Nonintrusive measurements of the telephone channel

Algorithms for a system which performs in-service nonintrusive measurements of telephone line parameters such as echo path delay, echo attenuation, sp eech and noise levels are presented in this paper, It forms the theoretical background of a hardware implementation of the ITU-T P.561 recommendations for nonintrusive assessment of quality of a telephone network for voice tr ansmissions. The type of input signal to the echo path, whether it is speec h, fax, or data, has to be determined during the measurement procedure. In the system, the relevant parameters are estimated by means of frequency- domain block-processing algorithms and the estimation process is controlled by algorithms monitoring the signal flow. The complexity of the echo path estimation algorithm is determined and compared to a standard procedure bas ed on the adaptive normalized least mean square (NMLS) algorithm. Simulations showing the performance of the system in single- and double-tal k situations are presented. Examples of results from the hardware real-time implementation operating on an international connection are also shown.