Since speech signals form an important and prevalent class of audio signals, it is expected that objective audio quality measurement tools will be of significant benefit to telecommunications equipment manufacturers, service providers, and end users. In light of the importance of the evolving digital cellular and PCS networks, a special focus involves the development of measurement tools for low-rate digital speech coding algorithms, and digital transmission over time-varying radio channels.
ITS research centers on careful modeling of the human hearing process coupled with attention to the higher-level perception and discrimination processes. Models and test results available in the literature are examined and incorporated when appropriate. The ITS approach is described by the block diagram in the above figure. The perceptual transformation block includes auditory band filtering along with outer and middle ear frequency response characteristics. These operations may be modeled by passing audio signals through the example filter bank shown on the above figure. Additional operations in the perceptual transformation include loudness transformations and frequency and time domain masking effects.
The distance measure block provides a comparison of the two perceptually transformed signals. After some additional processing, an auditory distance value is generated. This value can then be mapped to a user-oriented scale such as the five-point mean opinion score scale. An example of a possible mapping also appears on the above figure.