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diff --git a/site/posts/2025-06-26-ffmpeg-audio-cleanup.md b/site/posts/2025-06-26-ffmpeg-audio-cleanup.md
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+---
+title: FFmpeg audio cleanup
+description: A script to apply dynamic compression and noise reduction to audio files
+date: 2025-06-26
+slug: ffmpeg-audio-cleanup
+tags:
+  - technical
+  - ffmpeg
+---
+
+I recently needed to process 20+ phone audio recordings. The files are mp3 recordings in stereo, made in an environment with echoes and noise from fans/heaters.
+
+Although I could do it easily with [Tenacity](https://tenacityaudio.org/) I didn't want to use a manual process, since it would take days. So I tried using FFmpeg filters and Bash scripting.
+
+I found an FFmpeg filter called [compand](https://ffmpeg.org/ffmpeg-filters.html#compand) which lets you map an input decibel range to an output decibel range. I also used the [anlmdn](https://ffmpeg.org/ffmpeg-filters.html#anlmdn) filter to reduce noise, and the [highpass](https://ffmpeg.org/ffmpeg-filters.html#highpass) filter to help with clarity.
+
+I ran into a couple gotchas.
+
+1. `mpv` does something special for audio playback that prevents audio from clipping. `vlc` plays the file as it is.
+2. Because the compressor has an attack and decay (which is necessary for things to sound good) it can cause clipping if the volume rises sharply. Applying a `delay` parameter with half the duration of the attack length fixed this.
+
+Here is the script:
+
+```bash
+#!/bin/bash
+if [ "$#" == "0" ]; then
+  echo "Error: no arguments provided."
+  echo "Usage: $0 file1 file2 file3 ..."
+  echo "   or: $0 *.ext"
+  exit 1
+fi
+
+trap "exit" INT
+
+while [ "$#" != "0" ]; do
+  base="${1%%.*}"
+  ext="${1##*.}"
+  outfile="./normalized--$base.$ext"
+  if [ ! -f "$outfile" ]; then
+    echo "Processing $1"
+    ffmpeg -i "$1" -v warning -ac 1 -af "compand=attacks=0.3:decays=0.3:delay=0.15:points=-80/-300|-45/-25|-27/-15|0/-12|20/-12,anlmdn=s=10,highpass=f=500" -threads 4 "$outfile"
+  else
+    echo "Skipping $1, already processed."
+  fi
+  shift
+done
+```
+
+If this is useful to you please leave a comment or send an email, I would love to hear about it.
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