(Dual-tone multi-frequency signaling)
probably isn’t necessary to apologize for what seems at first
to be another ‘retro’ project, because even today, when dial
phones have yielded their place to cell phones and other advanced
devices, DTMF is still in use—Press ‘1’ to continue in English
‘7’ to delete your voice mail, etc. And in addition to familiar
consumer-related applications, DTMF remains alive and well in various
niches such as the amateur radio world.
The idea for this project came from
wondering whether or not Arduino could be made to generate two or more
tones concurrently. A quick Google search confirmed that
indeed it could.
Someone had even designed an Arduino midi player! The trick is to use a
special tone library (zip download). To
exercise Arduino’s multi-tone capability I decided to make a DTMF tone
generator. The application sketch (Arduino program code) is here.
As documented, Arduino-generated
tones are square waves having a duty cycle of 50%. To smooth the
waveform I used
a variant of a circuit from this page, having previously
tested component values at frequencies near the DTMF tones. At
first I listened to the tones using stereo headphones,
balancing the sound by ear, but subsequently used an oscilloscope
for objective adjustment. The
output of each filtered tone set (row and column tones) was adjusted
independently to have equal voltage at the lowest frequencies. In other
words the level at 697 Hz (filtered from DIO pin 12) was made equal to
that at 1209 Hz (filtered from DIO pin 11). The combined attenuated
then amplified using a boilerplate LM386 audio amplifier circuit.
Arduino generated DTMF tones sounded
okay when compared to
those generated by a wireless phone handset, but that is hardly proof
that they would work in a real signaling context. As it turns out there
is a test resource—there may be more
than one such—that reads back the numbers it hears. I dialed the
listed US test number and
placed the phone handset on the bench next to the DTMF speaker. The
link below presents excerpts from this test.
The DTMF test listener had no
trouble distinguishing the keys pressed, although the physical
positioning of speaker and microphone were not ideal, and I had made no
effort to improve or optimize the setup. In the video the camera was
aimed toward the
keyboard while I pressed keys, and toward the speaker as the ‘Test
system responded. However, to clarify, that response actually came from
the nearby handset’s
built-in speaker, not from the tones amplifier speaker.
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