What is DTMF? Science

Just push a button…

DTMF - does that make any sense to you? Read on to find out more in this week's An eye for an i...  

Communication can be a tricky business. We do it everyday, yet we never seem to be perfect at it - we are always learning. A miscommunication owing to an assumption in the conversion of units and a mission, that’s worth millions of dollars, fails. And yet, we are increasingly glued to our communication devices, taking them for granted. For we believe that the mistakes stem out of the human error and not the device or communication channels themselves…

Terms like “engaged tone,” “busy signal” and “dial tone” have crawled into our vocabulary, sometimes even when we are speaking in our local languages. Their usage seems so intrinsic that we hardly pause to think as to how they came about. But now that you are doing that, you might realise that the ‘tone’ and ‘signal’ indicate a relation to frequency.

Actually, not long ago, and we were still using rotary phones. Ask your grandparents and they might be able to tell you how it was when it took time to dial each digit, as they had to rotate a mechanical wheel or the rotary dial. In fact, it was only on November 18, 1963, that touch-tone services were introduced, marking in a way the beginning of the revolution that we are part of now.

Optional rollouts

The push-button telephone began as optional rollouts in Carnegie and Greensburg, United States. They were 10-button sets (* and # keys were added only in 1968) that slashed the dialling time drastically, changing everyday communication.

The dials were no match for touch-tone services and they were gradually replaced. Touch-tone, which was a former registered trademark of Bell Telephone Co., employed Dual Tone - Multi Frequency (DTMF) as the basis of the telephone. DTMF is the global standard for audible tones that represents the digits on a phone keypad.

Each button represents a frequency

So when you are making a phone call, you are pressing the different digits that comprise the phone number. Each row and column has a specified frequency. DTMF is a 4x4 matrix (the 4th column has kind of become obsolete these days) with each row representing a low frequency and each column corresponding to a higher frequency.

So if you are pressing the digit 1, it sends a sinusoidal tone for each of the two frequencies 697 Hz and 1209 Hz. Pressing 2 would evoke 697 Hz and 1336 Hz and in this way each digit can be uniquely identified, allowing the telephone company to switch the call accordingly. National telephone systems also designate specific frequencies to indicate the status of lines like busy signal and dial tone.

DTMF is here to stay?

Even as we move into the world of smartphones, DTMF continues to stay. As communication shifts from voice to data-driven services, including messaging and global positioning, keypads have evolved to facilitate the same. The 4x3 grid with the numbers from 0-9 along with * and #, however, remains the primary way in which calls are made.


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Printable version | Oct 28, 2021 5:58:02 PM | https://www.thehindu.com/in-school/sh-science/just-push-a-button/article6607637.ece

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