Convert M4V to AU — Free Online Tool
Convert M4V video files to AU audio format, extracting the audio stream and encoding it as 16-bit big-endian PCM — the native Sun AU format used on Unix systems. This tool strips the video entirely and produces a raw, uncompressed audio file compatible with legacy Unix applications and environments that expect the classic .au format.
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FFmpeg Command
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Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
M4V files typically carry AAC-encoded audio inside an MPEG-4 container, often alongside H.264 or H.265 video. During this conversion, FFmpeg discards the video stream entirely and decodes the AAC audio, then re-encodes it as 16-bit big-endian PCM (pcm_s16be) inside a Sun AU container. This is a full audio transcode — the compressed AAC data is decoded to raw PCM samples, which means the AU file will be significantly larger than the original audio track in the M4V but is fully uncompressed. The Sun AU format uses a simple fixed header structure with no support for metadata, chapters, or multiple tracks, so only the first audio stream from the M4V is carried over.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool. In the browser version this runs via FFmpeg.wasm compiled to WebAssembly, but the command is identical to what you would run in a terminal on your desktop. |
-i input.m4v
|
Specifies the input file — an M4V container that typically holds H.264 or H.265 video alongside AAC audio, as used by iTunes and iOS devices. |
-c:a pcm_s16be
|
Sets the audio codec to 16-bit signed big-endian PCM, which is the native and most compatible encoding for the Sun AU format. This decodes the compressed AAC audio from the M4V and re-encodes it as raw uncompressed PCM samples in big-endian byte order. |
output.au
|
Defines the output filename with the .au extension. FFmpeg uses this extension to automatically select the Sun AU container format, which wraps the pcm_s16be audio stream in AU's minimal fixed header structure. No video stream is written since AU is an audio-only format. |
Common Use Cases
- Extracting audio from iTunes-downloaded M4V content to feed into legacy Unix audio processing pipelines that accept .au files
- Preparing audio from iOS-compatible M4V videos for use with classic Unix tools like SoX or older Sun workstation software that reads the AU format natively
- Converting M4V interview recordings or lectures into uncompressed AU files for archival or analysis in academic Unix environments
- Stripping DRM-free M4V video to a flat PCM audio file for ingestion into audio editors or signal processing tools that prefer headerless-style raw formats
- Generating .au audio samples from M4V clips for embedding in older Java applets or web pages that used Sun AU as their audio format in the early internet era
Frequently Asked Questions
There is a one-way quality loss introduced at the decoding step. The AAC audio in your M4V is a lossy compressed format, and decoding it to PCM does not recover the audio data that was discarded during original AAC encoding. However, once decoded into the AU file as pcm_s16be, no further lossy compression is applied — the PCM data is an exact uncompressed representation of the decoded signal. Think of it as freezing the audio at its current fidelity rather than compressing it further.
AAC audio in M4V files is heavily compressed, typically at 128–256 kbps. The AU format stores audio as raw 16-bit PCM, which for stereo audio at 44.1 kHz works out to roughly 10 MB per minute — far larger than compressed AAC. This size increase is expected and is simply the cost of uncompressed audio storage. The AU file contains no video, so it won't include any of the M4V's visual content.
No. The Sun AU format has an extremely minimal header that stores only basic technical parameters like sample rate, bit depth, and channel count. It has no provision for title, artist, chapter markers, or any other metadata that your M4V may carry. All of that information is dropped during conversion. If preserving metadata is important, consider converting to a more capable audio format instead.
FFmpeg will select only the first (default) audio track from the M4V for conversion, since the AU format supports only a single audio stream. If your M4V contains multiple language tracks or a separate commentary track, those will not be included in the output. You can target a specific audio track by adding '-map 0:a:1' (for the second track, zero-indexed) to the FFmpeg command before the output filename if you need a different track.
To change the sample rate, add '-ar 22050' (or any target rate) before the output filename — for example, 'ffmpeg -i input.m4v -c:a pcm_s16be -ar 22050 output.au'. To use 8-bit unsigned PCM instead, replace '-c:a pcm_s16be' with '-c:a pcm_u8'. The AU format supports several PCM variants including pcm_s8, pcm_u8, pcm_alaw, and pcm_mulaw, which can reduce file size at the cost of dynamic range or fidelity.
Yes. On Linux or macOS you can run a shell loop: 'for f in *.m4v; do ffmpeg -i "$f" -c:a pcm_s16be "${f%.m4v}.au"; done'. On Windows Command Prompt, use: 'for %f in (*.m4v) do ffmpeg -i "%f" -c:a pcm_s16be "%~nf.au"'. This applies the same pcm_s16be encoding to each M4V file in the current directory and outputs a matching .au file for each.
Technical Notes
The Sun AU format (.au) is one of the oldest digital audio formats still in use, developed by Sun Microsystems for their Unix workstations. It uses a simple 28-byte minimum header followed by raw audio data, with no support for compressed modern codecs — all data must be PCM or G.711 variants (A-law/mu-law). The default codec in this conversion, pcm_s16be, stores 16-bit signed samples in big-endian byte order, which reflects AU's Unix/SPARC heritage where big-endian architectures dominated. M4V files may also contain FairPlay DRM protection applied by Apple; FFmpeg cannot process DRM-protected content, so only DRM-free M4V files (common with personal recordings or DRM-free purchases) will convert successfully. Since AU carries no container-level timing metadata, the audio plays back at whatever sample rate was detected during conversion — ensuring your source M4V has a clean, standard sample rate (44100 Hz or 48000 Hz) will give the most compatible output. The pcm_s16be codec provides effectively lossless storage of the decoded audio signal with a theoretical dynamic range of about 96 dB.