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Implement low pass filter.

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  • S Siddhartha

    Hi, I'm a novice and I'm learning.
    How would you manage to implement a 50 Hz LOW Pass Filter in this software?
    Thank you very much for the help.

    https://github.com/CieNTi/serial_port_plotter

    JKSHJ Offline
    JKSHJ Offline
    JKSH
    Moderators
    wrote on last edited by
    #2

    Hi @Siddhartha, and welcome.

    How would you manage to implement a 50 Hz LOW Pass Filter in this software?

    Which software? Qt, or the serial port plotter?

    Anyway, your first step is to learn the basics of Finite Impulse Response (FIR) filters. After that, you can implement it in C/C++. There is an example at https://ccrma.stanford.edu/~jos/fp/Definition_Simplest_Low_Pass.html

    Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

    S 1 Reply Last reply
    6
    • JKSHJ JKSH

      Hi @Siddhartha, and welcome.

      How would you manage to implement a 50 Hz LOW Pass Filter in this software?

      Which software? Qt, or the serial port plotter?

      Anyway, your first step is to learn the basics of Finite Impulse Response (FIR) filters. After that, you can implement it in C/C++. There is an example at https://ccrma.stanford.edu/~jos/fp/Definition_Simplest_Low_Pass.html

      S Offline
      S Offline
      Siddhartha
      wrote on last edited by
      #3

      @JKSH HI, True, what I need is to implement the Low Pass filter in the serial plotter.
      If I could implement it in this serial plotter that I attached it would be fabulous.

      This example that you give me how I could attach it to the serial plotter software that I attached?
      Where in all your classes should I implement the code and in what way?

      1 Reply Last reply
      0
      • fcarneyF Offline
        fcarneyF Offline
        fcarney
        wrote on last edited by
        #4

        This should help with the low pass filter:
        http://www.dspguide.com/
        You can download each chapter as a PDF. It covers just about every type of DSP you might need.

        How you implement this will be up to you.

        C++ is a perfectly valid school of magic.

        S 1 Reply Last reply
        2
        • fcarneyF fcarney

          This should help with the low pass filter:
          http://www.dspguide.com/
          You can download each chapter as a PDF. It covers just about every type of DSP you might need.

          How you implement this will be up to you.

          S Offline
          S Offline
          Siddhartha
          wrote on last edited by
          #5

          @fcarney Hello, the book is fabulous, the problem is that I am just learning and I do not know how to implement that filter in the serial plotter that I attached, I understand the biomedical signals but I do not understand how to implement the filter in the program.

          1 Reply Last reply
          0
          • mrjjM Offline
            mrjjM Offline
            mrjj
            Lifetime Qt Champion
            wrote on last edited by
            #6

            Hi
            In
            Mainwindow.cpp
            function
            void MainWindow::onNewDataArrived(QStringList newData)
            line 428
            it says
            ui->plot->graph(channel)->addData (dataPointNumber, newData[channel].toDouble());
            so that is where it adds the data.
            so you need something like

            double filteredData = DoLowPass(  newData[channel].toDouble() );
             ui->plot->graph(channel)->addData (dataPointNumber, filteredData );
            so that is where it adds the data.
            

            where DoLowPass should do the filtering.
            You can try with the simple version from the link @JKSH provided

            S 1 Reply Last reply
            2
            • mrjjM mrjj

              Hi
              In
              Mainwindow.cpp
              function
              void MainWindow::onNewDataArrived(QStringList newData)
              line 428
              it says
              ui->plot->graph(channel)->addData (dataPointNumber, newData[channel].toDouble());
              so that is where it adds the data.
              so you need something like

              double filteredData = DoLowPass(  newData[channel].toDouble() );
               ui->plot->graph(channel)->addData (dataPointNumber, filteredData );
              so that is where it adds the data.
              

              where DoLowPass should do the filtering.
              You can try with the simple version from the link @JKSH provided

              S Offline
              S Offline
              Siddhartha
              wrote on last edited by
              #7

              @mrjj Hello, thank you very much for answering, I tried to use your solution but I threw this error.
              0_1547654496959_Captura.PNG

              mrjjM 1 Reply Last reply
              0
              • S Siddhartha

                @mrjj Hello, thank you very much for answering, I tried to use your solution but I threw this error.
                0_1547654496959_Captura.PNG

                mrjjM Offline
                mrjjM Offline
                mrjj
                Lifetime Qt Champion
                wrote on last edited by
                #8

                @Siddhartha
                Hi
                well DoLowPass is just a fictional name.
                You have to create the function from JKSH link or from the book @fcarney linked.

                S 1 Reply Last reply
                1
                • mrjjM mrjj

                  @Siddhartha
                  Hi
                  well DoLowPass is just a fictional name.
                  You have to create the function from JKSH link or from the book @fcarney linked.

                  S Offline
                  S Offline
                  Siddhartha
                  wrote on last edited by
                  #9

                  @mrjj And what would it be like?

                  mrjjM 1 Reply Last reply
                  0
                  • S Siddhartha

                    @mrjj And what would it be like?

                    mrjjM Offline
                    mrjjM Offline
                    mrjj
                    Lifetime Qt Champion
                    wrote on last edited by
                    #10

                    @Siddhartha
                    Like shown in the link ?

                    /* C function implementing the simplest lowpass:
                     * 
                     *      y(n) = x(n) + x(n-1)
                     *
                     */
                    double simplp (double *x, double *y, 
                                   int M, double xm1)
                    {
                      int n;
                      y[0] = x[0] + xm1;
                      for (n=1; n < M ; n++) {
                        y[n] =  x[n]  + x[n-1];
                      }
                      return x[M-1];
                    }
                    
                    
                    S 1 Reply Last reply
                    2
                    • mrjjM mrjj

                      @Siddhartha
                      Like shown in the link ?

                      /* C function implementing the simplest lowpass:
                       * 
                       *      y(n) = x(n) + x(n-1)
                       *
                       */
                      double simplp (double *x, double *y, 
                                     int M, double xm1)
                      {
                        int n;
                        y[0] = x[0] + xm1;
                        for (n=1; n < M ; n++) {
                          y[n] =  x[n]  + x[n-1];
                        }
                        return x[M-1];
                      }
                      
                      
                      S Offline
                      S Offline
                      Siddhartha
                      wrote on last edited by
                      #11

                      @mrjj Thank you very much and I hope not to be annoying, but where would you place that code?

                      mrjjM 1 Reply Last reply
                      0
                      • S Siddhartha

                        @mrjj Thank you very much and I hope not to be annoying, but where would you place that code?

                        mrjjM Offline
                        mrjjM Offline
                        mrjj
                        Lifetime Qt Champion
                        wrote on last edited by
                        #12

                        @Siddhartha
                        Hi
                        well it could be a global function, but i think its better to make it a function of
                        MainWindow just like the other functions.
                        However, the simplp is a c function and uses a c array for the x,y but in plotter
                        the data is in
                        MainWindow::onNewDataArrived(QStringList newData)
                        which is a Stringlist so you cant just copy the code and have it working.

                        So first step is to read the text in link and understand how it works then change the code to use
                        the newData list instead as the c code expects other type of list and must be rewritten slightly.

                        1 Reply Last reply
                        0
                        • SGaistS Offline
                          SGaistS Offline
                          SGaist
                          Lifetime Qt Champion
                          wrote on last edited by
                          #13

                          Hi,

                          You may also find the DSPFilters project interesting.

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                          1 Reply Last reply
                          2
                          • JKSHJ Offline
                            JKSHJ Offline
                            JKSH
                            Moderators
                            wrote on last edited by JKSH
                            #14

                            @Siddhartha Before we go too deep, we need to know your starting point. Please tell us: When you said you're a novice, do you mean a novice in programming, or a novice in digital signal processing, or both?

                            (Note: Programming and signal processing are two very different fields. You'll need to learn both to implement a low pass filter in the plotter software)

                            Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                            S 1 Reply Last reply
                            4
                            • JKSHJ JKSH

                              @Siddhartha Before we go too deep, we need to know your starting point. Please tell us: When you said you're a novice, do you mean a novice in programming, or a novice in digital signal processing, or both?

                              (Note: Programming and signal processing are two very different fields. You'll need to learn both to implement a low pass filter in the plotter software)

                              S Offline
                              S Offline
                              Siddhartha
                              wrote on last edited by
                              #15

                              @JKSH Unfortunately, I'm new to both.

                              JKSHJ 1 Reply Last reply
                              0
                              • S Siddhartha

                                @JKSH Unfortunately, I'm new to both.

                                JKSHJ Offline
                                JKSHJ Offline
                                JKSH
                                Moderators
                                wrote on last edited by
                                #16

                                @Siddhartha said in Implement low pass filter.:

                                Unfortunately, I'm new to both.

                                No problem.

                                OK, here's a 5-step approach I recommend for you to reach your goal:

                                1. Learn how to build and run an existing program
                                2. Learn how to modify an existing program
                                3. Read about the basics of different types of low-pass filters
                                4. Convert a low-pass filter equation into C++ code
                                5. Integrate the C++ low-pass filter code into the serial port plotter (just like step #2)

                                Let's start with step #1. I see that you already have Qt Creator; are you able to build and run the serial port plotter software on your PC?

                                Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                                1 Reply Last reply
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