You will have all noticed: when you press one of the numbers on the phone keypad, a different sound is emitted depending on the key. Phone keypad button sounds were introduced into the 60s when the DTMF (Dual tone multifrequency), a multi-frequency coding system that assigns precise audio frequencies to digital codes. Each key is associated with a different sound signal because the device is thus able to signal 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. As soon as 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 American telephone operator AT&T (American Telephone and Telegraph Company) began looking for alternative phone number entry methods to 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 so successful for connecting remote telephone exchanges to make long distance calls that it decided to offer a version to its customers. This is how the DTMF system.
This method replaced rotary dials by introducing a keypad on phones with four columns and four rows. The figures of 1 has 9 were distributed between the first three rows and three columns, lo 0 it was located on the key of 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 the symbol respectively * (asterisk) e # (hashtag). As for the fourth column, the letters had been assigned to it HAS, B, VS And Dstill present today mainly on certain radio equipment used to control the internal network of telephone companies.
But how does the DTMF system work technically? Basically assign 8 different sound frequencies to the rows and columns of the keyboard: low frequency signals are assigned to the former; the latter receive high frequency signals.
When you press a key corresponding to a certain number or symbol, the phone generates a tone that simultaneously combines the high frequency signal coming from the column it is in with the low frequency sound signal relating to its corresponding line. For example, if the key is pressed 4one is issued combined acoustic signal of 1209 Hz and 770 Hz and sent to the telephone company, which is able to determine which button the impulse is coming from.
The use of two tones, rather than just one, is essential to ensure the proper functioning of the system, which is thus able to accurately distinguish the different numbers and symbols entered even if false signals should occur due to interference or background noise.
Telephones based on the DTMF system were made available to the public Monday November 18, 1963. When AT&T registered the trademark “Touch Tone” for DTMF technology and, unsurprisingly, phones supporting this system have been renamed Touch-tone phones.
How we use the DTMF system today
Despite its “age”, the DTMF system has survived to this day as a standard method of making telephone calls.
This meant that the numeric keypad layout of current landlines and smartphones is practically the same as that created for DTMF tone technology (with the exception of the fourth column in which the first four letters of the alphabet appear).
Additionally, the DTMF system allows the use of special codes to perform certain operations, including blocking calls and disabling caller ID. The DTMF system is also used to interact with phone menus of support numbers. Amazing what is hidden behind the tones of the digital keyboard!
