AN9789: Audio Quality Measurement Primer

This audio quality measurement Application Note is geared towards describing the methods for measurement of key audio specifications. Each analysis technique described here in, can be applied to audio ADCs, DACs, Analog Mixers, or any signal within the audio band. Audio quality is typically gauged by three main components. They are Signal to Noise Ratio, Total Harmonic Distortion, and Frequency Response. Audio quality, in all cases, is subjective and has been debated for decades. The inclusion of a weighting function, to account for the ear’s non-ideal perception of loudness at different frequencies, is one such example of the subjective quality of audio. In the early 1950s, transistors had just been invented. Audio amplifiers built with these devices offered better signal to noise, harmonic distortion, and frequency response than their vacuum tube predecessors. Circuits designed and built with vacuum tubes were still preferred by audio enthusiasts. The transistor amplifiers had lower power output, and when over-driven produced odd order harmonics. To the human ear, odd order harmonics have long been identified as sounding more unpleasant than even order harmonics. The vacuum tube amplifiers could be over-driven to produce more pleasing even-order harmonics while producing more power than the transistor amplifiers. Though the transistor amplifiers had better specifications, the tube amplifiers were superior in listening tests, and proved to have better subjective audio quality. Today’s audio circuits far outstrip their predecessors in most audio applications. Their specifications are very often beyond the range of human perception, yet the purists still debate.