Speech-volume measurements on telephone circuits

Knowledge of the electrical levels of speech signals at the input to speech-transmission systems is required for (a) study of the talking behaviour in telephone conversations (b) provision, from measurements under service conditions in the field, of basic data for estimating the power loading of multiplex transmission systems. For these purposes, an instrument has been designed which measures the r.m.s. voltage of the signal when integrated over a given period of several seconds. To cater for fragmentary speech, the total integration period is not fixed, the timing being stopped during silent periods. The period of `active¿ speech over which integration takes place is 10s. Accurate square-law integration over a sufficiently wide range of amplitude caters for the dynamic range of speech from a talker under conversation conditions. The operate and release levels, and operate and hangover times, of the timer switch have been chosen so that timing is not stopped during structural pauses, but only when a listener would have judged that the talker had stopped. Calibration is in decibels relative to 1V (dB V), and so conversion to decibels relative to 1mW (dBm) into an impedance of 600¿ requires the addition of 2.2dB to each reading. With this allowance, the reading may be regarded as `the long-term mean power while the talker is active¿. The instrument has been used to measure the distribution of speech volumes at the input to a transatlantic cable system and to make an estimate of the activity factor of the conversational speech so measured.