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What are DTMF Tones?

A touch tone telephone, which uses DTMF tones.
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  • Written By: Ken Black
  • Edited By: Bronwyn Harris
  • Last Modified Date: 01 November 2014
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Dual Tone Multi Frequencies (DTMF tones) are two different tones at two ends of a spectrum that are used to send information in telephonic communication mediums. The tones represent numbers 0-9 and the symbols * and #. A series of these numbers may be assigned to individuals as points of contact, as in a phone number, or the numbers can be used to relay other sorts of information.

While it may seem confusing, it should be noted that all numbers on a telephone are assigned two DTMF tones, and in all, seven different frequencies are assigned. Some numbers will have the same lower frequency, but different higher frequency, while others will have the same higher frequency but a different lower frequency. The range is between 697 Hz and 941 Hz for the lower frequencies and 1209 Hz and 1633 Hz for the higher frequencies. There are actually eight different tones, with a higher tone for the letters A, B, C, and D as well, though these are not used on a standard telephone.

DTMF is what makes touch tone dialing possible. In the earlier days of numbered telephone systems, the rotary dial was used, and dialing a phone number was accomplished by breaking up a steady tone. The number of breaks in the steady tone indicated the number that was desired. Since then, DTMF tones have made dialing a much faster and simpler process.

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The idea behind using two different tones to indicate a number was so that it would be more difficult to accidentally reproduce the sound and get an undesired result. While this may only be a slight inconvenience when dialing a number, it could be a much greater inconvenience when trying to enter personal information into a telephone system. It is impossible for the human voice to represent these tones.

In addition to being a much faster way to dial, DTMF tones also provide another major benefit: they enable the caller to communicate with a computer system using the numbered keypad. Many people are familiar with call systems that assign options to different numbers and ask the caller to press a number to indicate your desired option. In those cases, the system is actually recognizing the frequencies assigned to that specific number or group of numbers.

Now, DTMF tones are being replaced with powerful voice recognition software in many automated answering systems. This enables the user to speak naturally and be recognized, which could be of big benefit to those who have a hard time seeing the telephone keypad. Still, if the system fails to recognize a response, it may also give the option of using the telephone keypad.

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anon304707
Post 6

I've noticed that when dialing fast, the same digit repeatedly might end up making the DTMF recognition fail. Let's say the number I intend to enter is 433322298. It might happen that only 4332298 is recognized. I've seen this with an IVR system. How is this explained?

anon282938
Post 5

As for the 'inline decoder' for the fax, I don't think that's possible, since fax machines are given regular phone numbers. If they were to start giving fax machines their own special type of numbers you could call, and assign a special new set of frequencies to them. But then you wouldn't need your decoder.

Most folks I know end up having to get a second line, then just use it solely for their fax machine. Of course not everyone wants to pay extra money to have a dedicated line, especially if they don't use their fax machine enough to justify the cost of a dedicated line. Now days fax machines are starting to die out in favor for emails. Plus you can send faxes online now, rather than having to use a fax machine, which is more convenient and doesn't cost anything, so folks are able to totally do away with their fax machines altogether. I personally like fax machines though.

anon86755
Post 3

So, what are the frequencies for each digit?

anon72710
Post 2

There should some kind of little circuit that identifies the phone number from fax number.

This circuit should be between your phone and telephone line. That is some kind of DTMF decoder.

jschrader
Post 1

I am looking for a solution that will allow me to dial a fax number(s) and have it not ring the phone, but automatically send the fax. What do you think?

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