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OGV to M4A

Your video never leaves your browser. It decodes the soundtrack with the codec it already has, and this page writes the M4A. The picture is not in the file you get — this is the sound on its own.

…or drop one anywhere on this page.

The soundtrack as an M4A, the way Apple wants it.

AAC in an MP4 container, which is what an iPhone records and what Apple Music plays. Your browser's own AAC encoder does the work and nothing is downloaded to make it happen.

This is a second generation, and here is why

The sound inside an OGV is already compressed — its soundtrack is Vorbis. Getting it out means decoding it and encoding it again, and that second encode starts from what the first one left behind rather than from the original recording. So a lossy source becomes a lossy result twice over, and at a low bitrate you can hear it. Choosing a higher quality above costs size and buys back some of the difference; it cannot recover what the video threw away in the first place.

A stream copy would avoid this, and it is not what this does. Lifting the compressed track straight out of the container and dropping it into a new one, untouched, is lossless — but it needs a demuxer written for each container, which this site does not have. So rather than implying you are getting the original track back, the page says what it actually did. For most purposes — a podcast, a voice recording, something to listen to — the difference is not audible. For archiving a master, it is the wrong tool.

The sample rate is chosen, not discovered

This is worth being exact about, because most converters are not. A browser will not tell you what sample rate the soundtrack was recorded at. The one function that decodes audio always resamples to whatever rate it is asked for, and then reports that rate back — so the number it gives you is its own, never the file's. Measured here: a 44,100 Hz MP4 and a 48,000 Hz MP4 come back with identical rates, and the rate is whichever one was asked for.

The usual way round this is to read the rate out of the file by hand, and the usual way that goes wrong is reading it out of a video file, where the bytes that look like an audio header generally are not one. On one test clip that is 48,000 Hz throughout, that method reported 32,000 for the MP4, 44,100 for the WebM and 11,025 Hz for the MKV — and then resampled the sound down to match. Telephone quality, right length, right pitch, and nothing to say it had happened.

So this page asks you instead, defaults to 48,000 Hz because that is what video soundtracks almost always are, and says on screen that the number is the one you chose. If you know your source was 44,100, pick it and nothing is resampled at all.

The picture is gone, not hidden

M4A has nowhere to put a video track, so there is nothing in the downloaded file but sound. If it was the pictures you wanted, this browser will not help: it does not decode Theora, so an .ogv opens with no picture at all. The sound is a different matter, which is why this page exists and the picture pages do not take OGV.

How this works, and what it downloads

Your browser decodes the soundtrack and then encodes it again with its own AAC encoder, through WebCodecs. Nothing is downloaded and no encoder ships from here — which matters most for AAC, whose patents have not expired. This page writes the container round it: format code, no codec.

What does not come across

Chapters, subtitles, cover art and any second audio track — a commentary, another language — are not carried. Where a file has several audio tracks your browser hands over the one it would play, which is usually the first, and there is no way from here to ask for a different one.

AVI, MOV, WMV, FLV and MTS are refused, and that is not a shortcoming of this page: no browser will decode them. If you hand one over it says so plainly rather than producing a silent file.

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