I was reading a forum thread the other day and came across a post which claims that unlike MP3, other codecs such as Opus and AAC "[invent] audio data that's not accurate to the source" by "[synthesizing] high-end frequency data". Is this true?

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You can read up on how the Opus codec works here: http://opus-codec.org/

Opus is entirely open-source, so you can even read the source-code to determine how it works.

Here is a paper on how MP3 and AAC work: https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/conference/AES-17-Conference_mp3-and-AAC-explained_AES17.pdf

These information sources should provide the insight you are seeking.

And by the way, the post to which you refer is nonsense.

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