'NMR' and 'masking flag': Evaluation of quality using perceptual criteria

With the standardization of perceptual coding methods and the introduction of the first consumer products using these methods, digital audio is changing rapidly. This change has great effects on the methods used to measure and describe audio quality or the quality of digital systems. In recent years, the NMR (noise-to-mask ratio) technique has been used successfully to estimate the audibility of coding artifacts and other signal impairments. Frequency domain representations of both the input and the output of a digital system are used to derive the masking threshold and the error signal. The resulting values describe the estimated ratio of the noise to the masking threshold. They have proved to be much more useful for the objective assessment of subjective quality than traditional methods like SNR or THD.