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Telephone dialling challenge

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Back in the days when people still had faxes, it was not uncommon to be phoned by a fax machine, which would then whistle and peep at you. If you whistled back, it was possible to confuse it, so it would persist for longer before hanging up.

Today, alas, fax machines are far and few between. However, all phones these days use dual-tones, both for dialling and for allowing the user to enter numbers or select menu items. The dual-tone system uses pairs of audio-frequency tones in various combinations to represent digits (or other keys). The tones needn't be generated by the phone itself - any suitable sound-generator, held close to the phone's microphone, will work just as well.

Now. It is possible to whistle and hum at the same time, and it is also possible to whistle one note whilst humming another. Doing so in a controlled way is extremely difficult, but not completely impossible. It should therefore be possible to dial a telephone number using nothing but oneself (and a phone, obviously, but not its keypad).

MaxwellBuchanan, Feb 18 2019

Dtmf https://blogs.unime...hind-phone-numbers/
[theircompetitor, Feb 18 2019]

DTMF at Wikipedia https://en.wikipedi...frequency_signaling
A reasonable explanation [neutrinos_shadow, Feb 19 2019]


       There are in fact people who can do this having problem finding info, but has to do with ability to generate the two tones
theircompetitor, Feb 18 2019

       My dad perfected the "boop" made by supermarket barcode scanners. Saved/stole him literally pounds worth of stuff over the years. I'm sure he'd approve.
bs0u0155, Feb 18 2019

       //There are in fact people who can do this// That is very disconcerting.
MaxwellBuchanan, Feb 19 2019

       read the linked article -- I've seen better pages on that but they may have link-rotted. I was involved in building telephony systems late 90s early 2000s and I remember reviewing DTMF related material and seeing mention of that.
theircompetitor, Feb 19 2019

       //whistle and hum at the same time... Prime bagpipes territory then.   

       Umm, didn't phone phreaking occur somewhere between the Earth coalesced and the Teenage Mutant Turtles?
not_morrison_rm, Feb 19 2019

       Somewhere (I thought it was at Wikipedia (link) but it's not...) I read about why those particular frequencies are used. Something to do with no harmonics between any of them, so all tones and combinations of tones are completely unique and non-interfering.
Also, sometimes (in a completely random and uncontrolled way) I can whistle 2 tones at once, and there are people who can sing 2 notes at once.
neutrinos_shadow, Feb 19 2019

       So, a Scotsman piper dials up a Mongolian. The Mongolian says, hang on, let me conference in my Inuit pal...
RayfordSteele, Feb 19 2019


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