Improve gain handling

Now it's possible to set gain in gqrx.

src/main.c

@@ -40,11 +40,15 @@ static uint32_t lo_sin[LO_WORDS] __attribute__((__aligned__(1 << LO_BITS_DEPTH))
 #define RX_STRIDE (2 * IQ_SAMPLES)
 #define RX_BITS_DEPTH 12
 #define RX_WORDS (1 << (RX_BITS_DEPTH - 2))
+
+static_assert(RX_STRIDE * 4 < RX_WORDS, "RX_STRIDE * 4 < RX_WORDS");
+
 static uint32_t rx_cos[RX_WORDS] __attribute__((__aligned__(1 << RX_BITS_DEPTH)));
 static uint32_t rx_sin[RX_WORDS] __attribute__((__aligned__(1 << RX_BITS_DEPTH)));
 
-#define INIT_GAIN 256
+#define INIT_GAIN 120
 #define INIT_SAMPLE_RATE 100000
+#define INIT_FREQ 94600000
 
 #define NUM_GAINS 29
 static int gains[NUM_GAINS] = { 0,   9,	  14,  27,  37,	 77,  87,  125, 144, 157,
@@ -238,7 +242,7 @@ static const uint32_t samp_insn[4] = {
 
 static uint32_t null, one = 1;
 
-static void rf_rx_start(int rx_pin, int bias_pin, double freq, int rate)
+static void rf_rx_start(int rx_pin, int bias_pin)
 {
 	dma_ch_rx = dma_claim_unused_channel(true);
 	dma_ch_cp = dma_claim_unused_channel(true);
@@ -314,7 +318,7 @@ static void rf_rx_start(int rx_pin, int bias_pin, double freq, int rate)
 			      false);
 
 	/* Pacing timer for the sampling script trigger channel. */
-	dma_timer_set_fraction(dma_t_samp, 1, CLK_SYS_HZ / rate);
+	dma_timer_set_fraction(dma_t_samp, 1, CLK_SYS_HZ / sample_rate);
 
 	/* Sampling trigger channel. */
 	dma_conf = dma_channel_get_default_config(dma_ch_samp_trig);
@@ -349,7 +353,6 @@ static void rf_rx_start(int rx_pin, int bias_pin, double freq, int rate)
 	bias_init(rx_pin, bias_pin);
 	adder_init();
 
-	rx_lo_init(freq);
 	dma_channel_start(dma_ch_rx);
 	dma_channel_start(dma_ch_samp_trig);
 	watch_init(rx_pin);
@@ -436,7 +439,7 @@ static void rf_rx(void)
 
 		int delta = prev_transfers - dma_hw->ch[dma_ch_in_cos].transfer_count;
 
-		while (delta < RX_STRIDE * 2) {
+		while (delta < RX_STRIDE) {
 			delta = prev_transfers - dma_hw->ch[dma_ch_in_cos].transfer_count;
 			sleep_us(1);
 		}
@@ -458,11 +461,13 @@ static void rf_rx(void)
 
 		/*
 		 * Since the waveform is normally half of the time
-		 * above zero, we could halve once more.
+		 * above zero, we can halve once more.
 		 *
-		 * Instead we use 2/3 to provide 1/3 reserve.
+		 * This is not perfect, so we do not max out the base
+		 * gain but keep it slightly below the maximum to make
+		 * sure we do not overshoot often.
 		 */
-		max_amplitude = max_amplitude * 2 / 3;
+		max_amplitude = max_amplitude / 2;
 
 		/*
 		 * We are allowing the counters to only go as high
@@ -483,6 +488,11 @@ static void rf_rx(void)
 			I *= gain;
 			I /= max_amplitude;
 
+			if (I > 127)
+				I = 127;
+			else if (I < -128)
+				I = -128;
+
 			*blockptr++ = I + 128;
 
 			uint32_t sin_neg = *sin_ptr++;
@@ -497,7 +507,12 @@ static void rf_rx(void)
 			Q *= gain;
 			Q /= max_amplitude;
 
-			*blockptr++ = Q + 128;
+			if (Q > 127)
+				Q = 127;
+			else if (Q < -128)
+				Q = -128;
+
+			*blockptr++ = (uint8_t)Q + 128;
 		}
 
 		(void)queue_try_add(&iq_queue, block);
@@ -524,7 +539,7 @@ static void run_command(uint8_t cmd, uint32_t arg)
 	}
 }
 
-static bool check_command(void)
+static int check_command(void)
 {
 	static uint8_t buf[5];
 	static int pos = 0;
@@ -533,7 +548,7 @@ static bool check_command(void)
 
 	while ((c = getchar_timeout_us(0)) >= 0) {
 		if (0 == pos && 0 == c)
-			return true;
+			return 0;
 
 		buf[pos++] = c;
 
@@ -541,15 +556,16 @@ static bool check_command(void)
 			uint32_t arg = (buf[1] << 24) | (buf[2] << 16) | (buf[3] << 8) | buf[4];
 			run_command(buf[0], arg);
 			pos = 0;
+			return buf[0];
 		}
 	}
 
-	return false;
+	return -1;
 }
 
-static void do_rx(int rx_pin, int bias_pin, double freq)
+static void do_rx(int rx_pin, int bias_pin)
 {
-	rf_rx_start(rx_pin, bias_pin, freq, sample_rate);
+	rf_rx_start(rx_pin, bias_pin);
 	sleep_us(100);
 
 	dma_ch_in_cos = dma_claim_unused_channel(true);
@@ -583,7 +599,7 @@ static void do_rx(int rx_pin, int bias_pin, double freq)
 		/* Flush the queue */;
 
 	while (true) {
-		if (check_command())
+		if (0 == check_command())
 			break;
 
 		for (int i = 0; i < 32; i++) {
@@ -632,27 +648,19 @@ int main()
 
 	queue_init(&iq_queue, IQ_BLOCK_LEN, 256);
 
+	rx_lo_init(INIT_FREQ);
+
 	while (true) {
-		int c = getchar_timeout_us(0);
-
-		if (0 == c) {
-			continue;
-		} else if (1 == c) {
-			/* Tune to a new center frequency */
-			uint32_t arg;
-			fread(&arg, sizeof arg, 1, stdin);
-			arg = __builtin_bswap32(arg);
-
+		if (check_command() > 0) {
 			static const uint32_t header[3] = { __builtin_bswap32(0x52544c30),
-							    __builtin_bswap32(6),
+							    __builtin_bswap32(5),
 							    __builtin_bswap32(NUM_GAINS) };
 			fwrite(header, sizeof header, 1, stdout);
 			fflush(stdout);
 
-			gain = INIT_GAIN;
-			sample_rate = INIT_SAMPLE_RATE;
-
-			do_rx(10, 11, arg);
+			do_rx(10, 11);
 		}
+
+		sleep_ms(10);
 	}
 }

util/bridge.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 
-import binascii
 import struct
 from socket import (AF_INET, MSG_DONTWAIT, SO_REUSEADDR, SO_SNDBUF,
                     SOCK_STREAM, SOL_SOCKET, socket)
@@ -8,6 +7,31 @@ from socket import (AF_INET, MSG_DONTWAIT, SO_REUSEADDR, SO_SNDBUF,
 import click
 import serial
 
+COMMAND_NAMES = [
+    "reset",
+    "tune_freq",
+    "sample_rate",
+    "manual_gain",
+    "gain",
+    "ppm_offset",
+    "if_gain",
+    "test_mode",
+    "agc",
+    "direct_sampling",
+    "offset_tuning",
+    "11",
+    "12",
+    "gain_index",
+    "bias_tee",
+]
+
+
+def describe(cmd: int, arg: int):
+    try:
+        print("->", COMMAND_NAMES[cmd], arg)
+    except IndexError:
+        print("->", cmd, arg)
+
 
 @click.command()
 @click.option("-f", "--frequency", default=88200000, help="Frequency to tune to")
@@ -43,7 +67,7 @@ def bridge(frequency):
                     while len(cmd) >= 5:
                         fp.write(cmd[:5])
                         info = struct.unpack(">BL", cmd[:5])
-                        print("->", hex(info[0]), info[1])
+                        describe(*info)
                         cmd = cmd[5:]
 
                     data = fp.read(64)