Y4M to AU Converter
FileFlip converts Y4M to AU entirely inside your browser, using FFmpeg compiled to WebAssembly. The file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. That means no file-size limit, no queue and no account.
How do I convert a Y4M file to AU?#
- Drop your Y4M file onto the converter at the top of this page, or click it to pick one from your computer. You can add as many as you like.
- Leave the target set to AU and the conversion starts on its own. FileFlip downloads 30.8 MB of WebAssembly the first time and nothing after that, then runs FFmpeg on your own machine.
- Save the AU file. Your browser writes it straight to your downloads folder, because there was never a server holding it.
Does converting Y4M to AU lose quality?#
It depends on what is inside the file. Y4M is a container rather than a codec, so whether the AU conversion copies the streams across untouched or re-encodes them is decided by the codecs that particular file happens to use.
Is it safe to convert Y4M files online?#
Yes, because nothing is uploaded. FileFlip converts Y4M files inside the browser tab you already have open, using WebAssembly builds of the same engines a desktop converter would install. Your file is read from your own disk into the page's memory, and no request carrying it is ever made.
You do not have to take that on trust. Open your browser's network panel, run a conversion, and you will see the engine come down and your file go nowhere. There is no upload, no server-side copy, no retention window, and no account tied to what you converted.
How FileFlip works lists every engine, its version and its download size, and shows how to check for yourself that nothing leaves your machine.
What Y4M to AU actually does to the file#
Converting Y4M to AU runs on FFmpeg, downloads 30.8 MB of WebAssembly the first time and nothing after that, re-encodes or copies through depending on the codecs in the file, and drops encoded streams and video tracks.
| Engine | FFmpeg |
|---|---|
| Conversion path | One step: Y4M → AU |
| Downloaded to your browser | 30.8 MB, once, then cached by your browser |
| Quality | Decided by the codecs inside the file |
| You get back | One file |
| Where it runs | In a background worker, so the page stays responsive |
What survives the conversion
| What is in the file | This conversion | Why |
|---|---|---|
| Encoded streams | Dropped | The target format has nowhere to put it. |
| Video tracks | Dropped | The target format has nowhere to put it. |
What is a Y4M file?#
A Y4M file holds YUV4MPEG2 data, raw uncompressed video frames in planar YCbCr with a short plain-text header, the format codec developers feed into encoders like x264, x265 and vpxenc so nothing but the codec under test touches the pixels.
Use Y4M when testing or comparing video encoders, where the point is to remove every other codec from the pipeline so the one under test is the only variable.
Convert Y4M to a compressed format like MP4 for anything other than encoder testing, since the raw size makes it impractical to store or share.
What is an AU file?#
An AU file holds audio in Sun Microsystems' Unix workstation format, a plain header followed by raw sample data that was the standard system-alert and early web audio format before WAV and MP3 took over.
AU still shows up as the format Java's javax.sound and some Unix system tools default to for short sound effects, where its simplicity is the point.
Convert AU to WAV or MP3 when the audio needs to play in modern media players, browsers, or editing software that no longer ship AU support by default.
Y4M to AU questions people ask#
What is Y4M used for?
Y4M is the standard input format for testing and benchmarking video encoders such as x264, x265 and libvpx, because raw, uncompressed frames guarantee the encoder is working from an unaltered source rather than one already degraded by another codec.
Does a Y4M file have sound?
No, Y4M carries video only. It was designed purely as a way to pipe raw frames between video tools, with no provision for an audio track at all.
Why is a Y4M file so much bigger than the video it represents?
A Y4M file stores every pixel of every frame with no compression at all, unlike MP4 or MKV, which is exactly the point: encoder developers use it as clean, unaltered input so nothing but the codec being tested affects the result. A minute of 1080p footage can run to several gigabytes.
What creates an AU file?
AU files come from Unix and Sun workstation software, from Java's built-in sound APIs, and from older system sound effects, since AU was the default audio format on SunOS and NeXT machines. Websites in the 1990s also used it for embedded sound clips before WAV and MP3 became standard.