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48 lines
1.5 KiB
C++
48 lines
1.5 KiB
C++
/**
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* 3_fir.cpp
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* Written by Clyne Sullivan.
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*
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* The below code was written for applying FIR filters. While this is still essentially an overlap-
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* save convolution, other optimizations have been made to allow for larger filters to be applied
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* within the available execution time. Samples are also normalized so that they center around zero.
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*/
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adcsample_t *process_data(adcsample_t *samples, unsigned int size)
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{
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static adcsample_t buffer[SIZE];
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// Define the filter:
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constexpr unsigned int filter_size = 3;
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static float filter[filter_size] = {
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// Put filter values here (note: precision will be truncated for 'float' size).
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0.3333, 0.3333, 0.3333
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};
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// Do an overlap-save convolution
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static adcsample_t prev[filter_size];
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for (int n = 0; n < size; n++) {
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// Using a float variable for accumulation allows for better code optimization
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float v = 0;
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for (int k = 0; k < filter_size; k++) {
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int i = n - (filter_size - 1) + k;
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auto s = i >= 0 ? samples[i] : prev[filter_size - 1 + i];
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// Sample values are 0 to 4095. Below, the original sample is normalized to a -1.0 to
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// 1.0 range for calculation.
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v += (s / 2048.f - 1) * filter[k];
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}
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// Return value to sample range of 0-4095.
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buffer[n] = (v + 1) * 2048.f;
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}
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// Save samples for next convolution
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for (int i = 0; i < filter_size; i++)
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prev[i] = samples[size - filter_size + i];
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return buffer;
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}
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