DTMF (dual tone multi frequency) is the signal to the phone company that you generate when you press an ordinary telephone's touch keys. In the United States and perhaps elsewhere, it's known as "Touch-Tone®" phone (formerly a registered trademark of AT&T). With DTMF, each key you press on your phone generates two tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group. Here are the signals you send when you press your Touchtone phone keys:
In technical words: Dual Tone Multi-Frequency, or DTMF is a method for instructing a telephone switching system of the telephone number to be dialed, or to issue commands to switching systems or related telephony equipment.
The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as '1' ) will send a sinusoidal tone of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multifrequency. These tones are then decoded by the switching center to determine which key was pressed.
DTMF keypad frequencies table
DTMF event frequencies
The engineers had envisioned phones being used to access computers, and surveyed a number of companies to see what they would need for this role. This led to the addition of the number sign (#) and star (*) keys (also known as humphries), as well as a group of keys for menu selection: A, B, C and D. In the end, the lettered keys were dropped from most phones, and it was many years before the humphries became widely used for vertical service codes such as *67 in the United States and Canada to suppress caller ID.
Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive to network control. The levels of priority available were Flash Override (A), Flash (B), Immediate (C), and Priority (D), with Flash Override being the highest priority. For example, the A key is used on some networks to cycle through different carriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers. The A, B, C and D tones are used in amateur radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an
active phone line.
DTMF tones are also used by some cable television networks and radio networks to signal the local cable company/network station to insert a local advertisement or station identification.
Spectral Analysis of Dual-Tone Multi Frequency (DTMF)
References:
searchnetworking.com/
nemesis.lonestar.org/
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2 comments:
DTMF (Dual-tone Multi Frequency) is a tone composed of two sine waves of given frequencies. Individual frequencies are chosen so that it is quite easy to design frequency filters, and so that they can easily pass through telephone lines (maximum bandwith extends from 300 Hz to 3.5 kHz). DTMF was not intended for data transfer; it is designed for control signals only.
As we know that DTMf transmit most often, dedicated telephony circuits are used to generate DTMF (for example, MT8880). On the other hand, a microprocessor can do it, too.Just connect a RC filter to two output pins, and generate correct tones via software.However, getting the correct frequencies often requires usage of a suitable Xtal for the processor itself.
But let us know how to decode DTMF ,It is not easy to detect and recognize DTMF with satisfactory precision.
Here Rose somehow said correct that It is not easy to detect and recognize DTMF with satisfactory precision.so I want to share my some knowledge about DTMF, dedicated integrated circuits (IC's) are used. The functional solution for DTMF transmission and receiving by a microprocessor (a PIC in most cases) exists. It is complicated, so it is used only marginally.
Here Rose said telephony circuits are used to generate DTMF (for example MT8880). Most often, a MT 8870 or compatible circuit would be used.
Most decoders detect only the rising edges of the sine waves. So, DTMF generated by rectangular pulses and RC filters works reliably. The mentioned MT 8870 uses two 6th order bandpass filters with switched capacitors. These produce nice clean sine waves even from distorted inputs, with any harmonics suppressed
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