Convert MP4 to AU — Free Online Tool
Convert MP4 video files to AU audio format, extracting the audio stream and encoding it as 16-bit big-endian PCM (pcm_s16be) — the native uncompressed format for Sun AU files. This tool is ideal for Unix/legacy system compatibility or workflows requiring raw, uncompressed audio extracted from video.
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FFmpeg Command
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Estimated output:
Conversion Complete!
DownloadHow It Works
During this conversion, FFmpeg strips the video stream entirely from the MP4 container and extracts only the audio. The audio — which is most commonly AAC or MP3 inside an MP4 — is fully decoded and then re-encoded as uncompressed 16-bit big-endian PCM audio, written into a Sun AU container. Unlike remuxing, this is a full transcode of the audio stream: the compressed MP4 audio is decompressed to raw PCM samples. The AU format uses a minimal fixed header followed by raw audio data, which is why it has no support for chapters, multiple tracks, or subtitles that may have existed in the source MP4.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg command-line tool, which handles all media reading, stream decoding, encoding, and container writing for this conversion. |
-i input.mp4
|
Specifies the input MP4 file. FFmpeg reads the MP4 container, identifying the video and audio streams — typically H.264 video and AAC audio — and makes them available for processing. |
-c:a pcm_s16be
|
Sets the audio codec to pcm_s16be (signed 16-bit big-endian PCM), which fully decodes the compressed MP4 audio (e.g., AAC) into raw uncompressed samples stored in the byte order native to the Sun AU format. |
output.au
|
Defines the output filename and tells FFmpeg to write a Sun AU container. FFmpeg infers the AU format from the .au extension, automatically discarding the video stream since AU is an audio-only format. |
Common Use Cases
- Preparing audio extracted from MP4 video files for playback or processing on legacy Unix and Solaris workstations that natively support the AU format
- Supplying uncompressed audio samples from video recordings to older Java applications, which historically used AU as a default supported audio format via the Java Sound API
- Extracting a lossless PCM representation of an MP4's audio track for use in audio analysis pipelines that require raw, uncompressed sample data without any codec artifacts
- Converting MP4 narration or dialogue recordings to AU for ingestion into legacy telephony or IVR systems originally designed around Sun Microsystems infrastructure
- Archiving the audio component of MP4 files in an extremely simple, headerless-adjacent format that remains readable without modern codec libraries
Frequently Asked Questions
The conversion involves one generation of decoding loss if your MP4's audio is compressed (e.g., AAC or MP3), since that compressed audio must be fully decoded before being written as PCM into the AU file. However, the resulting AU file itself is uncompressed PCM, so no further quality loss occurs and the AU file will be a lossless snapshot of what the decoder produced. If your original MP4 audio was encoded at a high bitrate, the perceptible quality difference is typically minimal.
MP4 files typically store audio as AAC, which is a highly compressed lossy codec achieving good quality at 128–256 kbps. AU with pcm_s16be stores every audio sample as raw, uncompressed 16-bit data, which at 44.1 kHz stereo runs approximately 1.4 MB per second. A 10-minute MP4 with AAC audio around 10 MB could produce an AU file exceeding 100 MB — this is expected and is the nature of uncompressed PCM storage.
No. The Sun AU format has an extremely minimal header that stores only technical parameters: the data offset, data size, encoding type, sample rate, and channel count. It has no standardized fields for ID3-style metadata such as title, artist, album, or chapter markers. Any metadata present in the source MP4 will be lost in the output AU file.
Yes. The FFmpeg command uses pcm_s16be by default, but the AU format supports several other encodings including pcm_mulaw (G.711 mu-law, common in telephony), pcm_alaw (G.711 A-law), pcm_s8, and pcm_u8. To use mu-law, change the command to: ffmpeg -i input.mp4 -c:a pcm_mulaw output.au. Mu-law and A-law variants produce much smaller files but at significantly reduced audio fidelity, and are typically chosen only for telephony compatibility.
On Linux or macOS, you can run a shell loop: for f in *.mp4; do ffmpeg -i "$f" -c:a pcm_s16be "${f%.mp4}.au"; done. On Windows Command Prompt, use: for %f in (*.mp4) do ffmpeg -i "%f" -c:a pcm_s16be "%~nf.au". This applies the same pcm_s16be encoding to every MP4 in the current directory and saves each result as a corresponding AU file.
The video stream is automatically discarded by FFmpeg when writing to an AU output file, because the AU format is audio-only and has no mechanism to store video data. You do not need to explicitly pass a flag like -vn, as FFmpeg recognizes that the AU container cannot contain video and omits it. Only the first audio stream from the MP4 is written to the output.
Technical Notes
The Sun AU format (.au) originated with Sun Microsystems' NeWS and SunOS operating systems and was one of the earliest digital audio formats used on Unix workstations and across early internet browsers. Its structure is intentionally minimal: a 6-field header (magic number, data offset, data size, encoding ID, sample rate, channels) followed by raw audio data, making it trivial to parse without a media library. The default encoding produced by this tool — pcm_s16be — stores samples as signed 16-bit integers in big-endian byte order, which reflects the big-endian architecture of SPARC processors where AU originated. This byte order is the opposite of the little-endian PCM used in WAV files, so direct binary comparison between AU and WAV PCM data will appear byte-swapped. The AU format supports only a single audio track; multi-track or multi-language audio from an MP4 will be reduced to the default audio stream. Sample rates are stored as a 32-bit integer in the header, so virtually any sample rate from the source MP4 is preserved accurately. There is no support for floating-point PCM within standard AU encoding IDs, unlike formats such as WAV or AIFF.