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P25 -- Cleanup 1/2 Rate Decoding;

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lwvmobile
2023-10-02 08:58:03 -04:00
parent 9dfbcc886f
commit f274f74f44
1 changed files with 13 additions and 34 deletions
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47
src/p25_12.c
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@@ -3,7 +3,7 @@
* P25p1 1/2 Rate Simple Trellis Decoder
*
* LWVMOBILE
* 2023-09 DSD-FME Florida Man Edition
* 2023-10 DSD-FME Florida Man Edition
*-----------------------------------------------------------------------------*/
#include "dsd.h"
@@ -30,7 +30,10 @@ uint8_t p25_fsm[16] = {
13,2,1,14,
9,6,5,10 };
uint8_t p25_ecc[16] = {
//this is a dibit-pair to trellis dibit transition matrix (SDRTrunk and Ossmann)
//when evaluating hamming distance, we want to use this xor the dibit-pair nib,
//and not the fsm xor the constellation point
uint8_t p25_dtm[16] = {
2,12,1,15,
14,0,13,3,
9,7,10,4,
@@ -51,7 +54,6 @@ uint8_t find_min(uint8_t list[4], int len)
{
int min = list[0];
uint8_t index = 0;
int unique = 1; //start with flagged on, list[0] could be the min
int i;
for (i = 1; i < len; i++)
@@ -60,18 +62,14 @@ uint8_t find_min(uint8_t list[4], int len)
{
min = list[i];
index = (uint8_t)i;
unique = 1;
}
// else if (list[i] == min)
// {
// unique = 0; //only change to 0 if another non-unique match is found
}
// }
//NOTE: Disqualifying result on uniqueness can impact decoding
//let the CRC determine if the result is good or bad
//its not uncommon for two values of the same min
//distance to emerge, so its 50/50 each time
}
if (unique == 0)
return 0xF;
return index;
}
@@ -109,7 +107,6 @@ int p25_12(uint8_t * input, uint8_t treturn[12])
//convert constellation points into tdibit values using the FSM
uint8_t state = 0;
uint8_t temp_s = 0;
uint8_t tdibits[49]; //trellis dibits 1/2 rate
memset (tdibits, 0xF, sizeof(tdibits));
uint8_t hd[4]; //array of the four fsm words hamming distance
@@ -119,14 +116,6 @@ int p25_12(uint8_t * input, uint8_t treturn[12])
for (i = 0; i < 49; i++)
{
//just quickly peek for the location of the current point in the fsm
for (j = 0; j < 16; j++)
{
if (p25_fsm[j] == point[i])
temp_s = j&3; //set position to correct state output value
}
for (j = 0; j < 4; j++)
{
if ( p25_fsm[(state*4)+j] == point[i] )
@@ -147,20 +136,10 @@ int p25_12(uint8_t * input, uint8_t treturn[12])
//this method only seems to be reliable up to 1-2 bit errors...sometimes
for (j = 0; j < 4; j++)
hd[j] = count_bits ( ((nibs[i] ^ p25_ecc[(state*4)+j]) & 0xF ), 4 );
hd[j] = count_bits ( ((nibs[i] ^ p25_dtm[(state*4)+j]) & 0xF ), 4 );
min = find_min (hd, 4);
tdibits[i] = state = min;
if (min != 15) //unique value returned
{
tdibits[i] = state = min;
// irr_err--; //decrement 'corrected' point err
}
//use the predicted correct state/value at output instead -- last resort
else
tdibits[i] = state = temp_s;
temp_s = 0;
memset (hd, 0, sizeof(hd));
min = 0;
@@ -179,7 +158,7 @@ int p25_12(uint8_t * input, uint8_t treturn[12])
//trellis point/state err tally
// if (irr_err != 0)
// fprintf (stderr, " ERR = %d", irr_err);
// fprintf (stderr, " P_ERR = %d", irr_err);
return (irr_err);
}