My apartment building has a secured entrance. For someone to gain entry, they have to dial my code and it rings directly to my phone. I can chat with the person via the phone and when I let the person in, I dial “5” on the phone pad and it buzzes them in. (Actually, it sends notice to the entry system, and that creates its own buzz which then lets them in.)
Now, when I press the “5” it creates a dual tone of 770Hz and 1336Hz. So if I wanted to let someone in using my guitar playing the two notes, which two notes would they be and would I have to retune the guitar off of A=440Hz? (Assuming the system would accept the guitar tones).
770Hz is closest to G5 (783.99Hz) and 1336Hz is closest to E6 (1318.51Hz)[sup]1[/sup].
You’d need to re-tune them to be closer, as the bandpass filters on a DTMF decoder are fairly narrow.
[sup]1. In the standard 12-tone equal-tempered scale with A=440Hz.[/sup]
Looks like there’s an E note at about 330 Hz. Sharpen that to 335 and go up an octave and you’ll have your 770. Sharpen it to 334 and go up two octaves and you’ll have 1336. If the system is very precise, it would be tough to get the exact tones from the guitar.
One octave from 335 is 670 , not 770, I’m sure that was just a typo.
The interval from 770 to 1336 is about 9-1/2 semitones. You couldn’t do this unless your tuned two strings so they weren’t a correct interval from each other or you could perhaps bend one note a half of a halftone and not the other as long as you had the right base pitch.
Another matter for the DTMF decoder to decide if it likes the guitar’s waveform and harmonics. It’s probably expecting a squarewave which IIRC is first order harmonics… man, it’s been 25 years since I studies analog signal processing.
My cousin has worked for one or another telephone company for many years and he told me about this guy they knew was making calls from payphones without paying.
The company couldn’t figure out how he was doing it, so they hired him. The dude had discovered that at that time, the coins generated a computer type access sound and he had been whistleing it into the phone.