Split Audio into Multiple Streams with FFmpeg asplit

This article provides a straightforward guide on how to use the asplit filter in FFmpeg to duplicate a single audio stream into multiple identical streams. You will learn the basic syntax, see practical command-line examples for splitting audio, and understand how to route these split streams to different outputs or subsequent filters.

Understanding the asplit Filter

The asplit filter is the audio equivalent of the video split filter. It takes a single audio input stream and duplicates it into a specified number of identical output streams. This is particularly useful when you need to apply different audio effects to the same source simultaneously or output the same audio to multiple files or formats.

Basic Syntax

The basic syntax for the asplit filter within a filtergraph is:

[input_stream] asplit=N [output_stream_1][output_stream_2]...[output_stream_N]

• [input_stream]: The input audio stream label (e.g., [0:a]).
• asplit=N: The filter name and the number of output streams (N) you want to create. If you omit =N, FFmpeg defaults to 2.
• [output_stream_1]...: The unique labels you assign to each generated output stream.

Practical Examples

Example 1: Splitting Audio into Two Separate Files

If you want to split a single audio file into two identical streams and save them as separate files, use the following command:

ffmpeg -i input.mp3 -filter_complex "[0:a]asplit=2[out1][out2]" -map "[out1]" output1.mp3 -map "[out2]" output2.mp3

How it works: * -i input.mp3: Imports the source audio file. * -filter_complex: Initiates the complex filtergraph. * [0:a]asplit=2[out1][out2]: Takes the first audio stream ([0:a]), duplicates it into two streams, and labels them [out1] and [out2]. * -map "[out1]" output1.mp3: Directs the first split stream to output1.mp3. * -map "[out2]" output2.mp3: Directs the second split stream to output2.mp3.

Example 2: Splitting and Applying Different Filters

A common use case is splitting an audio stream, applying different filters to each clone, and then merging them or saving them separately.

The command below splits an audio stream, applies a highpass filter to the first stream, a lowpass filter to the second stream, and saves them into a single output file with multiple tracks:

ffmpeg -i input.wav -filter_complex "[0:a]asplit=2[a1][a2]; [a1]highpass=f=200[high]; [a2]lowpass=f=3000[low]" -map "[high]" high_output.wav -map "[low]" low_output.wav

How it works: * The filtergraph splits the input audio into [a1] and [a2]. * [a1]highpass=f=200[high] applies a highpass filter (cutting frequencies below 200 Hz) to the first split stream. * [a2]lowpass=f=3000[low] applies a lowpass filter (cutting frequencies above 3000 Hz) to the second split stream. * The final outputs are mapped to their respective destination files.