Configure FFmpeg Concat Filter Inputs and Streams

This article explains how to configure the input count and stream types using the FFmpeg concat filter. You will learn how to use the n, v, and a parameters to define the number of input segments, video streams, and audio streams, allowing you to seamlessly merge multiple media files into a single output.

To configure the concat filter in FFmpeg, you must define its parameters inside a -filter_complex graph using the syntax concat=n=X:v=Y:a=Z. These three parameters dictate how FFmpeg processes your input files:

• n (Segment Count): Specifies the number of input segments to join. The default value is 2.
• v (Video Streams): Specifies the number of output video streams per segment. The default value is 1.
• a (Audio Streams): Specifies the number of output audio streams per segment. The default value is 0.

Understanding the Input Mapping

The concat filter requires you to explicitly feed it the correct number of input streams based on your configuration. The total number of input streams you must pass into the filter is calculated as:
Total Inputs = n × (v + a)

The inputs must be ordered sequentially by segment. For each segment, you must specify the video stream(s) first, followed by the audio stream(s).

Example 1: Concatenating Video and Audio (1 Video, 1 Audio)

If you want to merge three separate video files, each containing one video stream and one audio stream, your parameters will be n=3, v=1, and a=1.

The total number of input streams needed is 3 × (1 + 1) = 6.

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex \
"[0:v][0:a][1:v][1:a][2:v][2:a]concat=n=3:v=1:a=1[outv][outa]" \
-map "[outv]" -map "[outa]" output.mp4

In this command: * [0:v][0:a] represent the video and audio of the first file. * [1:v][1:a] represent the video and audio of the second file. * [2:v][2:a] represent the video and audio of the third file. * [outv] and [outa] are the temporary labels assigned to the concatenated video and audio outputs, which are then mapped to the final output file.

Example 2: Concatenating Video Only (No Audio)

If you are merging two video files that do not have audio, set a=0. Your parameters will be n=2, v=1, and a=0.

The total number of input streams needed is 2 × (1 + 0) = 2.

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex \
"[0:v][1:v]concat=n=2:v=1:a=0[outv]" \
-map "[outv]" output.mp4

Important Requirements

When using the concat filter, all input segments must share identical attributes. If the attributes do not match, the filter may fail or produce corrupted output. Ensure your inputs have the same: * Video resolution (width and height) * Frame rate (fps) * Pixel format (e.g., yuv420p) * Audio sample rate * Audio channel layout (e.g., stereo or mono)