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NXDN: #299

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Additional check for LICH "offbits" that should all be 1's as sync condition;

Add 7-bit Parity Check for known DCR Lich ("RICH") Codes;

Make Tweaks to Help Incorporate Lich Parity and Offbits into NXDN Sync Determination;
This commit is contained in:
lwvmobile
2025-07-13 11:32:15 -04:00
parent ce805a7860
commit 8c34313720
1 changed files with 63 additions and 14 deletions
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77
src/nxdn_frame.c
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@@ -25,7 +25,8 @@
#include "dsd.h"
#include "nxdn_const.h"
#include <assert.h>
// #define NXDN_DEBUG_LICH //print LICH debug info on err on payload == 1
#define NXDN_LICH_OFFBITS //use the offbits to help determine sync status (disable if bad signal / bad sample)
void nxdn_frame (dsd_opts * opts, dsd_state * state)
{
@@ -94,13 +95,49 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
lich = 0;
for (int i=0; i<8; i++) lich |= (lich_dibits[i] >> 1) << (7-i);
lich_parity_received = lich & 1;
lich_parity_computed = ((lich >> 7) + (lich >> 6) + (lich >> 5) + (lich >> 4)) & 1;
lich = lich >> 1;
if (lich_parity_received != lich_parity_computed && lich != 0x4A) //may disable this if it causes issues
//debug lich as a 16-bit value (with encoding "dividing")
uint8_t lich_bits[16]; memset(lich_bits, 0, sizeof(lich_bits));
for (int i=0; i<8; i++)
{
if (opts->payload == 1) fprintf(stderr, " Lich Parity Error %02X\n", lich);
state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
lich_bits[(i*2)+0] = (lich_dibits[i] >> 1) & 1;
lich_bits[(i*2)+1] = (lich_dibits[i] >> 0) & 1;
}
uint16_t lich_bits_hex = (uint16_t)ConvertBitIntoBytes(lich_bits, 16);
UNUSED(lich_bits_hex);
//debug look at the "off bits" of the encoded lich, should be all 1's (8)
//disble this code if sync issues arise, this may not be ideal of marginal signal
uint8_t lich_off_hex = 0;
for (int i=0; i<8; i++)
lich_off_hex += lich_bits[(i*2)+1];
#ifdef NXDN_LICH_OFFBITS
if (lich_off_hex < 7) //allow up to 1 bit error
{
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf(stderr, " Lich Off Bit Fill Error: %d / 8; \n", lich_off_hex);
#endif
// state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
goto END;
}
#endif
uint8_t lich_full = lich;
lich_parity_received = lich & 1;
lich_parity_computed = ((lich_full >> 7) + (lich_full >> 6) + (lich_full >> 5) + (lich_full >> 4)) & 1;
lich = lich_full >> 1;
//special cases on DCR where parity is computed over 7 bits, and not 4 bits
if (lich == 0x4A || lich == 0x48 || lich == 0x46)
lich_parity_computed = ((lich_full >> 7) + (lich_full >> 6) + (lich_full >> 5) + (lich_full >> 4) + (lich_full >> 3) + (lich_full >> 2) + (lich_full >> 1)) & 1;
if (lich_parity_received != lich_parity_computed)
{
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf(stderr, " Lich Parity Error %02X / %04X\n", lich_full, lich_bits_hex);
#endif
// state->lastsynctype = -1;
goto END;
}
@@ -114,8 +151,11 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
//all inbound lich are even value (lsb is set to 0 for inbound direction)
if (lich % 2 == 0 && opts->p25_trunk == 1)
{
if (opts->payload == 1) fprintf(stderr, " Simplex/Inbound NXDN lich on trunking system - type 0x%02X\n", lich);
state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf(stderr, " Simplex/Inbound NXDN lich on trunking system - type 0x%02X\n", lich);
#endif
// state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
goto END;
}
@@ -243,11 +283,14 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
break;
default:
if (opts->payload == 1) fprintf(stderr, " false sync or unsupported NXDN lich type 0x%02X\n", lich);
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf(stderr, " false sync or unsupported NXDN lich type L: %02X / LH: %04X\n", lich, lich_bits_hex);
#endif
//reset the sacch field, we probably got a false sync and need to wipe or give a bad crc
memset (state->nxdn_sacch_frame_segment, 1, sizeof(state->nxdn_sacch_frame_segment));
memset (state->nxdn_sacch_frame_segcrc, 1, sizeof(state->nxdn_sacch_frame_segcrc));
state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
// state->lastsynctype = -1; //set to -1 so we don't jump back here too quickly
voice = 0;
goto END;
break;
@@ -264,8 +307,10 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
{
printFrameSync (opts, state, "IDAS D ", 0, "-");
}
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf (stderr, "L: %02X; ", lich);
fprintf (stderr, "L: %02X / LH: %04X; ", lich, lich_bits_hex);
#endif
}
else if (sacch2)
{
@@ -273,8 +318,10 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
{
printFrameSync (opts, state, "JPN DCR", 0, "-");
}
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf (stderr, "L: %02X; ", lich);
fprintf (stderr, "L: %02X / LH: %04X; ", lich, lich_bits_hex);
#endif
}
else if (voice || facch || sacch || facch2 || udch || cac)
{
@@ -283,8 +330,10 @@ void nxdn_frame (dsd_opts * opts, dsd_state * state)
printFrameSync (opts, state, "NXDN48 ", 0, "-");
}
else printFrameSync (opts, state, "NXDN96 ", 0, "-");
#ifdef NXDN_DEBUG_LICH
if (opts->payload == 1)
fprintf (stderr, "L: %02X; ", lich);;
fprintf (stderr, "L: %02X / LH: %04X; ", lich, lich_bits_hex);
#endif
}
//now that we have a good LICH, we can collect all of our dibits