PureMP3

A minimal macOS app for honest, high-quality MP3 conversion.

PureMP3 is a small macOS app for converting video and audio files to MP3 without pretending that lossy audio has magic compression rules.

It exists because converting a video to a clean MP3 should not require remembering FFmpeg flags, opening a heavyweight encoder, or pretending that a 320 kbps MP3 can be magically compressed while staying truly 320 kbps.

Drop files in. Pick a quality preset. Convert.

Demo

The Liquid Glass direction is inspired by Lucas Romero's CSS/SVG experiment, liquid-glass-effect-macos, translated here into native SwiftUI surfaces that still build on the current macOS 14 toolchain.

Highlights

• Drop audio or video files into a focused conversion queue
• Pick real LAME presets without digging through FFmpeg docs
• Use VBR Best, VBR Balanced, 320 kbps, 256 kbps, or 192 kbps
• Switch between full-window Glass and matte-black OLED display modes
• See the exact command PureMP3 will run
• Get warned before re-encoding an already lossy MP3
• Bundle FFmpeg and ffprobe inside the app for plug and play releases
• Keep the conversion policy in a tested Swift core module
• Ships with a custom generated macOS app icon

Why

MP3 file size is mostly:

duration x bitrate

If you keep real 320 kbps CBR MP3 quality, you cannot meaningfully shrink the file without changing something important:

• lower the bitrate
• use high-quality VBR
• change format
• re-encode and lose more quality

PureMP3 makes those tradeoffs explicit.

What It Does

• Converts video and audio files to MP3
• Supports high-quality LAME presets
• Shows the exact FFmpeg command
• Warns when the source is already MP3
• Keeps the interface intentionally small
• Uses a testable Swift core instead of hiding behavior inside the UI

Interface

PureMP3 is designed to stay out of the way:

• the queue is the product, not a settings maze
• quality choices are visible before conversion starts
• warnings are attached to the file they affect
• the FFmpeg command is visible instead of hidden behind a black box
• glass is used for hierarchy, depth, and interaction rather than decoration alone
• Glass mode uses full-window translucent surfaces with bright rim highlights
• OLED mode uses a matte-black surface system with lower glow and sharper contrast

Quality Presets

Preset	FFmpeg settings	Use when
VBR Best	-codec:a libmp3lame -q:a 0	You want excellent quality and smaller files than fixed 320 kbps
VBR Balanced	-codec:a libmp3lame -q:a 2	You want the best practical default
320 kbps	-codec:a libmp3lame -b:a 320k	You need maximum fixed MP3 bitrate
256 kbps	-codec:a libmp3lame -b:a 256k	You want very good music quality with smaller files
192 kbps	-codec:a libmp3lame -b:a 192k	You want a good general-purpose output size

The Rule PureMP3 Will Not Break

PureMP3 will not claim fake compression.

This is wrong:

ffmpeg -i myfile.mp3 -b:a 320k smaller.mp3

That re-encodes an already lossy MP3 into another MP3. It can lose quality, but it cannot restore quality or create a meaningfully smaller true 320 kbps file.

This is usually the better choice:

ffmpeg -i myfile.mp4 -vn -codec:a libmp3lame -q:a 2 myfile.mp3

Plug And Play FFmpeg

PureMP3 is built to ship with FFmpeg and ffprobe inside the app bundle, so normal users do not need Homebrew, Terminal, or a separate FFmpeg install.

Release app layout:

PureMP3.app/Contents/Resources/FFmpeg/bin/ffmpeg
PureMP3.app/Contents/Resources/FFmpeg/bin/ffprobe

Developer builds also support:

• PUREMP3_FFMPEG_DIR
• /opt/homebrew/bin
• /usr/local/bin
• /usr/bin

For release packaging, see Docs/DISTRIBUTION.md.

Build

git clone https://github.com/peterdsp/PureMP3.git
cd PureMP3
swift build
swift run PureMP3

To build a local .app bundle with the icon and bundled FFmpeg lookup:

script/download_ffmpeg_macos_arm64.sh
script/build_and_run.sh --verify

To create a local zipped app archive:

script/package_release.sh

Test

swift test

Architecture

PureMP3 is split into two layers:

PureMP3
+-- Sources
|   +-- PureMP3App
|   |   +-- SwiftUI views
|   |   +-- app state
|   |   +-- shell FFmpeg client
|   +-- PureMP3Core
|       +-- presets
|       +-- command building
|       +-- ffprobe parsing
|       +-- size estimation
+-- Tests
    +-- PureMP3CoreTests

The rule is simple: conversion policy belongs in PureMP3Core. The app can change shape, but the audio behavior stays tested.

Roadmap

• Real captured screenshots for each release
• Progress parsing from FFmpeg stderr
• Drag-to-reorder queue
• Conversion cancellation
• Metadata and album art preservation
• Opus, AAC, FLAC, and WAV outputs
• Homebrew cask
• Signed releases
• Localized interface
• Release pipeline for signed app bundles with bundled FFmpeg

Contributing

Contributions are welcome if they keep the app honest, small, and useful.

Good contributions:

• improve conversion correctness
• add tests around command generation
• make the UI clearer without adding clutter
• improve accessibility
• document real audio tradeoffs

Avoid:

• fake quality claims
• growth into a generic video editor
• hidden re-encoding behavior
• adding dependencies without a strong reason

License

PureMP3 is MIT licensed.

Bundled FFmpeg binaries keep their own license. FFmpeg licensing depends on the build configuration, so releases must include the correct FFmpeg notices, source links, and LGPL or GPL obligations.