AU to OPUS Converter

Converting 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 standardised on. It is a genuine leap forward — Unix-workstation audio out, an IETF-standard codec in — and the sections below cover the bitrate decision that actually matters, the sample-rate rules Opus imposes that catch people out with 8 kHz sources, and the playback gaps still worth checking before you send the file to someone.

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 applet audio and old Unix speech archives all work, and several files can be queued and converted with the same settings.
2. Choose How the Audio Is Sized: Under File Compression pick Quality Preset and choose from the dropdown (Highest through Lowest, with "Very High (Recommended)" as the labelled sweet spot), or switch to Custom Bitrate (starts at 128 Kbps), Constant Bitrate or Specific file size to hit an exact target. This is the setting that matters most — the walk-through below explains what to pick.
3. Set Audio Sample Rate, Audio Channel or Trim (Optional): Audio Channel downmixes to mono, which is the right call for any speech recording. Audio Sample Rate offers ORIGINAL plus fixed rates, though Opus only accepts a specific set — see the table below. Trim takes a Start time and Duration in seconds or HH:MM:SS.sss.
4. Convert and Download: Click Convert and save the .opus file individually or as a ZIP. Files upload over an encrypted connection, are converted on our servers and are deleted automatically a few hours later. No sign-up, no watermark.

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

AU files come in two very different flavours, and the bitrate that makes sense differs by a factor of four between them. Run ffprobe file.au or open VLC's Tools → Codec Information first: pcm_mulaw or pcm_alaw at 8000 Hz mono is the classic telephone-grade Sun capture, while pcm_s16be or deeper is a real linear-PCM master.

Sample Rates: What Opus Accepts

Opus is not a general-purpose resampler target. Its MDCT layer operates internally at 48 kHz and the codec is defined around a fixed ladder of rates — RFC 6716 covers narrowband through fullband at 8, 12, 16, 24 and 48 kHz. Anything you ask for outside that list gets snapped to the nearest one before encoding.

Worth knowing regardless of what you pick: an Ogg Opus stream reports 48 kHz when you probe it, because that is the rate an Opus decoder outputs. A file converted from an 8 kHz µ-law AU still contains only 8 kHz-worth of audio — the 48 kHz figure describes the decoder, not new information.

Common Errors and How to Fix Them

• "The .opus file won't open in an older app" — a .opus file is an Ogg stream with a different extension, and some older software matches on the extension rather than the contents. Renaming it to .ogg normally fixes that; if not, AU to MP3 plays essentially everywhere.
• "It sounds no better than the old file" — expected if the source was µ-law. Opus preserves what was captured, efficiently; it restores nothing an 8 kHz telephone-grade recording never had. The win is size and compatibility.
• "I set a high bitrate and it still sounds limited" — same cause. Encoding an 8 kHz source at 128 kbps produces a bigger file holding identical audio. Match the bitrate to the source using the table above.
• "The sample rate is not what I chose" — Opus only accepts 8, 12, 16, 24 and 48 kHz, so a request for 44.1 kHz is snapped to 48 kHz. That is by design, not a fault in the file.
• "I need an exact lossless copy" — Opus is lossy. For a bit-exact archive of a linear-PCM .au, keep the original or use AU to FLAC instead.

When This Doesn't Work

The weak spot with Opus is playback coverage on the far edges, not the conversion itself. Current desktop browsers are fine — Chrome from 33, Firefox from 15 and Edge from 14 all play Opus in the <audio> element. Android has decoded Opus since 5.0, with Ogg, MP4 and Matroska listed among its supported containers. Apple was the long holdout: Safari's support stayed partial for years and full playback on iOS only arrived with 18.4, so anything older in that ecosystem is a coin flip. A long tail of pre-2018 smart TVs, car head units and hardware players never added Opus at all. If the file has to land on unknown or elderly hardware, do not fight it — use AU to MP3 for universal compatibility, or AU to M4A when the destination is specifically Apple. And if the .au itself is truncated or corrupted, no output format will rescue it; the audio stream has to decode before it can be re-encoded.

Frequently Asked Questions

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

No — no encoder can add what was never captured. If the AU holds the classic 8-bit µ-law or A-law payload (Sun's companded default at 8 kHz mono), Opus reproduces that intelligibility very efficiently but cannot restore highs or dynamic range the compander discarded at recording time. If it holds linear PCM, you get a clean first-generation encode of an effectively lossless master, which is where Opus genuinely excels. What improves is file size and modern compatibility.

What bitrate should I pick for the Opus output?

Less than you would expect, because Opus is unusually efficient. For a linear-PCM stereo source, 96–128 kbps is transparent for most listeners — roughly where MP3 needs 128–160 kbps. For an 8 kHz µ-law speech source, 24–32 kbps mono already carries everything present in the file, and pushing higher just makes the file bigger without adding information. When unsure, leave Quality Preset alone and let the encoder scale to the material.

Why does my converted file show 48 kHz when the AU was 8 kHz?

Because that is what an Opus decoder outputs. Opus's transform layer runs internally at 48 kHz and the format is built so that any stream can be decoded at 48 kHz regardless of what went in, which is why probing tools report that figure. If you left Audio Sample Rate on ORIGINAL, the encoder still worked from your 8 kHz audio in narrowband mode — the file contains 8 kHz-worth of information wearing a 48 kHz label, not upsampled detail.

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

On Android and in modern browsers, yes: Opus decoding has shipped on Android since 5.0, and Chrome, Firefox and Edge have supported it for a decade. Apple is the catch — Safari's support was partial for years, with full playback arriving on iOS 18.4, so older iPhones and iPads are unreliable. Car head units and smart TVs from before roughly 2018 are a mixed bag. When guaranteed playback matters more than efficiency, convert to AU to MP3 instead.

Is Opus better than MP3 or AAC for an old .au recording?

At the bitrates these recordings deserve, comfortably. Opus was designed for exactly this territory: it handles speech at 16–32 kbps in a way MP3 cannot approach, and it stays ahead of AAC through the low and middle bitrate range. The only argument against it is reach — MP3 plays on anything with a headphone jack, and AAC is what Apple hardware treats as native. Choose Opus for efficiency and the modern web, AU to M4A for Apple, AU to MP3 for maximum compatibility.

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

More than most legacy conversions. This is not swapping one dated wrapper for another: a Sun/NeXT relic carrying audio/basic, with no tag fields and support limited to VLC-class tools, becomes a file built on the codec standardised as RFC 6716 in 2012 and used today by WebRTC, Discord, WhatsApp voice notes and YouTube. You get far better efficiency and current-software support, and for a µ-law source you give up nothing audible in return.

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

If the AU holds linear PCM and you care about preservation, keep the original or convert with AU to FLAC — both keep every sample, where Opus discards 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 worth preserving beyond the speech itself, so a low-bitrate Opus file is usually the whole job.

Can I convert an Opus file back to AU later?

Yes, Opus to AU reverses the direction, but not the quality loss. Once audio has been through Opus, rebuilding an .au around it wraps already-lossy audio in a much older container — and our AU output defaults to 8-bit G.711 µ-law unless you pick a PCM encoding, which would compand it a second time. Keep the source .au if its fidelity might matter again.

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

Your file is uploaded over an encrypted connection, converted on our servers, and deleted automatically a few hours after the job finishes — never shared, never made public, with no sign-up and no watermark. The practical limit on long recordings is upload size and time rather than the encode: a 60-second 8 kHz mono µ-law AU is only about 480 KB, and at 24 kbps mono the Opus version of that same minute works out to roughly 180 KB.