Speech Signal Power in the Switched Message Network

The characterization of speech signal power on Bell System message circuits is an essential step in the determination of signal power loading and crosstalk objectives. Knowledge of speech signal characteristics is also important to designers of a wide variety of telecommunications equipment. Speech levels at the class 5 office were last characterized in the 1960 Speech Volume Survey1 in terms of volume units (VU). In the years since 2695 the last survey, there have been substantial changes in the Bell System network. For example, the proportion of toll grade battery has been substantially reduced, the 300-type telephone set has been almost completely phased out, direct distance dialing is now virtually universal, and new loop and trunk design methods have been introduced. Also, in the intervening years, research in speech signal measurement has led to a new measure of speech level known as the equivalent peak level (EPL).2 This, together with advanced digital data acquisition technology, has facilitated the measurement of speech signal power with greater precision than was possible in 1960. This paper presents the results of a speech signal power survey made in 1975-1976. The measurements were made at 36 class-5-office main distributing frames (MDFs), which constitute a statistical sample of acceptable precision from all the MDFs within the Bell System. The class 5 (local or end) office MDF was selected as the measurement interface because it has access to all customer loops and all classes of local and toll traffic; dialed address information is readily available; only the customer's loop and station equipment is interposed between the customer and the point of measurement; and the customer's loop current may be measured.