A MEASUREMENT TECHNIQUE TO DETERMINE THE TRANSFER CHARACTERISTICS OF HANDS-FREE TELEPHONES

The transfer functions of hands-free telephones, mobile communciation systems but standard telephones with hand-sets as well can no longer be considered to be linear and time-invariant. Techniques, such as speech detectors, companding systems, procedures for noise suppression and particularly also echo cancellation are leading to the fact that the conditions of the systems are stable during particular periods only. Conventional measurement procedures, such as sine wave measurements are no longer suited for determining the transfer functions of such systems. Therefore, a new measurement procedure has been developed. An appropriate test signal is an integral part of the measurement procedure to ascertain the transfer functions of non-LTI (linear time-invariant) systems. On the one hand. such a signal must have properties similar to voice and on the other hand, it requires an exact technical definition. To meet all these requirements a test signal has been developed which is called composite-source signal. This test signal consists of three components: (1) Voiced sound 'artificial voice' according to CCITT Recommendation P.50 to activate the system under test; (2) Test signal, a pseudo-random-noise signal the length of which is exactly adapted to the length of the Fourier transform used; (3) Pause; The length of the pause has been chosen so that the repetition frequency of the complete sequence corresponds approximately to the average modulation frequency of voice. The above-described measuring sequence which can be fed into both talking and listening direction simultaneously allows to determine the transfer functions, time constants, loudness ratings but also specific parameters, such as adaptation time, etc. of echo cancelers. Various compositions of the three components can be used in order to meet the requirements of different measurement tasks. The corresponding measurements as compared to standard measuring methods are introduced and further measuring possibilities are discussed.