Fixed APRS pre-emphasis using custom FIR filter. Not sure on the deviation yet but it looks good in audacity

firmware/inc/ax25.h

@@ -42,11 +42,9 @@
 /**
  * The fm deviation setting for the si radio
  *
- * See https://github.com/bristol-seds/pico-tracker/blob/master/sim/si_fir_filter/si_fir_filter.ipynb
+ * More work required to determine this value..
- * for the source of these values
  */
-#define AX25_DEVIATION_MARK	1800 /* 1600 Hz */ /* TODO currently this deviation is used for both mark and space */
+#define AX25_DEVIATION		1800 /* corresponds to about ±2.5kHz maybe */
-#define AX25_DEVIATION_SPACE	4800 /* 3000 Hz */
 
 /**
  * How often our handler gets called

firmware/inc/si_trx_defs.h

@@ -242,6 +242,7 @@ enum {
   SI_MODEM_TX_NCO_TXOSR_20X		= (0x02 << 26),
   SI_MODEM_FREQ_DEV			= 0x0a,
   SI_MODEM_FREQ_OFFSET			= 0x0d,
+  SI_MODEM_TX_FILTER_COEFF8		= 0x0f,
   SI_MODEM_CLKGEN_BAND			= 0x51,
   SI_MODEM_CLKGEN_SY_SEL_0		= (0x00 << 3), /* low power */
   SI_MODEM_CLKGEN_SY_SEL_1		= (0x01 << 3), /* default */

firmware/src/aprs.c

@@ -101,7 +101,7 @@ uint8_t aprs_start(void)
   /* Encode the information field */
   /* Compressed Lat/Long position report, no timestamp */
   uint32_t information_len = sprintf(information,
-                                     "!%c%s%s%c%s%c/A=%06ld RTTY/434.6U8N2",
+                                     "!%c%s%s%c%s%c/A=%06ld TESTING",// RTTY/434.6U8N2",
                                      APRS_SYMBOL[0], /* Symbol Table ID */
                                      compressed_lat,
                                      compressed_lon,

firmware/src/si_trx.c

@@ -273,6 +273,25 @@ static void si_trx_modem_set_tx_datarate(uint32_t rate)
                           SI_MODEM_DATA_RATE,
                           rate & 0xFFFFFF);
 }
+/**
+ * Writes the coefficients of the modem tx filter
+ *
+ */
+static void si_trx_modem_tx_filter_coefficients(uint8_t* coeff_array)
+{
+  uint8_t buffer[4+9];
+
+  buffer[0] = SI_CMD_SET_PROPERTY;
+  buffer[1] = SI_PROPERTY_GROUP_MODEM; // group
+  buffer[2] = 9;
+  buffer[3] = SI_MODEM_TX_FILTER_COEFF8; // prop
+  /* Write filter coefficents 8 to 0 */
+  for (int i = 0; i < 9; i++) {
+    buffer[4+i] = coeff_array[8-i];
+  }
+
+  _si_trx_transfer(4+9, 0, buffer);
+}
 
 
 /**
@@ -428,7 +447,10 @@ void si_trx_reset(uint8_t modulation_type, uint32_t frequency,
   si_trx_set_frequency(frequency, deviation);
   si_trx_set_tx_power(power);
 
-  si_trx_modem_set_tx_datarate(3000);
+  uint8_t p_si_coeff[] = {0x6, 0x8, 0x1, 0xf2, 0xe4, 0xe7, 0xff, 0x1d, 0x2b};
+
+  si_trx_modem_set_tx_datarate(1600);
+  si_trx_modem_tx_filter_coefficients(p_si_coeff);
 
   /* RTTY from GPIO1 */
   si_trx_modem_set_modulation(SI_MODEM_MOD_DIRECT_MODE_SYNC, // ASYNC

firmware/src/telemetry.c

@@ -328,7 +328,7 @@ void telemetry_tick(void) {
 
           /* Radio on */
           si_trx_on(SI_MODEM_MOD_TYPE_2GFSK, _aprs_frequency,
-                    AX25_DEVIATION_MARK, APRS_POWER);
+                    AX25_DEVIATION, APRS_POWER);
           radio_on = 1;
         } else {
           /* Stop immediately */