How to Use FFmpeg amerge Filter to Merge Audio Tracks

This article provides a straightforward guide on how to use the amerge filter in FFmpeg to combine multiple audio streams into a single multi-channel audio track. You will learn the basic command syntax, how to merge two or more audio inputs, how to handle mismatched sample rates, and how amerge differs from the amix filter.

Understanding the amerge Filter

The amerge filter merges two or more audio streams into a single multi-channel stream. For example, if you have two separate mono audio files, you can use amerge to combine them into a single stereo (2-channel) audio file, where one input becomes the left channel and the other becomes the right channel.

Basic Syntax for Merging Two Audio Tracks

To merge two separate audio files into a single stereo output, use the following command:

ffmpeg -i input1.wav -i input2.wav -filter_complex amerge=inputs=2 -ac 2 output.wav

• -i input1.wav -i input2.wav: Specifies the two input audio files.
• -filter_complex amerge=inputs=2: Calls the amerge filter and specifies that there are 2 input streams.
• -ac 2: Forces the output to have 2 channels (stereo).

Merging Specific Audio Streams from Multi-stream Files

If you are working with video files that contain multiple audio tracks and want to merge them, you must explicitly map the audio streams:

ffmpeg -i input.mp4 -filter_complex "[0:a:0][0:a:1]amerge=inputs=2[aout]" -map 0:v -map "[aout]" -c:v copy output.mp4

• [0:a:0][0:a:1]: Selects the first and second audio streams of the first input file.
• [aout]: Labels the merged audio output.
• -map 0:v: Copies the video stream from the input file without re-encoding.
• -map "[aout]": Maps the merged audio to the output file.

Merging More Than Two Streams

You can merge three or more streams by increasing the inputs parameter and specifying the input labels accordingly:

ffmpeg -i in1.wav -i in2.wav -i in3.wav -i in4.wav -filter_complex "[0:a][1:a][2:a][3:a]amerge=inputs=4[aout]" -map "[aout]" output.wav

Handling Mismatched Sample Rates

The amerge filter requires all input streams to have the exact same sample rate (e.g., 44100 Hz or 48000 Hz). If the sample rates differ, the command will fail. You can resolve this by adding the aresample filter to synchronize the sample rates before merging:

ffmpeg -i input1.wav -i input2.wav -filter_complex "[0:a]aresample=async=1[a1];[1:a]aresample=async=1[a2];[a1][a2]amerge=inputs=2" output.wav

The Difference Between amerge and amix

It is important not to confuse amerge with amix: * amerge: Joins streams side-by-side into separate channels (e.g., merging two Mono tracks to create one Stereo track). * amix: Overlays/mixes the audio together into the same channels (e.g., mixing background music with a voiceover so both are heard simultaneously on both the left and right speakers).