To make AAC audio in FFmpeg, set the audio encoder explicitly with -c:a aac. For one compatible video file, a typical command is ffmpeg -i input.mp4 -c:v copy -c:a aac -b:a 192k output.mp4. For a playlist, use FFmpeg’s concat demuxer when the files have compatible streams; if they need normalization or re-encoding, use the concat filter instead.
What changes when you choose AAC
The output filename extension does not, by itself, guarantee an audio codec. FFmpeg selects a default encoder based on the output muxer when you do not specify one, so -c:a aac makes the audio choice explicit. FFmpeg documents a native AAC encoder and its options in the FFmpeg codec documentation.
If the input audio is MP3 and the output must be AAC, FFmpeg has to decode the MP3 and encode the decoded audio as AAC. That is transcoding, not streamcopy, and it can introduce another lossy generation. If the source audio is already AAC and the destination supports it, copying that stream may avoid unnecessary re-encoding. FFmpeg defines transcoding as decoding a stream and encoding it again; streamcopy forwards compressed packets without decoding or encoding, so it is fast and causes no quality loss from re-encoding (FFmpeg command-line documentation).
Encode one video with AAC audio
Copy compatible video; encode the audio
ffmpeg -i input.mp4 -c:v copy -c:a aac -b:a 192k output.mp4Here -c:v copy preserves the existing video stream without re-encoding it, while -c:a aac encodes audio as AAC. The -b:a 192k value is an illustrative bitrate, not a universal optimum. Choose settings based on the source, listening needs, and destination requirements; the official documentation does not establish one best bitrate for every use.
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Allow video re-encoding when copying is unsuitable
ffmpeg -i input.mp4 -c:a aac -b:a 192k output.mp4Because this command does not specify -c:v, FFmpeg uses the output muxer’s default video encoder after stream selection. That may re-encode the video and take longer. For predictable results, name an appropriate video encoder explicitly when re-encoding. Streamcopy also cannot apply filters, and it may fail if the target container cannot carry the source stream or necessary stream information.
Choose how to join playlist files
FFmpeg offers two main approaches. The concat demuxer reads files sequentially and adjusts timestamps so one follows another; the concat filter is the recommended route when re-encoding is needed. Neither method makes arbitrary files automatically compatible. Inspect the inputs first, especially if codecs, stream layouts, or durations differ. FFmpeg’s FAQ on joining video files explains the distinction.
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| Approach | Best fit | Re-encoding | Important limitation |
|---|---|---|---|
| Concat demuxer | Inputs with compatible stream layouts when you want to avoid re-encoding. | May avoid it; you can still encode audio to AAC in the output command. | Files are processed in sequence and timestamps adjusted. Differences in stream lengths can create gaps. |
| Concat filter | Inputs that need re-encoding, filtering, or stream normalization. | Required for the streams passed through the filter. | Specify suitable encoders and ensure the inputs are made compatible for the filter workflow. |
Build a playlist with the concat demuxer
1. Create the playlist file
Make a plain-text file named playlist.txt with one entry per file, in the order you want them joined:
file 'track1.mp4'
file 'track2.mp4'Use paths that resolve from where you run FFmpeg, or provide full paths. The concat demuxer expects its own file-list syntax; this is not an ordinary playlist format for a media player.
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2. Run the AAC command
ffmpeg -f concat -safe 0 -i playlist.txt -c:v copy -c:a aac -b:a 192k output.mp4This is a starting template, not a tested command for every set of files. The -safe 0 option permits paths outside the concat demuxer’s restricted safe subset. Use it only with a playlist file you control. The command may need explicit stream mapping, a video encoder, or different container settings, depending on the files and destination. If FFmpeg rejects the inputs or their stream layouts differ, use a concat-filter workflow and re-encode the streams that need it.
Check the output and diagnose common failures
- The output audio is still MP3: make sure the output command includes
-c:a aacand that you are checking the newly written output file, not the original input. - FFmpeg cannot copy the video into the output: the source video stream may not be compatible with the target container, or the muxer may lack required information. Select a suitable output container or deliberately re-encode video with a specified encoder.
- The playlist is rejected: check that the text file uses
file 'path'entries and that each path is correct. If the inputs have incompatible streams, switch to the concat filter and normalize/re-encode as needed. - There are gaps between tracks: the concat demuxer adjusts timestamps, but differing stream lengths can create gaps. Inspect duration and stream alignment; use a filter-based workflow if you need to normalize the sources.
- Processing takes longer than expected: audio encoding is required when converting MP3 to AAC. If video is also being re-encoded because you omitted
-c:v copy, that can add substantial processing work.
FFmpeg documentation is version-sensitive and reflects the build and options available in the installed version. The commands here are illustrative; verify behavior against your actual files and intended playback destination.
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Or let it run in the cloud
Encoding and assembling a playlist with FFmpeg is the do-it-yourself route. If your goal is to keep a pre-recorded playlist live on YouTube around the clock, StreamNeo is a separate option: upload your recording or build a playlist, add your YouTube stream key, and go live. StreamNeo loops uploaded videos from the cloud, so nothing has to stay on at home. It streams the upload as made, up to 4K 60fps, at one price per slot; it also automatically recovers if YouTube drops the stream. The first day is free with no card. Monthly: $9.99 per month. StreamNeo is for YouTube streams from uploaded videos, not camera broadcasts. See StreamNeo, then start the free first day.
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