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[SOLVED] What happens in DTMF when a key is pressed?

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moonnightingale

moonnightingale

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Aspermy understanding
when a key is pressed both the frequencies are added
is it correct

for example when 1 is pressed then

697 and 1209 Hz are added to make one waveform
 

Re: what happens in DTMF

As per my understanding ...
the frequencies of the waves are not added..
the amplitudes of the waves of those two frequencies get added....
if you add the frequencies the resulting wave will be of sinusoidal in nature with higher frequency...but in this case resulting wave is not sinusoidal..
Hope you understand...:)
 

Re: what happens in DTMF

It will be like this :
MultiTone1.png

Mr Srikanth1555, the way you replied is little bit funny...:)
 

Re: what happens in DTMF

yeah...its not sinusodal...
do u think its a sine wave?
we get the above waveform by adding two sine waveforms....
 

Re: what happens in DTMF

I think what Srikanth1555 is saying is correct although worded strangely. The frequencies are NOT added, in other words you don't get a single tone of 697 + 1209 = 1906Hz. what you do get is a complex waveform which is the sum of the two wave shapes. Consider that one wave may be rising while the other is falling and as they are doing so at different rates and not necessarily in phase, the waveform will be a repeating pattern but not symetrical. Filtering at the DTMF receiver still allows the two tones to be individually detected.

Brian.
 

Re: what happens in DTMF

I know the frequencies are not added.
I meant to say two waveforms are added and some time they have constructive addition and some time destructive

if for example

tone1= sin(2xpix697xt)
tone1= sin(2xpix1209xt)

DTMF tone= tone1+tone2

Is it correct??
 

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