Compress Old Audio Files

August 27, 2022

We the pass of the years one keeps storing files, music in my case.

A few thousands.

But technology improved in this sense and new encoders exist that can compress (loosely) the same audio with the same quality but at a much smaller bit rate, and therefore, resulting in much smaller files.

Quick and dirty script follows!

Encoders & containers

The current documentation of FFmpeg gives a ranking of the encoders available, from best (left) to not so good (right):

libopus > libvorbis >= libfdk_aac > libmp3lame >= eac3/ac3 > aac > libtwolame > vorbis > mp2 > wmav2/wmav1

So the idea is to re-encode an arbitrary audio file to libopus:

ffmpeg -i $fn -acodec libopus $name

For the output file ffmpeg can save the audio into a container. Different container files exists (.m4a, .mp4, .ogg, .flv) but today’s internet stream platforms use .webm.

⊕ You could write directly .opus, the raw encoded audio, without a container and get even smaller files.

I’ll go with that.

Bit rates

One of the things of better encoders is that we can use a lower bit rate without losing quality, as long as we are in the expected numbers for which the encoder was designed.

For libopus we need bit rates of 32kbps or larger.

Doing a quick, totally subjective test, ~50k is the smallest bit rate that maintains the same quality of my files (which range between 96k and 128k MP3).

ffmpeg -i $fn -acodec libopus -b:a 50k $name.webm

Final tweak, I have some videos that I only care their audio. We can instruct ffmpeg to get those with the “no video” option (-nv).

Scripting

The compress_audio.sh script follows:

#!/bin/bash

set -euo pipefail
shopt -s nocaseglob

if [ "$#" != "1" ]; then
    echo "Usage $0 <target dir>"
    exit 1
fi

cd "$1"
echo  "Processing $(pwd)..."

IFS=$'\n'
for fn in $(ls *.mp3 *.ogg *.wav *.wma *.m4a *.avi *.opus); do
    name=$(basename -- "$fn")
    ext="${name##*.}"
    name="${name%.*}"

    # Take the input (-i) file, ignore any video stream (-vn)
    # and change its audio codec (-acodec) tto the codec libopus
    # and change its audio bitrate (-b:a) to 50k
    # and save it under the same name but with .webm extension
    ffmpeg -i "$fn" -vn -acodec libopus  -b:a 50k "$name.webm"

    rm "$fn"
done

The script processes the files inside a folder. Any error and the processing will stop and fail so the input file will not be deleted (rm "$fn").

The script does not work recursively, instead I use find.

find . -type d -exec compress_audio.sh {} \;

This is on purpose: if compress_audio.sh fails, that folder is skipped (no file is further processed or removed) but the whole recursive scan keeps on (find will not stop on an error).

Results

The ~3k audio files with a total size of ~20GB was processed in ~2hs with a final total size of ~7GB, a ~65% reduction. Not bad.

That only counted for audio files. Extracting the audio from videos and deleting the videos additionally saved a few hundred of megabytes more.

Related tags: misc, ffmpeg