DTMF is a signalling system for identifying the keys say the number dialed on a pushbutton or DTMF keypad. DTMF (Dual tone multi frequency) as the name suggests uses a combination of two sine wave tones to represent a key. These tones are called row and column frequencies as they correspond to the layout of a telephone keypad.
A DTMF keypad (generator or encoder) generates a sinusoidal tone which is mixture of the row and column frequencies. The row frequencies are low group frequencies. The column frequencies belong to high group frequencies. This prevents misinterpretation of the harmonics. Also the frequencies for DTMF are so chosen that none have a harmonic relationship with the others and that mixing the frequencies would not produce sum or product frequencies that could mimic another valid tone. The high-group frequencies (the column tones) are slightly louder than the low-group to compensate for the high-frequency roll off of voice audio systems.          The row and column frequencies corresponding to a DTMF keypad have been indicated in the above figure. DTMF tones are able to represent one of the 16 different states or symbols on the keypad. This is equivalent to 4 bits of data, also known as nibble.

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 When you press the buttons on the keypad, a connection is made that generates two tones at the same time. A "Row" tone and a "Column" tone. These two tones identify the key you pressed to any equipment you are controlling. If the keypad is on your phone, the telephone company's "Central Office" equipment knows what numbers you are dialing by these tones, and will switch your call accordingly. If you are using a DTMF keypad to remotely control equipment, the tones can identify what unit you want to control, as well as which unique function you want it to perform.
When you press the digit 1 on the keypad, you generate the tones 1209 Hz and 697 Hz. Pressing the digit 2 will generate the tones 1336 Hz and 697 Hz. Sure, the tone 697 is the same for both digits, but it take two tones to make a digit and the decoding equipment knows the difference between the 1209 Hz that would complete the digit 1, and a 1336 Hz that completes a digit 2.
Two-Way Radios:
Just as you dial your telephone to reach another person, you can use transmit DTMF signals over a radio, and turn things on and off, flash lights, control motors, cameras, activate warning systems, turn on irrigation systems, open gates, and in general control the world! You may have heard some tones at the start of some National News Broadcasts. These are DTMF tones that send out at the start of the broadcast to transfer (or alert to transfer) their audio onto the local affiliates airwaves. Basically, it turns on a master switch. Used over two-way radios, you can transmit a DTMF "phone number". You have the same "phone number" programmed in a decoder hooked up to a radio receiver at a remote location. When the decoder sees its "phone number" come in over the radio, it wakes up and gets to work controlling the things you have hooked it up to.With the better decoders (of course all the Genave decoders are the better ones) you are able to take phone numbers a step further
Bhanu Namikaze

Bhanu Namikaze is an Ethical Hacker, Web Developer, Student and Mechanical Engineer. He Enjoys writing articles, Blogging, Solving Errors and Social Networking. Feel Free to let me know any of your concerns about hacking or let me know if you need any more methods on hacking anything. Enjoy Learning

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