Evaluation of Nonlinear Digital Audio Systems Through Transparency Reduction Using the Combined Test Signal

The well known and established methods used for distortion measurements cannot be used for detecting the errors that appear in digital audio systems based on perceptual coding and decoding. A test stimulus has to be used that has the time-varying properties similar to real signals such as speech and music. Furthermore, the procedure of quality evaluation has to include an estimate of the temporal behaviour of human hearing. For the purpose of evaluation of digital audio systems based on perceptual coding, a new combined test signal was developed with the cumulative distribution of amplitude following the Gaussian distribution. Input and output signals are transformed from the objective domain to the excitation domain. Transparency reduction is defined as a measure of distortion, being the ratio of error excitation and stimulus excitation.