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jogerh
jogerh
Associate III
April 13, 2017
Question

How to accelerate a running average ?

  • April 13, 2017
  • 5 replies
  • 1543 views
Posted on April 13, 2017 at 09:10

Dear Community,

to realize a nearly ideal low pass (without phase shift) – filter I need to perform a symmetric running average operation using the STM32F4 core. This operation takes 3000 sampled points of raw data and averages for each filtered point 100 of those raw data points ( +/- 50). Thus at least 290000 additions plus 2900 division are required. Currently the complete process takes about ~30 ms @ 144 MHz clock speed and I think it can be significantly accelerated. I would be glad if someone could give a suggestion what I can do to accelerate this operation.

Here is a piece of Code:

short int Even_Moving_average(short int * data_array, short int windowsize, short int position) {

 int count = 0;

 int average = 0;

 //check 

 if(position - 50 < 0)       return 0;

 if(position + 50 + 1 > BLOCK_SIZE -1) return 0;

 for(count = 0; count < 100 + 1; count++) {

  average = average + data_array[count + position - 50];

 }   

 return (short int)(average/101);

}

//call

 for(count = 50; count < BLOCK_SIZE - 52; count ++) {  

    result[count] = Even_Moving_average(BASEwaveOUT,100,count);  

 }

// takes  ~ 30 ms ;-(

    This topic has been closed for replies.

    5 replies

    Jan Waclawek
    Jan Waclawek
    Visitor II
    April 13, 2017
    Posted on April 13, 2017 at 09:35

    Remember the sum. Except the first and last 50, which you solve separately as lead-in and lead-out, for each new point you then you don't need to perform 100 additions, only subtract subtract x[pos - 50] add x[pos + 50].

    Try it with pencil and paper on a short average of 3 or 4.

    JW

    jogerh
    jogerhAuthor
    Associate III
    April 13, 2017
    Posted on April 13, 2017 at 11:25

    Dear Jan,

    thank you very much for giving this hint – this clearly accelerated everything. After implementing your suggestion this time intensive step now takes only 3.7 ms – its now 8 times faster than before  and this is great !

    Andrei Chichak
    Andrei Chichak
    Lead
    April 14, 2017
    Posted on April 14, 2017 at 08:56

    This statement:

      average = average + data_array[count + position - 50];

    might be tightened up by using a pointer, don't allocate space for average count unless you're going to do the calculation. There's less messing around with the initial check if they are brought together into one statement.

    #include <stdint.h>

    int16_t Even_Moving_average( int16_t data_array[], int16_t windowsize, int16_t position) {

    //check

       if (((position - 50) < 0) || ((position + 50 + 1) > (BLOCK_SIZE - 1))) {

          return 0;

       }

       int32_t average;

       int16_t* data;

       data = &data_array[position - 50];

       for(int16_t count = 0, average = 0; count < 100 + 1; count++) {

          average = average + (uint32_t) *data++;

        }

        return ((int16_t)(average/101));

    }

    But make sure that the optimizer is turned on. Your divide is probably being turned into a multiply anyway using 9s-complement math unless your compiler really sucks.

    It's probably test code, but windowsize is never used.

    Andrei

    Andrei Chichak
    Andrei Chichak
    Lead
    April 14, 2017
    Posted on April 14, 2017 at 09:04

    Orrrrr, there's the 

    arm_mean_f32

    DSP function on the F4. You could use the force.

    A

    jogerh
    jogerhAuthor
    Associate III
    April 20, 2017
    Posted on April 20, 2017 at 10:57

    Dear Andrej,

    using 'the force' sounds promising - I will try to do so.

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