MPG is the same program stream, under its shorter name. M4A is an MPEG-4 container holding AAC audio, which is what a voice memo and a music store purchase both are. ffmpeg sits between the two, and this page says exactly what happens to an MPG file on the way to becoming an M4A one.
What runs when an MPG file becomes M4A
ffmpeg held to a single thread, on a machine we rent and watch. Your MPG file is uploaded once, ffmpeg runs once, the M4A comes back, and neither file is kept. MPG to M4A is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the MPG into the M4A
The sound, and nothing but the sound. ffmpeg passes -vn on every audio target, so the MPG video stream is dropped at the demultiplexer and never reaches an encoder at all. That is why this pair is quick: the picture is the expensive part of an MPG file, and this pass does not look at it.
The running time. The M4A plays for exactly as long as the MPG did, because nothing on this path trims, cuts or speeds anything up.
The sample rate and the channel layout of the MPG audio track, carried into the M4A unchanged wherever the encoder accepts them, and resampled only where it cannot.
What MPG to M4A costs you
The picture, all of it, and on purpose. The MPG video stream is dropped before any encoder sees it, and the M4A that comes back is sound alone. Nothing in the M4A remembers there was ever an image, and no later conversion can put one back.
Cover art carried inside the MPG. ffmpeg counts an attached picture as a video stream, so the same -vn that drops the film drops the artwork with it. Keeping it is worse: on several audio containers the mux then fails outright with an error nobody can read.
Any second audio track, and the subtitles. ffmpeg selects one stream out of the MPG, and an M4A file has nowhere to put a subtitle track in any case.
Exactness, one more time. The audio inside the MPG was compressed once already, and M4A compresses the decoded result again. Two lossy passes do not cancel each other out, they stack.
Where MPG and M4A files come from
MPG. The same format as MPEG, under the extension DOS allowed. Which of the two a file carries says nothing about its content.
If a model is going to read the M4A
An M4A file is a waveform, and a language model reads text, so handing an assistant this M4A directly gets you nothing. The M4A becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. MPG to M4A is the step before it: get the sound out of the MPG in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running MPG to M4A against the real engine with a real file, ffmpeg wrote 27,637 bytes of M4A in 0.2 seconds, machine otherwise idle. That is one MPG file on one day and not an average, which is why the number is given together with the file that produced it.
The MPG to M4A verdict was reached by ffprobe, meaning the container was actually demultiplexed, recognised under the right name, and carried at least one stream. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the M4A file has anything inside it.
The same MPG file, sent somewhere else
Other outputs the probe measured out of an MPG file, so the cost of choosing M4A can be read against something. Sizes do not compare across engines, because each family was probed with its own MPG witness file.
MPG to GIF: 184,860 bytes in 0.1 seconds, verified by a format signature.
MPG to MP3: 24,468 bytes in 0.1 seconds, verified by ffprobe.
MPG to MP4: 49,380 bytes in 0.3 seconds, verified by ffprobe.
MPG to WAV: 265,038 bytes in 0.1 seconds, verified by ffprobe.
MPG to FLAC: 64,941 bytes in 0.1 seconds, verified by ffprobe.
MPG to OGG: 13,540 bytes in 0.2 seconds, verified by ffprobe.
Other ways into M4A, and what they measured
Among the published routes into M4A, MPG is the number 13 largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
OGG to M4A: 28,012 bytes in 0.1 seconds.
OPUS to M4A: 27,734 bytes in 0.2 seconds.
WAV to M4A: 27,633 bytes in 0.2 seconds.
WEBM to M4A: 27,734 bytes in 0.2 seconds.
WMA to M4A: 27,998 bytes in 0.1 seconds.
What we will not pretend about MPG to M4A
An MPG file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MPG to M4A run that passes 240 seconds is killed, and the ffmpeg process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The MPG file behind the measurement on this page was 3 seconds of 320 by 240 test pattern at 25 frames a second, with a sine wave on the audio track, built by ffmpeg for the probe itself. That is why MPG to M4A came out at 0.2 seconds, and it is not what your file will do. The measurement proves the pair works and says almost nothing about how long yours will take.
MPG to M4A costs half a credit per conversion, whatever the length of the MPG. That is the same as an image or a document here, and it is a measurement rather than a gesture: an audio target on this engine is done in under two seconds, where a video target can take minutes.
An MPG longer than 60 minutes is refused before encoding. ffprobe measures the MPG first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the M4A.
MPG to M4A waits at the audio door, which is 2 slots wide and gives up after 5 seconds rather than making you wait twenty times the length of the work. Until the 30th there was one door for everything, and a single video transcode starved every audio conversion for its whole duration: an M4A that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the MPG. No transcription, no scene detection, no summary: MPG to M4A is a container and codec operation, and the M4A says exactly what the MPG said, in a different wrapper. The sound is the whole payload.
One MPG file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MPG to M4A: what people ask
What actually converts my MPG file to M4A?
ffmpeg does it, held to a single thread, on a machine we rent and watch. Not a browser trick and not somebody else service: the MPG file is uploaded once, ffmpeg runs once, the M4A comes back, and neither file is kept afterwards.
How long does MPG to M4A take?
The probe measured 0.2 seconds for 27,637 bytes of M4A, on a 3 second file. An audio target scales far better than a video one: three minutes of source through this engine takes under a second, because only the sound is encoded. Past 240 seconds any run is stopped, and an audio one does not get close.
What do I lose going from MPG to M4A?
The one to know about first: The picture, all of it, and on purpose. The MPG video stream is dropped before any encoder sees it, and the M4A that comes back is sound alone. Nothing in the M4A remembers there was ever an image, and no later conversion can put one back.
Does the M4A still have the video in it?
No, and that is the entire point of the pair. The MPG picture is dropped before any encoder touches it, so what you download is sound only. If you wanted to keep the picture you wanted a video target, and it is priced differently because it costs differently.
Is MPG to M4A free?
There is a free allowance every month, and one MPG to M4A conversion costs half a credit against it however long the MPG runs. An audio target is charged per conversion and not per minute, because the engine encodes three minutes of sound in well under a second.