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VK1ET : Changes to modesInitError()

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Restrict Syndromes to exclude DF bits. Do NOT introduce syndromes for
errors in the first 5 bits (DF field)

Use the --aggressive flag to see if we populate the two-bit error
syndromes. If --aggressive is not specified, then we don't attempt to
fix two bit errors.
This commit is contained in:
Malcolm Robb 2013-05-21 13:25:33 +01:00
parent 640d63a0d1
commit 0a0ba165db
1 changed files with 8 additions and 4 deletions
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12
dump1090.c
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@ -1126,6 +1126,7 @@ uint32_t modesChecksum(unsigned char *msg, int bits) {
pCRCTable++;
theByte = theByte << 1;
}
rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
}
@ -1268,9 +1269,9 @@ void modesInitErrorInfo() {
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
memset(msg, 0, MODES_LONG_MSG_BYTES);
/* Add all possible single and double bit errors */
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
// Add all possible single and double bit errors
// don't include errors in first 5 bits (DF type)
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
msg[bytepos0] ^= mask0; // create error0
@ -1279,7 +1280,9 @@ void modesInitErrorInfo() {
bitErrorTable[n].pos0 = i;
bitErrorTable[n].pos1 = -1;
n += 1;
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
if (Modes.aggressive) {
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1; // create error1
@ -1297,6 +1300,7 @@ void modesInitErrorInfo() {
bitErrorTable[n].pos1 = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
}
msg[bytepos0] ^= mask0; // revert error0
}