Mixing and mastering is the stage in audio production where they take all of the individually recorded tracks containing the different instruments and takes, combine them together into one track, and do all of the audio processing needed to make everything sound the best it can – things like adjusting the eq of each part, adding effects like reverbs, and so on.
Remastering is where they redo the mastering process in an attempt to fix any flaws an album has sonically, and adjust the end result to be more suited to modern styles and listening.
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