How does a Touch-Tone phone fight background noise interference?

My father worked for the telephone company and he would occasionally demonstrate how phones worked. One lesson I remember is how you could use sound to dial phone numbers. I was pretty young, so this seemed like magic to me, but my sister who was older, wiser and more inquisitive asked if it was possible for the phone to hear a noise and dial a number by mistake. He explained how this was not easy because when you pushed the button you were actually “hearing” two tones being played at the same time.

I had always remember him saying this, but never really delved deeper into what he meant until yesterday when I was reading an old copy of Popular Mechanics and found this explanation that sums up the lesson I think he was trying to teach us.



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5 thoughts on “How does a Touch-Tone phone fight background noise interference?

  1. I’ve known about the tone thing since the mid-80s, thanks to mr wizard (one of my top fav shows back in the day). Here’s the clip:

    When I was in my teens we had a push button phone, BUT the lines or switchboard or whatever in town wasn’t setup to handle tones, so we had to set it to pulse mode. Bear in mind that this was the late 80s/early 90s, so the lines not being able to handle tones was quite antiquated.

  2. TheSixMillionDollarJedi says:

    Isn’t this what David Lightman does with the tape recorder while locked in the infirmary in Wargames? It’s not a phone but it’s touch a touch pad with tones that he records.

    Retroist, did you ever ask your dad about the pop can tab and the pay phone? That never seemed to work.

  3. timepilot84 says:

    I discovered this when I was a kid. If you pressed two buttons in the same row or column, it would only play the one tone common to both buttons.

  4. Max Power says:

    Similarly, rotary (pulse) dialing simulated tapping the switchhook. You can dial a phone by pressing the switchhook in the same pattern as the rotary dial would for the number.
    At a college job, I had a phone at my desk with no buttons – it was intended for incoming calls only. I was able to make outgoing calls with the switchhook.

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