Friday, 26 February 2016

Why touch-tone phones needed both # and *

I was curious about finding the answer to this rather basic (but just as inconsequential), question (also an obsolete one, in this day of smartphones). At any rate, I was only able to arrive at an answer by piecing together information from a variety of sources. So, to make things simpler for anyone else who ever wonders the same thing, I'm putting up my answer here. It's a deduction, so I may be wrong, but it makes enough sense that I'm fairly confident in it.

The first thing to be clear about is the purpose of the two buttons, which we so rarely use. Interestingly, the engineers who designed them weren't entirely sure what they would be used for, either, but they could anticipate that they might be necessary for some future type of telephone-computer interaction, and toured a number of businesses canvassing for ideas about what applications they might develop and what they would need. They did know that the phone company would be implementing vertical service codes, which were distinct from phone calls and would require a special prefix. They could have defined a reserved numeric prefix (like the later infamous 10 10 numbers), but using the * key turned out to be more pragmatic. (I still remember the *67 and *69 VSCs from my youth.)

The biggest use of having a non-digit input was that it enabled input of variable-digit sequences in applications, such as a conference id, or to allow more then ten choices if you need them for a voice menu (and I pity you if you do). By ending with the # key, the system can distinguish 1 from 10 and know when you've finished making your choice.

I knew that much, but what irked me is why there had to be two symbols. Neither one of the two uses for the keys requires the use of both of them, so what's the point of making two?

The answer lies in the way the touch-tone system was built, a system called dual-tone multi-frequency signalling (DTMF). Unpacking the name, there are two tones that are sent simultaneously with each key press (a low tone and a high tone). Since it would have been science fiction in the 1960s to use a microprocessor to emit the proper tones, the solution had to be built using basic electronics. And, the elegant way to do that (once ergonomic research, and the need to maintain the rotary phone's alphabetical order, had dictated that a 3-row square of digits was the layout to use), was to have the row of the keypad determine the low tone and the column to determine the high tone. So voilà, the first touch-tone phones had a layout that looked like this:

So, pressing 5 generates a tone at 1336 Hz and 770 Hz, etc. But what any efficiency-minded engineer notices is that the 941 Hz tone is underused, serving only for 0. So this is the key reason why two function keys were added: it didn't cost anything in terms of complexity to do so: there were essentially already two unused buttons built in to the system. If there had been an added cost to adding two keys, the need would have had to be justified and perhaps they wouldn't have been implemented. As it was, since they were revisiting the original design anyway, they already had cause to want to future-proof things. So in fact, it isn't so much that they needed to add * and # to the keypad, it's more the case that they were essentially already there. (In fact they added not two function keys but 6, adding a fourth column with keys A-D at 1633 Hz which was only ever used by the military, to set call priority. Again, the choice brings symmetry, since there are 4 rows and 4 columns in the full design, and no leftover unused frequency pairs.)

And, in point of fact, having two function keys would have allowed some large organisations to implement their own, internal vertical functions, prefixed with the # key, while still allowing access to the phone company's VSC's on the star key. (In practice large organisations would use a PBX which probably wouldn't allow direct access to the external telephone network anyway—as anyone who remembers dialing 9 before making an outgoing call from their hotel room would well remember.)

As for why the symbols * and # were chosen, it seems largely to do with their familiarity, since standard typewriters had them, although if they had only chosen @ instead of *, as they might well have done, we would have had an uncanny precursor to Twitter!

Posted by jon at 7:01 PM in Computers 
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