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I was walking past a bar the other day and heard music coming out. I could immediately tell that the music was live, being played in the bar at that moment. This is something I have often noticed, I can tell whether music is recorded or being played live. 

This seems to extend to recordings of live music as well. I have often found myself identifying that the musical background of, for example, a youtube video was being played at the time and was recorded using the same microphone that recorded the ambient sound of the video.

First of all, I assume that I am not alone and that any musician or music lover can also make this distinction. So, why can we do this? What is it that allows us to recognize live performances? I assume it has something to do with the way they are recorded (or not as the case may be) and that a good sound engineer would be able to fix things so that I couldn't tell but what does it depend on?
