A Digital Simulation of the Telephone System

In recent years, an increasing number of systems for speech communications have been proposed which must operate over both wideband channels and standard telephone connections.1"5 Included among such systems are waveform coders, speech analysis-synthesis systems, and systems for man-machine communication by voice--e.g., speech 839 recognizers or speaker verifiers. To investigate the capabilities of such systems in a controlled manner, it is useful to be able to simulate the effects of the telephone channel on the speech signal as well as the other signal processing parts of the system. The conventional approach to telephone simulations of speech processing systems is to repeatedly dial up a new line for each input utterance to obtain a reasonable distribution of lines. However, not only is this method clumsy, but it does not guarantee good statistical sampling of telephone lines. As an alternative, it would be desirable to substitute a controlled simulation of a telephone channel which attempted to model the system from the handset to the earphone--that is, the telephone carbon button, the telephone line, the switching, and the receiver. Figure 1 is a set of block diagrams that illustrate the various types of simulations one can consider using. The simplest simulation, in Fig. la, is an "end-to-end" simulation in which the input signal (to either the telephone line or the black box simulation) is obtained directly from a talker, and the simulation output is sent directly to a listener.