Because each number on the telephone keypad corresponds to a different sound

You will all have noticed it: when we press one of the numbers on the telephone keypad, a different sound is emitted depending on the key. Phone keypad button sounds were introduced in the 60's when the DTMF (Dual-Tone Multi-Frequency), a multi-frequency coding system that assigns precise audio frequencies to numeric codes. Each key is associated with a different sound signal because in this way the device is able to report to the telephone network the number or symbol pressed on the telephone keypad. Each button on the keyboard is associated with a specific pair of frequencies, one high and one low. The moment a button is pressed, the phone generates these two frequencies at the same time and the telephone network uses these tones to identify which button was pressed.

What is the DTMF system and how it works

But when was this system developed and who invented it? In the 1950sthe US telephone operator AT&T (American Telephone and Telegraph Company) began looking for alternative phone number input methods to the pulse dialing method. Do you know the “classic” entry of numbers using the iconic rotating ring present on “old” phones? In practice they were looking for a way to replace it.

AT&T noted that the method of MF signaling (multi-frequency) was having so much success connecting distant telephone exchanges to make long-distance calls that it decided to offer a version of it to its customers. This was how the DTMF system.

This method replaced rotary dials by introducing a keyboard to telephones that had four columns and four rows. The numbers from 1 to 9 were distributed among the first three rows and three columns, lo 0 it was located on the key in the second column, in the fourth row, and on the sides of the latter (therefore always in the fourth row) were positioned the keys bearing, respectively, the symbol * (asterisk) e # (hashtag). As for the fourth column, the letters had been assigned to it TO, B, C And Dstill present today mainly on some radio equipment for controlling the internal network at telephone companies.

Credits: WebArchive.

But how does the DTMF system technically work? Basically assigning 8 different sound frequencies to the rows and columns of the keyboard: low frequency signals are assigned to the former; the latter are assigned high frequency signals.

When you press a key corresponding to a certain number or symbol, the telephone generates a tone that simultaneously combines the high-frequency signal coming from the column in which it is located with the low-frequency sound signal relating to its corresponding row. For example, if the key is pressed 4a is issued combined acoustic signal of 1209 Hz and 770 Hz and sent to the telephone company, which is able to determine the button from which the impulse comes.

The use of two tones, rather than just one, is essential to guarantee the correct functioning of the system, which is thus able to accurately distinguish the different numbers and symbols typed even if false signals were to occur due to interference or background noise.

Telephones based on the DTMF system were made available to the public Monday 18 November 1963. At the time the AT&T company registered the trademark “Touch Tone” for DTMF technology and, not surprisingly, the phones that supported this system were renamed Touch-Tone phones.

How we use the DTMF system today

Despite its “age,” the DTMF system has survived to this day as the standard method for making phone calls.

This meant that the numeric keypad layout of current landlines and smartphones themselves is practically the same as created for DTMF tone technology (with the exception of the fourth column in which the first four letters of the alphabet appeared).

Furthermore, the DTMF system allows the use of special codes to perform certain operations, including call blocking and turning off caller ID. The DTMF system is also used for interact with the telephone menus of the assistance numbers. Amazing what is hidden behind the numeric keypad tones!