 seconds × 44,100 samples/sec = 132,300 samples
Example Command
To loop the 3-second segment (from second 2 to 5) a total of 5 times using the calculated sample values, run the following FFmpeg command:
ffmpeg -i input.wav -filter_complex "aloop=loop=5:size=132300:start=88200" output.wavTips for Precise Looping
Determine the Sample Rate: If you do not know the sample rate of your input file, find it using
ffprobe:ffprobe -v error -select_streams a:0 -show_entries stream=sample_rate -of default=noprint_wrappers=1input.wavAudio Length: Note that the output duration will automatically expand to accommodate the newly added loops.