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I'm using ffmpeg to encode audio files as AAC at 256kbps. However, the actual bitrate of the resulting file is rarely 256. It's usually a number close to 256 like 241, 255, 257, 254. In one case it was 125.

What's going on? Is this is a property of AAC or ffmpeg? If I ask ffmpeg for mp3 at 320kbps I always get 320. I thought for a while that the AAC encoder was using 256 as an upper limit and only using the bitrate it needs but then I got a few 257 files so that can't be it.

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Both AAC and MP3 are not always encoded with constant bit rate but they support variable bit rate encoding too.

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