Devices for Controlling Amplitude Characteristics of Telephonic Signals

T HE transmission of speech energy over electrical circuits is attended by the interesting and sometimes difficult problem of preserving the original signal in spite of limitations in the transmission medium. These limitations include load carrying capacity, interference with other service, noise, change in attenuation with time and many others. Because of special limitations it is sometimes desirable to alter the amplitude characteristics of the speech or other signal energy without, of course, materially lowering its intelligibility. In high quality systems the peak voltage from some speech sounds of a given talker may be over 30 db (some 30 times) higher than from his weakest sounds when there is very little inflection in the speech. Loudness changes for emphasis will increase this range of intensities. Ordinary message systems do not have to contend with quite so wide a range of instantaneous voltages from a single talker, but different talkers under extreme terminal conditions produce about a 45 db range of average voltage, which is additive to that for a single talker. Consequently, a voltage range of about 70 db (over 3000 to 1) must be considered for message circuits. In order to accommodate such ranges of intensity to certain transmission media such as radio links a new family of automatic devices has been developed. In general all of these contain amplifiers or attenuating networks whose loss or gain is changed according to some function of the applied input and which may have a variety of time sequences in their control circuits.