What's all this 'fidelity' stuff about anyway?

Nearly everybody has heard the term "high fidelity".  Most understand that fidelity is a desirable thing to have, and many believe that the word 'fidelity' is synonymous with 'quality'. In any casual (non-technical) setting, they are the same.  But what does an audio engineer mean when he speaks about fidelity?

Audio quality is a subjective measurement that is different for each person.  We all know quality when we hear it, but when I listen to a 128k bitrate mp3, I may hear beautiful music, while somebody that is used to 320k files will think it's terrible.  Yet another person might be fine with 96k.  A bit of sound (a signal) can have hissing, scratching, ringing, echoes, and any number of other effects (collectively known as artifacts) that can affect quality.  But how do you quantize that?  Obviously a sound with a lot of unpleasant artifacts is of lower quality than one that doesn't have them, but can you turn that into numbers?  

Okay, yeah, that was a rhetorical question.  The answer, of course, is yes.  Over the years, audio engineers (people much smarter than me) have invented a number of characteristics and properties of a signal (metrics, in scientific parlance) that can be calculated mathematically, and that indicate how good or bad the signal is going to sound.  These metrics, taken collectively, determine the fidelity of the signal.  Over the next several articles, I'm going to go over several of the more important fidelity metrics and try to explain in fuzzy detail what each metric means, what a signal sounds like when you get it right (or wrong!), and why it is important for high fidelity.

Audio index