Audio restoration is the process of 'cleaning up' audio files that have sonic anomalies repairable with today's audio technology.  Common restoration practices include removal of clicks and pops from records, and eliminating hiss from cassette and reel to reel analog tapes.   Making custom 'noise prints' of hiss, hum, buzz and background noise from a recording, the scrupulous audio engineer can eliminate these artifacts while maintaining the audio's original sonic luster.