Monthly Archives: December 2008

DTMF Touchtone Telephone Tones Relating to Notes on a Piano

A Reader Asks

 

“Which Piano Notes do Touch-Tone Dial Frequencies Generate?”

 

There are 8 frequencies that, when set in an X-Y Matrix combine to produce 16 dual tones. The frequencies are set in two groups. (All frequencies are in Hz).

 

High Group

 

1209

1336

1477

1633

 

Low Group

 

941

852

770

697

 

The combinations and what they create are as follows:

 

1=697+1209

2=697+1336

3=697+1477

A=697+1663

4=770+1209

5=770+1336

6=770+1477

B=770+1663

7=852+1209

8=852+1336

9=852+1477

*=941+1209

0=941+1336

#=941+1477

D=941+1663

 

SIDEBAR — The letters A, B, C and D are not commonly found on your typical telephone set. They are used for special signaling and switching operations the consumer might need with special software or home equipment.

 

Let’s use the given that A4 is 440Hz and Middle C is 256Hz. These also happen to be standard audio calibration frequencies used for certain calibrations and setups.

 

Cheating a bit an equation was not used to calculate the frequency vs. musical notes. I did a comparison against some technical information I have in my Standards and Practices tech files. This will get us in the ballpark. Most of these have a tolerance of +/- 50 cents. Here is what we discover:

 

1209 (High side of D6)

1336 (High side of E6)

1477 (Low side of F#6)

1633 (Low side of G#6)

 

941 (Low side of A#5)

852 (High side of G#5)

770 (Low side of G5)

697 (Low side of F5)

 

As you can see the frequencies do not match musical notes exactly. This is because the frequencies were chosen by design to avoid dialing mistakes by familiar ambient sounds picked-up by the telephone’s receiver (microphone). Technically, if the frequencies matched exactly, misdialing might occur if there was music playing while one was dialing. The frequencies were also chosen so as not to interfere with other telephone tones used in the network for operations like billing, call transfers and other telemetry used to advance telephone calls. -33-

 

 

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