Method and apparatus for evaluating audio distortion

An apparatus for evaluating an audio distortion in an audio system, wherein an input and an output audio signal from the audio system includes N samples, which comprises:

• first estimation means for estimating a power density spectrum of the input digital audio signal to the audio system, wherein the first estimation means include means for windowing the input audio signal to compensate for the lack of spectral selectivity at low frequency regions thereof and the power density spectrum of the input digital audio signal, X(k), is determined as follows:
• ##EQU6## wherein N is a positive integer, w(n)=x(n).multidot.h(n), x(n) is the input digital audio signal, .omega. is 2.pi.kn/N, k=0,1,2, . . . , N-1and n=0,1,2, . . . , N-1, and a weight factor for the windowing means, h(n), is represented as follows:
• h(n)=.sqroot.8/3.multidot.1/2{1-cos (2.pi.n/N)};
• means for determining a masking threshold depending on the power density spectrum of the input digital audio signal;
• second estimation means for estimating a power density spectrum of an error signal representative of the difference between the input digital audio signal and its output digital audio signal from the audio system, wherein the second estimation means include means for windowing the error signal to compensate for the lack of spectral selectivity at low frequency regions thereof;
• and
• third estimation means for estimating a power density spectrum of the error signal which exceeds the masking threshold and for calculating a perceptual spectrum distance representative of the audio distortion, wherein the perceptual spectrum distance(PSD) is calculated as follows:
• ##EQU7## wherein E(k) is the power density spectrum of the error signal, and M(k) is the masking threshold, k=0,1, . . . , N-1.