DTMF stands for Dual Tone Multiple Frequency. It is a tone consisting of two frequencies superimposed. Individual frequencies are chosen such that it is easy to design filters and easy to transmit the tones through a telephone line having bandwidth of approximately 3.5 kHz. DTMF was not intended to be used for data transfer, it was meant to be used for sending the control signals along the telephone line. With standard decoders it is possible to send 10 beeps per second i.e., five bits per second. DTMF standard specifies 50ms tones and 600ms duration between two successive tones.
Note that the last column is not commonly seen in the telephones that we used, but telephone exchanges use them quite often. Nowadays, DTMF is used for dialing the numbers in telephones, configuring telephone exchanges etc. A CB transceiver of 2.7 MHz is normally used to send floating codes. DTMF was designed to be able to send the codes using microphone. Each beep (or digit you dial on the telephone) is composed of two concurrent frequencies, which are superimposed on amplitude. The higher of the two frequencies is normally aloud by 4dB, and this shift is termed as twist. If the twist is equal to 4dB, the higher frequency is loud by 4dB. If the lower frequency is loud, then the twist is said to be negative.
Generating DTMF
DTMF signals can be generated through dedicated ICs or by using RC networks connected to a microprocessor. MT8880 is an example of a dedicated IC. But getting the latter method work is a bit difficult if high accuracy is needed. The crystal frequency needs to be sacrificed for a non standard cycle length. Hence this method is used for simple applications. Most often, a PIC micro could be used for the above purpose.
Decoding DTMF
Detecting DTMF with satisfactory precision is a hard thing. Often, a dedicated IC such as MT8870 is used for this purpose. It uses two 6th order band-pass filters using switched capacitor filters and it suppresses any harmonics. Hence they can produce pretty good sine waves from distorted input. Hence it is preferred. Again microprocessors can also be used, but their application is limited.
3 comments:
Dear blogger,
Great post.. I was wondering if one could simulate a keypress on a phone by playing the right DTMF signal sound from, say PC loudspeaker and direct that sound to the phone?
DTMF signalling .. thanks for the informative article, I have looked around and it seems that DTMF generators have sunk without trace .. the only one I can find is by Holtek [9200]do you know any others that are still current... I am really suprised I would have thought there would have been loads of devices available given the fact that DTMF is used all over the place and the simplicity of not having to generate within a microprogram!!
Cheers
Jon
we want to DTMF type fixed line phone by using to insert a coin(desktop pay phone) if possible circuit design ok for us or what way you can help us we want 1000 pc per month in india.
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