M4A to Opus
Nothing you choose here leaves your browser. There is no server to send it to — the conversion happens on your own device, and nothing is uploaded, stored or seen by anyone.
What this conversion costs you
Re-encoding AAC as Opus: smaller, and a little further from the original. Both are lossy, so this is a second generation and not a repack. Opus is more efficient, so the file gets smaller at the same perceived quality — useful for voice recordings in particular, where Opus is far ahead. For music you already like the sound of, leaving it as an M4A is the better answer.
Your sample rate, kept
This is the part other browser-based converters get wrong without telling you. The web audio decoder resamples everything to your device's own rate — usually 48,000 Hz — so a 44,100 Hz file, which is every CD rip and most MP3s, comes back with its 44,100 samples a second interpolated into 48,000. The audio is subtly changed and nothing says so.
This page reads the sample rate out of the file's own header first, then decodes at that rate, so the samples are the ones you started with. The table above says which rate it found and confirms it was kept. Where a file's header cannot be read, it says that instead of pretending.
How it works, and why nothing is uploaded
Your browser decodes the M4A itself, and then encodes the Opus itself as well — through WebCodecs, which hands a page the encoders your operating system already carries. Nothing is downloaded at all, not even the once-off encoder the MP3, Ogg and FLAC pages offer. This site ships no Opus encoder and never will; it borrows the one already on your machine. What is written here is the container around it — the Ogg file structure — which is plain format code with no codec in it. One thing to know before you start: Opus only runs at 48,000 Hz. If your file is 44,100 Hz — which every CD rip and most MP3s are — the encoder resamples it, and this page will tell you so. That is the codec's doing rather than a choice made here, and it cannot be avoided while producing Opus at all. The file keeps a note of the rate it came from.
Where to go next
- Opus to M4A — the same conversion the other way
- M4A to MP3 — the format everything has played for thirty years
- M4A to WAV — uncompressed, enormous, and readable by anything
What this does not do
It converts whole files and does not trim, join, change the volume or alter the tags — the artist and title in your original do not survive, because the audio is decoded to raw samples and encoded again. It does not write AAC or M4A: those patents have not expired, and this site will not open that question for a format people already have.
Nothing is uploaded, so it cannot work on a file that is not on the device you are using, and a long recording will use a lot of memory while it is being converted.