How to Downmix 7.1 to 5.1 Audio with FFmpeg

This article provides a quick guide on how to downmix a 7.1 surround sound audio track to a 5.1 channel layout using FFmpeg. You will learn the exact command-line syntax to perform this conversion efficiently while preserving audio quality and copying the video stream without re-encoding.

The Basic Downmix Command

The simplest and most common way to downmix audio in FFmpeg is by using the -ac (audio channels) option. Setting this to 6 tells FFmpeg to output 5.1 audio (which consists of 6 channels: Front Left, Front Right, Center, LFE, Surround Left, and Surround Right).

Run the following command in your terminal:

ffmpeg -i input.mkv -c:v copy -c:a aac -ac 6 output.mkv

Command Breakdown: * -i input.mkv: Specifies your input video file containing the 7.1 audio. * -c:v copy: Copies the video stream directly without re-encoding it, saving time and preserving original video quality. * -c:a aac: Encodes the output audio to the AAC format (you can change this to ac3, eac3, or flac depending on your needs). * -ac 6: Downmixes the 8 channels of the 7.1 audio into the 6 channels of 5.1 audio. * output.mkv: The name of your output file.

Advanced Downmix Using the Pan Filter

If you want precise control over how the 7.1 channels (which include side and back speakers) are mapped and mixed into the 5.1 channels, you can use FFmpeg’s pan audio filter. This prevents volume loss and ensures the back channels are correctly merged into the surround channels.

Run the following command:

ffmpeg -i input.mkv -c:v copy -af "pan=5.1|FL=FL|FR=FR|FC=FC|LFE=LFE|SL=0.5*SL+0.5*BL|SR=0.5*SR+0.5*BR" -c:a aac output.mkv

How the Pan Filter Works here: * FL=FL / FR=FR / FC=FC / LFE=LFE: Keeps the Front Left, Front Right, Center, and Subwoofer channels exactly as they are. * SL=0.5*SL+0.5*BL: Mixes 50% of the Side Left (SL) and 50% of the Back Left (BL) channels into the final Surround Left channel. * SR=0.5*SR+0.5*BR: Mixes 50% of the Side Right (SR) and 50% of the Back Right (BR) channels into the final Surround Right channel.