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DTMF Amplitude for the phone line

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foxbrain

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i'm using ht9200 to generate dtmf tones i can hear it and i saw it in the scope but without any effect to the line....
i think that there should be a standard amplitude for the dtmf signal?
so what should it be?
 

Can you describe "without any effect to the line...."?

How you connect generator with line? Did you use transformer? Impedance ?
 

i tried these 2 circuits as interfacing but the one with pcd when i connect it with the amplifier i hear noise and without it the line doesn't takes any effect
 

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It has a certain amplitude. There is a mathematical formula which you can see here:

DTMF.png

Also you can find the typical amplitude of the DTMF by reading the datasheet of ICs which produce DTMF like MT8870 or MT8880.
 

You'll find exact DTMF specifications in regulations like ETSI EN 300 001. In a short, required DTMF level is in a -10 dBm range.
 

There is also another amplitude parameter called "Twist" which is the difference between the high tone level and the low tone level. It also varies from country to country but is typically around 2 to 3 dB.

Brian.
 

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