1 files changed, 47 insertions, 0 deletions
diff --git a/examples/3_fir.cpp b/examples/3_fir.cpp
new file mode 100644
index 0000000..b6d8751
--- /dev/null
+++ b/examples/3_fir.cpp
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+/**
+ * 3_fir.cpp
+ * Written by Clyne Sullivan.
+ *
+ * The below code was written for applying FIR filters. While this is still essentially an overlap-
+ * save convolution, other optimizations have been made to allow for larger filters to be applied
+ * within the available execution time. Samples are also normalized so that they center around zero.
+ */
+
+Sample* process_data(Samples samples)
+{
+    static Samples buffer;
+
+	// Define the filter:
+	constexpr unsigned int filter_size = 3;
+	static float filter[filter_size] = {
+        // Put filter values here (note: precision will be truncated for 'float' size).
+        0.3333, 0.3333, 0.3333
+	};
+
+    // Do an overlap-save convolution
+    static Sample prev[filter_size];
+
+    for (int n = 0; n < SIZE; n++) {
+        // Using a float variable for accumulation allows for better code optimization
+        float v = 0;
+
+        for (int k = 0; k < filter_size; k++) {
+            int i = n - (filter_size - 1) + k;
+
+            auto s = i >= 0 ? samples[i] : prev[filter_size - 1 + i];
+			// Sample values are 0 to 4095. Below, the original sample is normalized to a -1.0 to
+            // 1.0 range for calculation.
+            v += (s / 2048.f - 1) * filter[k];
+        }
+
+        // Return value to sample range of 0-4095.
+        buffer[n] = (v + 1) * 2048.f;
+    }
+
+    // Save samples for next convolution
+    for (int i = 0; i < filter_size; i++)
+        prev[i] = samples[SIZE - filter_size + i];
+
+    return buffer;
+}
+