Array Filter Help!!

Projects Programming
1

Hi All,

Im fairly new to programming the arduino, i have experience in VB and C#.

I am currently doing a university project which requires me to meaure the frequency of a signal and apply a 2nd order low pass filter to the signal. (bessel filter)

I already have managed to measure the frequency and have called it the variable Freq.

What i now need to do is pass each measured Freq value into an array, then apply the bessel filter to it (basically just a formula to each Freq value based on other previous values in the array).

so something like this (for 100 samples): (how i think id do it in vb)

For (intcount 1 to 100) " this loop passes the frequency data into the array"
{
Array(intcount) = freq;
}

for (intcount 1 to 100)
{
FilteredArray(intcount) = 0.009(array(n)) + 0.01(array(n-1)) +0.009(array(n-1)) +1.7(filteredarray(n-1)) -0.7(filteredarray(n-2))
}

serial.write(filteredarray) " allows me to see the values for the filtered array
serial.write(array) " allows me to see values of the original array

any way as i odnt have much experience in programming arrays itno the arduino, i was hoping that one of you could perhaps suggest a way to do this^?

Thanks in advance!! :slight_smile:

2

Look at this and see whether you can apply any of the ideas to your problem

float rawData[100];
float filteredData[100];

void setup()
{
  for (int f = 0; f < 100; f++)
  {
    rawData[f] = measureFrequency(); 
  }

  for (int f = 0; f < 100; f++)
  {
    filteredData[f] = (0.009 * rawData[f]) + (0.01 * rawData[f-1]) + (0.009 * rawData[f-1]) + (1.7 * filteredData[f-1]); 
  }

  for (int f = 0; f < 100; f++)
  {
    Serial.println(rawData[f]);
  }

  for (int f = 0; f < 100; f++)
  {
    Serial.println(filteredData[f]);
  }
}

void loop()
{
}

int measureFrequency()
{
  int freq = 0;
  //code here to measure frequency and put it in the freq variable
  return freq;
}

Arrays in C are indexed from zero and there is no bounds checking so be careful not to stray outside the declared array dimensions.

3 
serial.write(filteredarray)  " allows me to see the values for the filtered array

No, it won't. Dumping binary data that way is not going to prove useful with most serial monitor applications.

4

Careful! This line will attempt to access out-of-bounds array elements during the first pass (e.g. rawData[-1])

filteredData[f] = (0.009 * rawData[f]) + (0.01 * rawData[f-1]) + (0.009 * rawData[f-1]) + (1.7 * filteredData[f-1]); 
  }
5

So it does.
It's just as well that I warned myself :slight_smile:

6

Can you tell us how you developed those filter coefficients?