Electronic – Anticipating processing delay in embedded systems

embeddedprogrammingsignal processing

Background:

I have been working with the STM32F3 discovery board and I am trying to implement some signal processing algorithms from the ground up. My problem is shown below in the graph. The more taps I add to an FIR filter, the longer the delay between the input signal and the DAC output signal. For the audio signals I am using, a delay over 40 us will cause distortion that can be heard, limiting my filtering capabilities. It also makes it difficult to design the filter because the sampling rate is limited by the computation time (the next input cannot be processed until the previous is sent to the output).

A quick test of the time delay between an input signal and uC DAC output

Questions:

Are there hardware or software structures that I should be making use of that improve performance?

If I cannot improve performance, should I design my filter with a sampling rate that takes into account the number of operations I need to perform between samples and the operation time from the datasheet?

I haven't tried the CMSIS libraries since I wanted to get a better feel for the actual algorithms but would these libraries improve performance?

Code Snippets:
(Generalized for IIR Filters)

while (1)
{
    dreadADC();
    FilteredValue=filter(ADC1ConvertedValue);
    DAC_SetChannel2Data(DAC_Align_12b_R, FilteredValue);
}
uint16_t filter(__IO uint16_t nValue)
{

    for(i=(Taps-1);i>0;i--)
    {
        x[i]=x[i-1];
        y[i]=y[i-1];
    }
    x[0]=nValue;
    y[0]=0;
    for(j=1;j<Taps;j++)
    {
         y[0]= y[0]+(b[j]*x[j])-(a[j]*y[j]);
    }
    y[0]=y[0]+(b[0]*x[0]);
    return y[0]+1024;
}

ADC Timing Setup 

ADC_RegularChannelConfig(ADC1, ADC_Channel_7, 1, ADC_SampleTime_1Cycles5);

DAC Timing Setup

TIM_TimeBaseStructure.TIM_Period = 2-1 ;          
TIM_TimeBaseStructure.TIM_Prescaler = 1-1;       
TIM_TimeBaseStructure.TIM_ClockDivision = 1-1;

Thanks for your time and I would appreciate any guidance!

Best Answer

Find a library that implements FIR filters in assembly. I am 100% sure there is a DSP library for STM32.

Alternativelly write your own code that uses advanced features on the core.

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