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4min chapter
Phone Tones
Twenty Thousand Hertz
The Bell System's Touch Tone Dialing
AT&T invented touch-tone dials in the early 20th century. The public didn't like them because they started at the bottom and went up instead of top to bottom. Bell developed a system where buttons produced two tones, one for each row or column. These were called dual-tone multi-frequency (DTMF) tones. DTMF tones are different from MF tones due to their odd pitches.
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