In my last legacy Public Switched Telephone Network (PSTN) post I covered pulse or rotary dial service. Let's look at dual tone multi frequency (DTMF) dialing service in this post.
The most commonly used method for inputting a number in the US and Europe is now the dual-tone-multifrequency (DTMF) signaling method. DTMF telephones are also commonly known as Touchtone telephones. These phones also use numerical keypads but offer an even faster way to signal the number to call by sending tones on the telephone line. The DTMF phone uses a 12-button keypad. When a button is pressed on the keypad an electric contact is closed and two oscillators generate two tones at specific frequencies.
Telephone DTMF Keypad
These tones combine to form one sound to the listener, just like when two different musical notes on an instrument are played at exactly the same time. The combined tones are a signal for the button that was pressed on the keypad. The frequencies used are illustrated in the keypad diagram. For example, notice when the number 8 is pressed the frequencies 852 Hz and 1336 Hz are combined to form the number 8 tone.
For the central office to accept tones from a caller, the tones must be at least 50 milliseconds long and also be separated by a 50 millisecond pause. DTMF phones offer much more rapid dialing of numbers than rotary pulse methods with the average phone number taking 10 to 15 times less time to dial using a Touchtone phone. Not only are Touchtone phones faster, they are also more reliable because they do not depend on as many moving parts as a rotary phone.
In my next legacy PSTN post, I'll describe how a telephone is made to ring.
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