convert-audio-mono

disable-model-invocation	true
name	convert-audio-mono
description	Use when the user wants to convert a video or audio file's stereo audio to mono, or fix audio that only plays in one ear.

Convert Audio to Mono

Overview

This skill converts the stereo audio channel of a media file (typically video or audio) to mono. It optimizes the process by copying the video stream without re-encoding to preserve video quality, and handles silent channels by extracting the active channel rather than a simple downmix.

When to Use

Use when a video or audio file's sound plays in only one ear/channel, or when a mono mix is requested.

Core Pattern

1. Analyze Channel Volumes: Run ffmpeg -i <input> -filter_complex "[0:a]astats=metadata=1" -f null - to identify the peak and RMS volume of each channel. Note: Do not use reset=1 in the filter, as it resets stats per frame.
2. Determine Extraction Strategy:
   • If one channel is silent (volume ~ -90 dB or less) and the other has sound, isolate the active channel to avoid a 6 dB volume drop from normal averaging:
     • Left channel only: -af "pan=mono|c0=c0" (or FL)
     • Right channel only: -af "pan=mono|c0=c1" (or FR)
   • If both channels have active sound, use standard downmixing:
     • -ac 1
3. Execute Conversion: Run ffmpeg, copying the video stream to save time and preserve quality: ffmpeg -i <input> -c:v copy -af "<filter>" -c:a aac -b:a 256k <output>

Common Mistakes

• Re-encoding video: Re-encoding the video stream degrades quality and takes significant time. Always use -c:v copy.
• Averaging silent channels: A simple -ac 1 downmix of a one-eared track averages it with silence, dropping the overall volume by 6 dB. Always analyze first and use the pan filter for single-channel isolation.