TABLE OF CONTENTS We've looked at state-of-the-art high-speed inter-IC communications protocols, but we'll see there's still life in the 1960's touch-tone signaling technology. (touch-tone was a registered service mark of AT&T, but is now a public domain term.) We'll see how to decode touch-tone signals with a special purpose decoder IC and build an automatic remote control answering machine to receive touch-tone commands.
Touch-tone is an example of dual-tone multifrequency (DTMF) signaling. As the name suggests, touch-tone involves simultaneously sending two tones from a set of multiple tones. As shown in Figure 17-1, each row and each column in a standard keypad is assigned a unique audio tone frequency. (Only a few specialty telephones have column 4 keys.) When you press a button, its row and column frequencies are simultaneously sent. Figure 17-2 shows the resulting envelope waveform, and Figure 17-3 shows the corresponding spectrum display. Figure 17-3 shows the spectrum of the "*" key, involving simultaneous transmission of 941 Hz and 1209 Hz tones.
Touch-tone is not the only example of DTMF used within the telephone network. At one time, the Bell System extensively used "inter-office" DTMF another 4 4 matrix system but with tone frequencies incompatible with touch-tone to set up calls within its network. Inter-office multifrequency signaling along with other "in-band" signaling however, turned out to be vulnerable to hacking and has largely been replaced by more efficient...
