DTMF Decoder

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Problem Description

A  test program to read and display DTMF decoded digital output from a hardware device interface.

Background & Techniques

This program was the result of a request from a  fellow who wanted me to adapt my Oscilloscope program to "listen" for the  Dual-Tone Multi-Frequency (DTMF) "Touchtone" signals and display the digits transmitted.   I never learned exactly why he wanted this, but it did sound like an interesting project.  He even offered to pay $500 for the project, but we couldn't agree on payment terms.  While investigating the feasibility, I found that,  while decoding individual digits would be easy, it would take considerable work to monitor long strings and detect the gaps between digits.

While studying the problem I ran across ads for hardware boards which did the hard work of converting analog signals into digits that could be read by a computer.   I emailed my  son, the family Electrical Engineer, asking what would be involved  to  implement that and a couple of weeks later he sent the device pictured above.  The General Purpose Input/Output (GPIO) board accepts digits from the DTMF decoder card and outputs up to 8 serial bits at a time  to a USB port through a device driver which emulates serial read/write operations. 

I installed an open source program that makes Windows serial port communication functions available in Pascal and accessible in Delphi.  My program monitors digital input streams and displays the string of digits received when a one second "gap" occurs.  A new string is started after each display.     For debugging,  the on/off states of bits in the  last byte received  are displayed and the individual bits can be set or cleared. 

 DTMF audio strings for testing were provided by my Sound Generator V3 program which can generate touch-tone strings with specified sound length of each digit and inter-digit gap lengths and feeds them to the earphone jack. 

The good news is that the program is now working with digits and gaps as short as 50ms! The bad news is that the email address of the original requester is no longer valid.  If anyone knows  how to get in touch with Claudio, formerly at  crbbotucatu@ig.com.br, let him know that this setup is available to purchase for less than half of his original $500 offer.   Inquiries from anyone else with interest in acquiring it, are also welcome.   

Programmer's Notes:

The source code for this test program will provide the details for reading/writing the Numato GPIO board port settings with the SensaSer serial interface unit.  Most of the code was a matter of trial and error .  I still have not discovered where the last digit received is stored, but it is always displayed when monitoring is started, even after resetting the bits, purging buffers, and trying every other trick i could think of.    If anyone can shed light on this, please let me know.

Running/Exploring the Program 

bulletDownload  executable
bulletDownload source 

Suggestions for Further Explorations



Original:  May 26, 2016

Modified:  May 15, 2018

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