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DTMF Detection

Module by: Jacob Fainguelernt. E-mail the author

Summary: This module shows the implementation of a DTMF detector using the DSK6713. The detector is desgined using Simulink.

Introduction

The Dual-tone multi-frequency (DTMF) signaling method was developed originally to replace pulse (Dial Pulse or DP in the U.S.) and loop disconnect (LD) signaling methods to dial numbers, in order to shorten the holding time of the switch during dialing , specially for long distance calls.

DTMF signaling uses two tones to represent each key on the touch pad. There are 12 distinct tones1 (Please refer to Table 1). When any key is pressed the tone of the column and the tone of the row are generated. As an example, pressing the '5' button generates the tones 770 Hz and 1336 Hz. The frequencies were chosen to avoid harmonics: no frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies.

Table 1: DTMF Frequencies
. 1209 Hz 1336 Hz 1477 Hz
697 Hz 1 2 3
770 Hz 4 5 6
852 Hz 7 8 9
941 Hz * 0 #

Table 1- DTMF frequencies

The low frequencies are referred as rows, The higher frequencies are referred as columns.

This example will show the implementation of a DTMF receiver using the DSK6713. The DTMF receiver will be based on the simulation model used in the Simulink demo. A graphic user interface (GUI) will also be created to activate the DSK6713. DTMF digits will be sent from the PC sound card.

Related Files

Simulation

The Simulation Model

Figure 1 shows the model used to simulate DTMF detection and generation.

Figure 1: DTMF Simulation
Figure 1 (graphics1.png)

The DTMF generator is composed by 7 sine wave generators, corresponding to the 7 DTMF frequencies. The oscillators are grouped as described in Table 1. The oscillators’’ outputs are multiplexed and summed (Please refer to Figure 2). Upon selection of a digit to be transmitted, the correspondent oscillators are activated.

Figure 2: DTMF Generation
Figure 2 (graphics2.jpg)

The DTMF detection comprises two processes, the frequency detection algorithm itself, and a leading edge detection subsystem.

The frequency detection subsystem measures the energy present at the seven frequencies, and calculates the digit by taking the component of maximum energy in the lower and upper frequency groups. The process is shown in Figure 3. This process runs continuously; however the calculated digit is reported only if its energy is above a threshold as determined by the leading edge detection subsystem. The DTMF detection process is shown in Figure 4.

Figure 3: Frequency Detection
Figure 3 (graphics3.jpg)
Figure 4: DTMF Detection
Figure 4 (graphics4.jpg)

Running the Simulation

  • Open the simulation model “DTMF.mdl”
  • In order to select the digit double click the DTMF generator subsystem and enter the digits to be sent as follows:
Figure 5: "Dialing" DTMF Digits
Figure 5 (graphics5.jpg)
  • Activate the simulation, the detected digits will be displayed as shown:
Figure 6: Displaying the Detected DTMF Digits
Figure 6 (graphics6.jpg)

Real Time Implementation

The real-time implementation environment is shown in Figure 7. The DTMF generation will be performed in MATLAB® and sent to the DSK through the PC sound card. The detected digits will be reported through RTDX. A graphic user interface (GUI) will be built to activate the transmitter, and display the reported digits.

Figure 7: Real Time Implementation
Figure 7 (graphics7.jpg)

The DTMF receiver will be based on the DTMF receiver from the simulation model, with the following changes:

  • Samples will be read from the Analog to Digital Converter, and filtered by a DC removal block2
  • Digits will be reported through RTDX.
  • Addition of the DSK6713 target preference

The DTMF Receiver

  • Open the simulation model “DTMF.mdl”, remove all the blocks besides the DTMF receiver block.
  • Open the Simulink ® library browser and add the "C6713DSK".
Figure 8: The C6713DSK target Block
Figure 8 (graphics8.png)
  • Add the “Analog to Digital”” converter (ADC) to your model:
Figure 9: The ADC target Block
Figure 9 (graphics9.png)
Figure 10: The ADC Block Configuration
Figure 10 (graphics10.png)
  • The “TO RTDX” will be used to report the detected digits to the PC. Add it to your model:
Figure 11: The "To RTDX" Block
Figure 11 (graphics11.png)
Figure 12: The "To RTDX" Block Configuration
Figure 12 (graphics12.png)
  • Add the “Digital Filter” and “Gain” blocks for DC removal:
Figure 13: The Digital Filter Block
Figure 13 (graphics13.png)
Figure 14: The Digital Filter Block Configuration
Figure 14 (graphics14.png)
Figure 15: The Gain Block
Figure 15 (graphics15.png)
Figure 16: The Gain Block Configuration
Figure 16 (graphics16.png)
  • The DTMF receiver model is shown Figure 17:
Figure 17: DTMF Receiver
Figure 17 (graphics17.png)
  • Save the model as "DTMF_DSK6713.mdl".
  • Built the model.

The User Interface

Functionality

The GUI will perform the following functions:

  • Initialization: Initialize the RTDX interface and loading the DTMF Receiver program we have built in the last section to the DSK.
  • Generate the DTMF signal samples: The GUI will enable the user to select a sequence of 11 digits3 to be transmitted. It will create the samples of the DTMF signal based on this selection, and send them to the sound card.
  • Display the detected digits reported by the DSK

Description

The desired GUI is shown in Figure 18

Figure 18: DTMF GUI
Figure 18 (graphics18.png)

It will enable the user to:

  • Dial the digits: every digit dialed is added to a specific buffer, and displayed under the “Dialed Digits” label.
  • Delete the last digit
  • Clear all the digits
  • Send: Create the DTMF signal from the digits selected4. A dual-tone signal will be created for each digit. Silence will be inserted between consecutive digits. This signal is sent to the sound blaster using the “sound” command in MATLAB.
  • Display: Digits reported by the DSK will be displayed under the “Detected Digits” label.

The Procedure

  • Activate the GUIDE (Graphic User Interface Integrated Development Environment in MATLAB.
  • Open a new GUI and name it DTMF
  • Add 12 Push buttons for the keypad and set their String property to the proper values (0-9,*,#) using the property inspector.
  • Add the Send, Clear and Delete buttons
  • Add 2 Edit Texts objects for display
  • You may now add the labels (static texts):
    • “Dialed Digits”
    • “Detected Estimation”
    • “DTMF Transmitter”
  • Press the “play” button to open the MATLAB script. Name the script “DTMF.m”.
  • Enter the Initialization (DTMF_OpeningFcn ) as shown:

function DTMF_OpeningFcn(hObject, eventdata, handles, varargin)
modelName = gcs;
CCS_Obj = connectToCCS(modelName);
% Identify RTDX channel names/modes
chan_struct(1).name = 'OutputDigit';
chan_struct(1).mode = 'r';
handles.RTDX_chan1=chan_struct(1);
% Identify RTDX host buffer parameters
RTDX_config_struct.Buffsize= 32768;
RTDX_config_struct.Nbuffers = 4;
RTDX_config_struct.Mode = 'continuous';
CodegenDir = fullfile(pwd, ['DTMF_RT' '_c6000_rtw']);
OutFile = fullfile(CodegenDir, ['DTMF_RT' '.out']);
%Load is needed for RTDX setup
CCS_Obj.load(OutFile,20);
% Set up RTDX
r = setupRTDX(CCS_Obj, chan_struct, RTDX_config_struct);
handles.pipe=r;
handles.CCS_Obj=CCS_Obj;
handles.output = hObject;
CCS_Obj.run;     
% Enable all RTDX channels
r.enable('all');
%The buffer that stores the user input digits
A=[];
handles.A=A;
% flag that tells if the gui needs to clear the dialed string or 
%the next digit joins the previous string: 
handles.initFlag=0;
handles.maxSize=11;
handles.freqTable=generateTable();
% Update handles structure
guidata(hObject, handles);

This function will run after activating the GUI. It will then load the DSK, initialize the RTDX interface, and initialize a table of samples for each digit, by calling the generateTable auxiliary function:


function res=generateTable()
t=(1:256*6)/8192;
res(1,:)=sin(2*pi*697*t)+sin(2*pi*1209*t);
res(2,:)=sin(2*pi*697*t)+sin(2*pi*1336*t);
res(3,:)=sin(2*pi*697*t)+sin(2*pi*1477*t);
res(4,:)=sin(2*pi*770*t)+sin(2*pi*1209*t);
res(5,:)=sin(2*pi*770*t)+sin(2*pi*1336*t);
res(6,:)=sin(2*pi*770*t)+sin(2*pi*1477*t);
res(7,:)=sin(2*pi*852*t)+sin(2*pi*1209*t);
res(8,:)=sin(2*pi*852*t)+sin(2*pi*1336*t);
res(9,:)=sin(2*pi*852*t)+sin(2*pi*1477*t);
res(10,:)=sin(2*pi*941*t)+sin(2*pi*1209*t);
res(11,:)=sin(2*pi*941*t)+sin(2*pi*1336*t);
res(12,:)=sin(2*pi*941*t)+sin(2*pi*1477*t);
  • Enter the handle Button function:

      function handleButton(hObject,handles,k)
%get the digits buffer
A=handles.A;
%if we have to erase so we are doing it here:
if handles.initFlag==1
    handles.initFlag=0;
    A=[];
end
%check if the string is too long with the new digit:
if length(A)==handles.maxSize
    fprintf('Error: too many digits\n');
    return;
end
%concatenate the new digit to the existing buffer
A=[A k];
handles.A=A;
set(handles.text1, 'String', strrep(strrep(strrep(int2str(A),'10','*'),'11','0'),'12','#'));    
guidata(hObject,handles);
 
% invoked when the user dials '1':
function pushbutton1_Callback(hObject, eventdata, handles)
handleButton(hObject,handles,1);
% invoked when the user dials '2':
function pushbutton2_Callback(hObject, eventdata, handles)
handleButton(hObject,handles,2);
. . .

This function will be activated, every time the user pushes a digit button in the keypad. The digit will be displayed under the “Dialed Digits” label.

  • The following function will be activated by the “Send” button:

%input: digits and handle to the signals table
%output: buffer that contains the DTMF samples for the digits.
function res=buildXmt(handles,A)
res=[];
for k=1 : length(A)
    freqTable=handles.freqTable;
    %place the next digit and then zeros (space)
    res=[res freqTable(A(k),:) zeros(1,256*6);];
end
 
%The Send Button Handler:
function pushbutton13_Callback(hObject, eventdata, handles)
r=handles.pipe;
A=handles.A;
padding=0;
if length(A)==0
    return
end
% if dialed less than 11 digits pad to 11 digits
if length(A)<11
     padding=11-length(A);
    A=[A ones(1,11-length(A))];
end
%build the Send Buffer
xmt=0.5*buildXmt(handles,A);
%Transmit DTMF signals buffer
sound(xmt,8192)
%Wait for end of detection
pause(4)
%read the digits from the rtdx:
num_of_msgs = msgcount(r,'OutputDigit');
r.flush('OutputDigit',num_of_msgs-1);
 y=r.readmsg(handles.rtdx_chan1.name, 'double',1);
y=y(1:end-padding);
 
%Update the upper label with the digits read from dsk estimation
set(handles.text4, 'String', strrep(strrep(strrep(int2str(y),'10','*'),'11','#'),'20','*')); 
handles.initFlag=1;
guidata(hObject, handles);

It will generate the DTMF samples Buffer, based on the “User Dialed” information. The samples will be sent to the sound-card. The program will then monitor the RTDX channel to receive the digits reported by the DSK.

  • Enter the functions activated by the “Clear” and “delete” push buttons:

% Delete one digit.
function pushbutton14_Callback(hObject, eventdata, handles)
A=handles.A;
if length(A)==0
    return
end
A=A(1:end-1);
set(handles.text1, 'String', strrep(strrep(strrep(int2str(A),'10','*'),'11','0'),'12','#')); 
handles.A=A;
guidata(hObject, handles);
 
 
%Clear all the dialed digits.
function pushbutton15_Callback(hObject, eventdata, handles)
A=handles.A;
A=[];
set(handles.text1, 'String',
strrep(strrep(strrep(int2str(A),'10','*'),'11','0'),'12','#')); 
handles.A=A;
guidata(hObject, handles);

Running the Example

The system used to run this example is shown in Figure 7.

Activate the GUI (DTMF.fig). Enter an array of digits and compare them to those detected by the DSK.

Things to Try

The original algorithm (simulation) runs in a "noiseless" environment. You are invited to check the performance of the real-time model, and eventually change the algorithm to reduce its sensitivity to noise.

You may also add new buttons for DTMF parameters (level, frequency deviation, etc …), and test the DTMF receiver performance against these parameters.

MATLAB and Simulink are registered trademarks of The MathWorks, Inc. See www.mathworks.com/trademarks for a list of additional trademarks. Other product or brand names may be trademarks or registered trademarks of their respective holders.

Footnotes

  1. The DTMF method defines four additional digits: “A”, “B”,”C” & “D”. Most of the systems today do not use those digits.
  2. This block is necessary to convert the unipolar format of the samples to a 2’s complement format for processing by the DSP.
  3. If the user selects less than 11 digits, the sequence is padded with DTMF digit '1'.
  4. The DSK operates with a buffer of 11 digits so if the user dialed less then 11 digits we are padding them to 11 in the MATLAB.

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