Improve FIR filter a bit

src/main.c

@@ -25,7 +25,7 @@
 #define CLK_SYS_HZ (300 * MHZ)
 
 #define LO_PIN 9
-#define RX_PIN 8
+#define RX_PIN 13
 #define FB_PIN 5
 #define PSU_PIN 23
 
@@ -384,34 +384,52 @@ static void rf_rx_stop(void)
 
 inline static int next_sample(const uint32_t *buf, int *h)
 {
-	int x1 = buf[0] - buf[1];
+	int x3 = 2 * (buf[0] - buf[1]);
 	int x2 = 0;
-	int x3 = buf[5] - buf[4];
-	int x4 = 0;
+	int x1 = 2 * (buf[5] - buf[4]);
+	int x0 = 0;
 
-	int sample = 18 * h[9] + 14 * h[8] - 32 * h[7] - 33 * h[6] + 106 * h[5] + 362 * h[4] +
-		     565 * h[3] + 565 * h[2] + 362 * h[1] + 106 * h[0] - 33 * x1 - 32 * x2 +
-		     14 * x3 + 18 * x4;
+	const int c[] = { 4, 2, -8, -9, 19, 55 };
 
+	int sample = 0;
+	sample += (c[0]) * h[10];
+	sample += (c[0] + c[1]) * h[9];
+	sample += (c[0] + c[1] + c[2]) * h[8];
+	sample += (c[0] + c[1] + c[2] + c[3]) * h[7];
+	sample += (c[1] + c[2] + c[3] + c[4]) * h[6];
+	sample += (c[2] + c[3] + c[4] + c[5]) * h[5];
+	sample += (c[3] + c[4] + c[5] + c[5]) * h[4];
+	sample += (c[4] + c[5] + c[5] + c[4]) * h[3];
+	sample += (c[5] + c[5] + c[4] + c[3]) * h[2];
+	sample += (c[5] + c[4] + c[3] + c[2]) * h[1];
+	sample += (c[4] + c[3] + c[2] + c[1]) * h[0];
+	sample += (c[3] + c[2] + c[1] + c[0]) * x3;
+	sample += (c[2] + c[1] + c[0]) * x2;
+	sample += (c[1] + c[0]) * x1;
+	sample += (c[0]) * x0;
+	sample *= gain;
+	sample /= 256;
+
+	h[10] = h[6];
 	h[9] = h[5];
 	h[8] = h[4];
 	h[7] = h[3];
 	h[6] = h[2];
 	h[5] = h[1];
 	h[4] = h[0];
-	h[3] = x1;
+	h[3] = x3;
 	h[2] = x2;
-	h[1] = x3;
-	h[0] = x4;
+	h[1] = x1;
+	h[0] = x0;
 
-	return gain * sample / 565;
+	return sample;
 }
 
 static void rf_rx(void)
 {
 	const uint32_t base = (uint32_t)rx_cos;
-	int prevI[10] = { 0 };
-	int prevQ[10] = { 0 };
+	int prevI[11] = { 0 };
+	int prevQ[11] = { 0 };
 	int pos = 0;
 
 	while (true) {