Before revision: Text Messaging the Def-Tone System (DTS) This relay system became known commonly as the Def-Tone System (DTS) because the tones representing letters of the alphabet were eventually carried in tones outside the range of human hearing.
After revision: Text messaging and the Def-Tone System (DTS) This relay system became known commonly as the Def-Tone System (DTS) because the tones representing letters of the alphabet were eventually carried in tones outside the range of human hearing.