how do phone trees/call systems work? How does the computer recognize which number you are pressing?
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If you’re old, like me, you’ll be familiar with the term “touch-tone.” This refers to something called *dual-tone multi-frequency signaling* (DTMF), and this is how phone trees work.
The next time you play with your phone, open the keypad and press some buttons. You should hear your phone play a *tone* when you *touch* the buttons — hence, touch-tone.
These are actually two tones played atop one another, which is why certain buttons sound similar. For example, pressing the number 1 makes your phone play a 697Hz tone and a 1209Hz tone together; the number 2 plays 697 and 1336Hz.
At the far end of the line, a digital signal processor can “hear” which two tones are coming in, and therefore it knows what button was pressed.
Strictly speaking, cell phones don’t use DTMF for dialing anymore, but they still do for purposes like you describe.
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