PureMP3

A small macOS app for honest, high-quality audio conversion.

PureMP3 converts video and audio files to MP3 or lossless FLAC without hiding the tradeoffs behind vague compression claims.

Drop files into the queue, choose a real quality preset, pick an output folder, then convert. The app shows the FFmpeg command it will run and warns when a source is already a lossy MP3.

Current Design

PureMP3 is now a compact, fixed-size macOS window built around the conversion queue.

• The header keeps the product title, display mode switcher, Add files, and Convert actions visible.
• The left sidebar contains quality presets, output folder selection, and the bitrate truth note.
• The main panel is either a drop target empty state or the active conversion queue.
• Queue rows show input names, output names, duration when available, lossy MP3 warnings, and conversion status.
• The bottom command bar previews the exact FFmpeg command for the first queued file.
• The window supports two display modes: Glass and OLED.

The interface is intentionally narrow in scope. Quality, output, queue state, and command visibility stay on one screen.

Highlights

• Drag audio or video files into the app, or add them with the file picker
• Convert MP4, M4A, WAV, FLAC, and MP3 inputs to MP3 or lossless FLAC
• Use FLAC Lossless Ultra or LAME presets for VBR Best, VBR Balanced, 320 kbps, 256 kbps, and 192 kbps
• Switch between translucent Glass surfaces and a darker OLED mode
• Preview the FFmpeg command before conversion
• Warn before re-encoding an already lossy MP3
• Choose the output directory from the app
• Reveal completed output files in Finder
• Bundle FFmpeg and ffprobe inside release builds
• Keep conversion policy in a tested Swift core module

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.

Quality Presets

Preset	FFmpeg settings	Use when
Lossless Ultra	-vn -codec:a flac -compression_level 12	You want lossless FLAC output and accept larger files
VBR Best	-vn -codec:a libmp3lame -q:a 0	You want the highest LAME VBR quality, usually smaller than fixed 320 kbps
VBR Balanced	-vn -codec:a libmp3lame -q:a 2	You want the best practical default
320 kbps	-vn -codec:a libmp3lame -b:a 320k	You need the maximum fixed MP3 bitrate
256 kbps	-vn -codec:a libmp3lame -b:a 256k	You want very good music quality with smaller files
192 kbps	-vn -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

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.

For more detail, see Docs/ARCHITECTURE.md.

Roadmap

• 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 and notarized 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.