Who came up the the sounds dialed numbers make on a telephone?
Are they universal tones?
Do range musically?
Do you think they’ll ever change/disappear?
The touch-tone system (officially known as DTMF (dual tone multi frequency)) was invented at Bell Labs. Each button is actually a combination of two tones, one for the row and one for the column. The frequencies were selected so that no two were harmonics of each other, which makes detecting the frequencies at the switching equipment easier. Though regular telephones have three columns and four rows of buttons, a 4x4 grid of frequencies was created, and some private telephone systems (including an obsolete military system called AUTOVON) used the fourth column. (Labeled A, B, C, and D; not related to the numbers associated with each digit.)
The frequencies are:
1209Hz 1336Hz 1477Hz 1633Hz
697Hz 1 2 3 A
770Hz 4 5 6 B
852Hz 7 8 9 C
941Hz * 0 # D
Thus, the tone for 5 would be 770Hz and 1336Hz played simultaneously. Doing it in this way makes phones easier to build, since you only need to generate eight frequencies instead of 12 or 16, and the tone decoders at the other end only need eight bandpass filters; the combination from one of each group yielding which button was pushed.
The DTMF system is standard in most parts of the world, and there’s no good reason to change it as it works pretty well.
9878999888999987899988987
A telephone FAQ (search for the question “What frequencies do touch tones use for which numbers?”) should answer your first three questions.
As to when they will disappear: They will disappear when all telephone lines will have changed from analog to digital (ISDN or a successor technology). In digital phone systems numbers are not signalled by tones but are inaudibly included in the data stream. Of course phone manufacturers might theoretically then chose to send tones to the handset loudspeaker to provide feedback that a button has been pressed, but that is not the case with the types of ISDN phones that I have used to date.
interesting…very cool…thanks
…with fleece as white as snow :)
I would say it makes detecting the correct tones more accurate, not easier. Just a minor nitpick, cause I’m me.
What friedo says is true. in some systems (or at least in Sweden) there is also an ‘R’ button. This is treated separately, and in fact equates a very short ‘click’, or a opening of the line. (It can be emulated by ‘hanging up’ for a fraction of a second.)
That’s how the old-fashioned rotational dial phones worked, by the way. The numbered disc would disconnect the line for a short duration several times as it moved back from the number ‘dialed’. (Bonus point for anyone who can guess where that word came from :).)
Which means that, if your phone company still supports dial phones (I think most still do), you can call a number by repeatedly tapping the hang-up button.
I don’t have any links to the circuitry here, but the contacts in the phone’s dial don’t disconnect the entire phone from the line or act quite as if you were tapping the switch hook. Some dials actually have two or three sets of contacts that are activated when dialing.
As for a button labeled “R” - In America, that tends to be Redial. The “tap switchook briefly” function is called Flash, and is usually either labeled as Flash or “F”
Another reason why they are duel tone is to prevent false dialling caused by stray tones or whistles on the line. Thus both tones for each number have to be received before the equipment will react.
Could you tell us what are the two frequencies for this “R” button? I suspect it may be A, B, C or D in the chart above. I doubt there would be different frequencies just for that button.
It’s not likely to be any frequencies at all, and instead, simply signals the phone’s circuitry to execute a switchhook flash.
Nope!
As I tried to explain earlier, it’s just a short ‘click’ (can be emulated by tapping the hang-up button once).
Perhaps not - digital cell phones still use DTMF during a call, because you need DTMF to interact with voice menus. (“For sales, press 1. For technical support, press 2…”)
Do ISDN phones generate DTMF when you press the numbers during a call, or do they send digits through the data stream (which are presumably translated into DTMF at the phone company)?