Replace BPF with LPF after I/Q conversion

src/main.c

@@ -382,10 +382,36 @@ static void rf_rx_stop(void)
 	dma_t_samp = -1;
 }
 
+inline static int next_sample(const uint32_t *buf, int *h)
+{
+	int x1 = 2 * (buf[0] - buf[1]);
+	int x2 = 0;
+	int x3 = -2 * (buf[4] - buf[5]);
+	int x4 = 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;
+
+	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[2] = x2;
+	h[1] = x3;
+	h[0] = x4;
+
+	return sample >> 10;
+}
+
 static void rf_rx(void)
 {
 	const uint32_t base = (uint32_t)rx_cos;
-	int prev1 = 0, prev2 = 0, prev3 = 0, prev4 = 0;
+	int prevI[10] = { 0 };
+	int prevQ[10] = { 0 };
 	int pos = 0;
 
 	while (true) {
@@ -416,48 +442,16 @@ static void rf_rx(void)
 		/*
 		 * Since every 2 samples add to either +1 or -1,
 		 * the maximum amplitude in one direction is 1/2.
-		 */
-		int64_t max_amplitude = CLK_SYS_HZ / 2;
-
-		/*
+		 *
 		 * We are allowing the counters to only go as high
-		 * as sampling rate, but the I/Q conversion splits
-		 * samples between two channels.
+		 * as sampling rate.
 		 */
-		max_amplitude /= sample_rate * 2;
+		int64_t max_amplitude = CLK_SYS_HZ / 2 / sample_rate;
 
 		for (int i = 0; i < IQ_SAMPLES; i++) {
-			int sI = 0, sQ = 0;
-			int pos, neg;
-
-#define next_sample ((-(pos - neg) + prev2 * 2 - prev4))
-
-			// +I
-			pos = *cos_ptr++;
-			neg = *cos_ptr++;
-			sI += next_sample;
-			prev4 = prev3, prev3 = prev2, prev2 = prev1, prev1 = pos - neg;
-
-			// +Q
-			pos = *cos_ptr++;
-			neg = *cos_ptr++;
-			sQ += next_sample;
-			prev4 = prev3, prev3 = prev2, prev2 = prev1, prev1 = pos - neg;
-
-			// -I
-			pos = *cos_ptr++;
-			neg = *cos_ptr++;
-			sI -= next_sample;
-			prev4 = prev3, prev3 = prev2, prev2 = prev1, prev1 = pos - neg;
-
-			// -Q
-			pos = *cos_ptr++;
-			neg = *cos_ptr++;
-			sQ -= next_sample;
-			prev4 = prev3, prev3 = prev2, prev2 = prev1, prev1 = pos - neg;
-
-			int64_t I = sI / 4;
-			int64_t Q = sQ / 4;
+			int64_t I = next_sample(cos_ptr + 0, prevI);
+			int64_t Q = next_sample(cos_ptr + 2, prevQ);
+			cos_ptr += 8;
 
 			I *= gain;
 			I -= (max_amplitude * 181) / 256;