FLV is the Flash Video container, orphaned the day the plugin that gave it a name was switched off. AAC is Advanced Audio Coding on its own, lossy, and the encoding that replaced MP3 nearly everywhere it mattered. ffmpeg sits between the two, and this page says exactly what happens to an FLV file on the way to becoming an AAC one.
What runs when an FLV file becomes AAC
ffmpeg held to a single thread, on a machine we rent and watch. Your FLV file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept. FLV to AAC 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 FLV into the AAC
The sound, and nothing but the sound. ffmpeg passes -vn on every audio target, so the FLV 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 FLV file, and this pass does not look at it.
The running time. The AAC plays for exactly as long as the FLV did, because nothing on this path trims, cuts or speeds anything up.
The sample rate and the channel layout of the FLV audio track, carried into the AAC unchanged wherever the encoder accepts them, and resampled only where it cannot.
What FLV to AAC costs you
The picture, all of it, and on purpose. The FLV video stream is dropped before any encoder sees it, and the AAC that comes back is sound alone. Nothing in the AAC remembers there was ever an image, and no later conversion can put one back.
Cover art carried inside the FLV. 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 FLV, and an AAC file has nowhere to put a subtitle track in any case.
Exactness, one more time. The audio inside the FLV was compressed once already, and AAC compresses the decoded result again. Two lossy passes do not cancel each other out, they stack.
If a model is going to read the AAC
An AAC file is a waveform, and a language model reads text, so handing an assistant this AAC directly gets you nothing. The AAC becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. FLV to AAC is the step before it: get the sound out of the FLV in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running FLV to AAC against the real engine with a real file, ffmpeg wrote 27,485 bytes of AAC in 0.2 seconds, machine otherwise idle. That is one FLV file on one day and not an average, which is why the number is given together with the file that produced it.
The FLV to AAC 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 AAC file has anything inside it.
The same FLV file, sent somewhere else
Other outputs the probe measured out of an FLV file, so the cost of choosing AAC can be read against something. Sizes do not compare across engines, because each family was probed with its own FLV witness file.
FLV to GIF: 173,530 bytes in 0.1 seconds, verified by a format signature.
FLV to MP3: 24,677 bytes in 0.1 seconds, verified by ffprobe.
FLV to MP4: 48,269 bytes in 0.3 seconds, verified by ffprobe.
FLV to WAV: 267,342 bytes in 0.1 seconds, verified by ffprobe.
FLV to FLAC: 156,775 bytes in 0.1 seconds, verified by ffprobe.
FLV to OGG: 9,138 bytes in 0.2 seconds, verified by ffprobe.
Other ways into AAC, and what they measured
Among the published routes into AAC, FLV is the sixth largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
M4A to AAC: 27,317 bytes in 0.2 seconds.
MKV to AAC: 27,699 bytes in 0.2 seconds.
MOV to AAC: 27,317 bytes in 0.2 seconds.
MPG to AAC: 27,223 bytes in 0.2 seconds.
OGG to AAC: 27,598 bytes in 0.1 seconds.
What we will not pretend about FLV to AAC
An FLV file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An FLV to AAC 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 FLV 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 FLV to AAC 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.
FLV to AAC costs half a credit per conversion, whatever the length of the FLV. 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 FLV longer than 60 minutes is refused before encoding. ffprobe measures the FLV first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the AAC.
FLV to AAC 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 AAC that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the FLV. No transcription, no scene detection, no summary: FLV to AAC is a container and codec operation, and the AAC says exactly what the FLV said, in a different wrapper. The sound is the whole payload.
One FLV file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
FLV to AAC: what people ask
What actually converts my FLV file to AAC?
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 FLV file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept afterwards.
How long does FLV to AAC take?
The probe measured 0.2 seconds for 27,485 bytes of AAC, 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 FLV to AAC?
The one to know about first: The picture, all of it, and on purpose. The FLV video stream is dropped before any encoder sees it, and the AAC that comes back is sound alone. Nothing in the AAC remembers there was ever an image, and no later conversion can put one back.
Does the AAC still have the video in it?
No, and that is the entire point of the pair. The FLV 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 FLV to AAC free?
There is a free allowance every month, and one FLV to AAC conversion costs half a credit against it however long the FLV 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.