This is one I happen to know- in the early days of touch tone dialing, it was feared that 10-key operators would be able to type numbers too quickly for the system to process, so the keypad was deliberately turned upside down to prevent this.
divemedic is correct.
The Touch-Tone (more properly called Dual Tone Multi-Frequency or DTMF) system is specified as must accept tones greater than or equal to 40 mS long and must reject tones less than 20 mS long (analog bandpass filters were originally used for the tone detection and the spec enabled that type of system to actually work).
Other bits of trivia are that the row and column frequencies are as follow:
1209 1336 1477 1633
697 1 2 3 A
770 4 5 6 B
852 7 8 9 C
941 * 0 # D
Hence the digit 1 is composed of the two tones of 697 and 1209 Hz. # with 941 and 1477 Hz.
A rather odd selection of frequency, yes? It has to do with the avoidance of harmonics from the low frequency group showing up in the high frequency group and simulating a valid digit. If the selection was not so carefully made voice, music, background noise, etc. could simulate a valid digit while you were dialing and result in a wrong number. I have the original AT&T paper from the 1960s in my collection of DTMF information.
It didn’t completely solve the problem (20 mS must reject helps some too as well as numerous other criteria) and DTMF receivers were carefully tested to make sure they could avoid such digit simulations. Test for resistance to such digit simulations was called “Speech Immunity Testing” even though music was actually a tougher problem than speech.
Also of interest is the “extra” column of A, B, C, D in that matrix. It never took off in the U.S. but some European phones had those buttons available for the anticipated remote control of devices and things like “Telephone Banking”.
Also of possible interest is:
1) Once upon a time when I used an alias it was Mr. DTMF.
2) I am the sole inventor on a patent for a DTMF receiver.
3) I wrote my Masters Thesis on the speech immunity of DTMF receivers.
4) The Danish Telephone company once wrote a report on a DTMF receiver that I designed saying it was the only one in the world that met their specifications for end-to-end signaling.
5) An executive of a central office telephone equipment supply company once called me the world’s foremost expert on DTMF. I didn’t disagree with him.
I think it is time to shut up before I bore everyone to death because I could go on for hours and hours about DTMF…
Uh…. no.
At that time Al Gore was doing that I thought the future was with the telephone and related systems.
One should be very cautious of me predicting the technological future. I’m very good at solving problems but very poor at deciding which problems should be solved.
divemedic: In all honesty, I thought you were pulling my leg (in the same manner that the QWERTY keyboard was supposedly intended to “slow down” typists – not really, though), but it looks like my cynicism got the better of me.
I wonder if we could start a movement to get things standardized again… probably not – after all, we Western-worlders are still using rail gauges based on the distance between the wheels of just-barely-post-wheel-invention wagons… Joe: Well, that is a whole lot of information I never knew about how our phone system worked, as well as about you as an individual… consider me thankful and humbled . Of course, it still seems somewhat stupid to me that we resorted to reorienting the keys in order to force people to slow down, rather than have them simply semi-Pavlovianly learn by having their calls fail if they typed too quickly. I mean, once they got used to the upside-down format, I would imagine that their typing speed would go back to whatever it was before the transition.
Anywise, thanks for the information!
And, for some reason, my brain just thought about the possibility of using those ABCD “numbers”, and expanding them to allow for a hex-based phone number system (given how quickly we are undoubtely handing out phone numbers these days), but never mind that – most people could not do the math.
random quotes
“Toleration and liberty are the foundations of a great republic.” by Frank Lloyd Wright
This is one I happen to know- in the early days of touch tone dialing, it was feared that 10-key operators would be able to type numbers too quickly for the system to process, so the keypad was deliberately turned upside down to prevent this.
divemedic is correct.
The Touch-Tone (more properly called Dual Tone Multi-Frequency or DTMF) system is specified as must accept tones greater than or equal to 40 mS long and must reject tones less than 20 mS long (analog bandpass filters were originally used for the tone detection and the spec enabled that type of system to actually work).
Other bits of trivia are that the row and column frequencies are as follow:
1209 1336 1477 1633
697 1 2 3 A
770 4 5 6 B
852 7 8 9 C
941 * 0 # D
Hence the digit 1 is composed of the two tones of 697 and 1209 Hz. # with 941 and 1477 Hz.
A rather odd selection of frequency, yes? It has to do with the avoidance of harmonics from the low frequency group showing up in the high frequency group and simulating a valid digit. If the selection was not so carefully made voice, music, background noise, etc. could simulate a valid digit while you were dialing and result in a wrong number. I have the original AT&T paper from the 1960s in my collection of DTMF information.
It didn’t completely solve the problem (20 mS must reject helps some too as well as numerous other criteria) and DTMF receivers were carefully tested to make sure they could avoid such digit simulations. Test for resistance to such digit simulations was called “Speech Immunity Testing” even though music was actually a tougher problem than speech.
Also of interest is the “extra” column of A, B, C, D in that matrix. It never took off in the U.S. but some European phones had those buttons available for the anticipated remote control of devices and things like “Telephone Banking”.
Also of possible interest is:
1) Once upon a time when I used an alias it was Mr. DTMF.
2) I am the sole inventor on a patent for a DTMF receiver.
3) I wrote my Masters Thesis on the speech immunity of DTMF receivers.
4) The Danish Telephone company once wrote a report on a DTMF receiver that I designed saying it was the only one in the world that met their specifications for end-to-end signaling.
5) An executive of a central office telephone equipment supply company once called me the world’s foremost expert on DTMF. I didn’t disagree with him.
I think it is time to shut up before I bore everyone to death because I could go on for hours and hours about DTMF…
OK Joe, that is pretty cool. But did you invent the internet?
Uh…. no.
At that time Al Gore was doing that I thought the future was with the telephone and related systems.
One should be very cautious of me predicting the technological future. I’m very good at solving problems but very poor at deciding which problems should be solved.
divemedic: In all honesty, I thought you were pulling my leg (in the same manner that the QWERTY keyboard was supposedly intended to “slow down” typists – not really, though), but it looks like my cynicism got the better of me.
. Of course, it still seems somewhat stupid to me that we resorted to reorienting the keys in order to force people to slow down, rather than have them simply semi-Pavlovianly learn by having their calls fail if they typed too quickly. I mean, once they got used to the upside-down format, I would imagine that their typing speed would go back to whatever it was before the transition.
I wonder if we could start a movement to get things standardized again… probably not – after all, we Western-worlders are still using rail gauges based on the distance between the wheels of just-barely-post-wheel-invention wagons…
Joe: Well, that is a whole lot of information I never knew about how our phone system worked, as well as about you as an individual… consider me thankful and humbled
Anywise, thanks for the information!
And, for some reason, my brain just thought about the possibility of using those ABCD “numbers”, and expanding them to allow for a hex-based phone number system (given how quickly we are undoubtely handing out phone numbers these days), but never mind that – most people could not do the math.