Does ffmpeg pass a file through if it already conforms to the given transmux options?

Question

Assuming I have an MP3 file with this qualities:

• mono
• 44100 sample rate
• 64k constant bitrate

Question: If I provide that file to ffmpeg as input, using the command line arguments necessary to transmux it to an output file matching the aforementioned values, which of the following should I expect from the output result?:

• A worse quality version of the original file, because it's be re-compressed in a lossy way.
• The same file, because ffmpeg didn't need to do anything.
• Something else entirely...?

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Extra context:

I'd like to batch transmux a set of provided audio files. But there's a chance that some of those files already conform to a given standard.

I'm wondering if I need to check for those maximum values before handing the file to ffmpeg (to avoid unnecessary processing) or if that would be wasteful because ffmpeg is smart enough to handle that case?

Answer 1

ffmpeg will reencode file if you do not specify -c:a copy