AU to OPUS Converter

Convert AU to Opus: What This Tutorial Covers

This walks through moving a legacy Sun .au (or .snd) recording into Opus, the open royalty-free codec the modern web, messaging apps, and streaming services rely on. It is genuinely a forward move — old Unix-workstation and Java-applet audio out, the current IETF-standard codec in — and below it covers the one honest catch (what conversion can and cannot recover) plus the playback gap on older hardware that trips people up.

How to Convert AU to Opus

1. Upload Your AU File: Drag and drop your .au or .snd file onto the page, or click "+ Add Files" to browse. Sun/SPARCstation system sounds, early Java 1.0/1.1 applet audio, and old Unix speech archives all work, and you can queue several at once to migrate a whole folder.
2. Set the Bitrate: The output stays on the Opus codec; open Advanced Options and pick a Quality Preset (Highest down to Lowest), or switch to Custom Bitrate, Constant Bitrate, or Variable Bitrate to set an exact value. This is the setting that matters most — see the walk-through below.
3. Adjust Audio Channel, Audio Sample Rate, or Trim (Optional): Leave Audio Channel and Audio Sample Rate on "Original" to copy the source layout, or downmix to mono and lower the rate for a smaller file. Use Trim (Start time + Duration) to keep only part of a longer recording.
4. Convert and Download: Click Convert and save your .opus file individually or as a ZIP. No sign-up, no watermark.

Walk-through: Picking an Opus Bitrate for Your AU Source

What bitrate to choose depends entirely on what is inside the .au, and AU files come in two very different flavors. The classic Sun default is 8-bit µ-law at 8 kHz mono — telephone-grade audio with usable bandwidth capped near 4 kHz. But the AU container also stores uncompressed linear PCM at 16, 24, or 32-bit, which is effectively a lossless master. These two cases call for different settings:

• µ-law / A-law source (the telephone-grade case): Opus cannot restore highs or dynamic range the µ-law compander discarded when the file was first recorded — no codec can add detail that was never captured. What Opus does do is store that same intelligible speech far more efficiently. A Constant Bitrate of 24–32 kbps mono, or the Variable Bitrate 24k-40k range, is plenty; pushing higher just makes a bigger file holding the same limited audio.
• Linear-PCM source (the good case): This is a clean first-generation encode of a lossless master, and Opus is excellent at it. For stereo PCM, the 96k-128k Variable Bitrate range is transparent for most listeners — Opus at 96 kbps lands roughly where MP3 needs 128 kbps. There is rarely a reason to exceed 160 kbps.

Not sure which flavor you have? Run ffprobe file.au or open it in VLC's Codec Information panel: "pcm_mulaw" or a 8000 Hz / mono signature means telephone-grade; "pcm_s16be" or higher means linear PCM and the good case.

Common Errors and How to Fix Them

• "The .opus file won't play on my iPhone or older Android" — native Opus support is uneven on older hardware. Android plays the bare .opus extension from Android 10 onward (earlier versions need it wrapped in .ogg), and the standalone file is unreliable in the iOS default player before iOS 18.4. If a device refuses it, convert to AU to MP3 instead, which plays virtually everywhere.
• "Output sounds no better than the old file" — expected if the source was µ-law. Opus preserves what was there efficiently; it restores nothing the original 8 kHz recording never captured. The win is a smaller, modern, web-ready file, not higher fidelity.
• "Bitrate is set high but the file still sounds limited" — same cause. Setting 128 kbps on an 8 kHz µ-law source stores telephone-grade audio in a larger container; it cannot regain lost bandwidth. Match the bitrate to the source instead.
• "I wanted to keep an exact lossless copy" — Opus is lossy. For a true archival master from a linear-PCM .au, keep the original or use AU to FLAC for lossless compression with tagging instead.

When This Doesn't Work

The biggest gotcha with Opus is playback support, not the conversion itself. Every current desktop browser plays Opus — Chrome 33+, Firefox 15+, and Edge 14+ in the <audio> element — and Android handles the .opus extension natively from Android 10. The weak spots are Apple's ecosystem and a long tail of older devices: Safari only gained full Opus playback on iOS 18.4 (macOS support stayed partial for years), and some pre-2018 smart TVs and legacy car infotainment systems never added it. If your target is one of those, do not fight it — convert to AU to MP3 for universal compatibility. And if you need a true lossless master rather than a small shareable file, Opus is the wrong target entirely; keep the PCM .au or use AU to WAV. The conversion can also fail if the .au is corrupted or only partially downloaded, in which case the audio stream may not decode cleanly.

Frequently Asked Questions

Will converting my AU file to Opus improve the sound quality?

No — and the reason depends on what is inside the AU. If it holds the classic 8-bit µ-law or A-law audio (Sun's telephone-companded default at 8 kHz mono), Opus reproduces that intelligibility but cannot restore highs or dynamic range the compander discarded at recording time. If it holds linear PCM (16/24/32-bit), you get a clean first-generation Opus encode of a lossless master — that is the case where Opus shines. Either way you keep what you had; encoding never adds detail the original capture lacked.

Is converting an old Unix AU to Opus actually worth it?

Yes, more so than most legacy conversions. Unlike re-packaging one dated format into another, this is a real modernization: a Sun/NeXT workstation relic with no metadata and limited player support becomes Opus, the open codec standardized by the IETF as RFC 6716 that WhatsApp, Discord, YouTube, and WebRTC all use. You get far better efficiency and broad current-browser support in exchange for a file the modern web actually understands.

What bitrate should I pick for the Opus output?

Less than you would expect, because Opus is very efficient. For a linear-PCM stereo source, 96-128 kbps is transparent for most listeners — Opus at 96 kbps is roughly on par with MP3 at 128 kbps. For an 8 kHz µ-law speech source, 24-32 kbps mono is already plenty and anything higher just inflates the file. In our testing, a 60-second 16-bit/44.1 kHz stereo PCM AU (about 10 MB) encoded to a 96 kbps Opus near 0.7 MB with no audible loss, while an 8 kHz µ-law AU re-encoded to a 24 kbps mono Opus at full speech intelligibility.

Will the Opus file play on my iPhone, Android phone, or car stereo?

Usually on Android and modern browsers, less reliably on Apple and older hardware. Android plays the bare .opus extension from Android 10 onward; Chrome, Firefox, and Edge all play Opus on the desktop. Apple is the catch — Safari only gained full Opus playback on iOS 18.4, and older car and TV systems are a mixed bag. If you need guaranteed playback on any device, convert to AU to MP3 instead.

Should I keep my AU file as an archival master, or is Opus fine?

If the AU holds linear PCM and you care about a true lossless master, keep the original .au or convert it to AU to FLAC — both preserve every sample exactly, where Opus is lossy and discards some data by design. Opus is the right target when you want a small, modern, shareable file rather than an editing or preservation master. For a µ-law AU there is little to preserve beyond the speech itself, so Opus at a low bitrate is usually all you need.

Can I convert an Opus file back to AU later?

Yes — Opus to AU reverses the process. Remember it is not a round-trip to the original quality, though: once audio has been through Opus encoding, converting back to AU rebuilds the .au container around the already-lossy audio and cannot recover detail Opus dropped. Keep your source .au if you may need the original fidelity.

How are my files handled, and how long do you keep them?

Files are uploaded over an encrypted connection, converted on our servers, and deleted automatically a few hours after conversion — never shared, never made public, with no sign-up and no watermark.