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Strong preamble detection even with --aggressive.

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To try to decode messages with a fundamentally flawed preamble is mostly
a useless waste of CPU time.

The new aggressive mode still detects a sensible percentage of
additional messages because of the error tolerance and two-bits fixes
but does not waste your CPU time.
This commit is contained in:
antirez 2013-01-26 13:03:24 +01:00
parent 184953ff92
commit 977493cf63
2 changed files with 29 additions and 32 deletions
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1
README.md
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@ -195,7 +195,6 @@ sending DF17 packets), but can detect a few more messages.
The algorithm in aggressive mode is modified in the following ways:
* Preamble detection is weakened to be more liberal in what is consdered valid.
* Up to two demodulation errors are tolerated (adjacent entires in the magnitude
vector with the same eight). Normally only messages without errors are
checked.

60
dump1090.c
View file

@ -1361,38 +1361,36 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
continue;
}
if (!Modes.aggressive) {
/* The samples between the two spikes must be < than the average
* of the high spikes level. We don't test bits too near to
* the high levels as signals can be out of phase so part of the
* energy can be in the near samples. */
high = (m[j]+m[j+2]+m[j+7]+m[j+9])/6;
if (m[j+4] >= high ||
m[j+5] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage(
"Too high level in samples between 3 and 6",
msg, m, j);
continue;
}
/* The samples between the two spikes must be < than the average
* of the high spikes level. We don't test bits too near to
* the high levels as signals can be out of phase so part of the
* energy can be in the near samples. */
high = (m[j]+m[j+2]+m[j+7]+m[j+9])/6;
if (m[j+4] >= high ||
m[j+5] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage(
"Too high level in samples between 3 and 6",
msg, m, j);
continue;
}
/* Similarly samples in the range 11-14 must be low, as it is the
* space between the preamble and real data. Again we don't test
* bits too near to high levels, see above. */
if (m[j+11] >= high ||
m[j+12] >= high ||
m[j+13] >= high ||
m[j+14] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage(
"Too high level in samples between 10 and 15",
msg, m, j);
continue;
}
/* Similarly samples in the range 11-14 must be low, as it is the
* space between the preamble and real data. Again we don't test
* bits too near to high levels, see above. */
if (m[j+11] >= high ||
m[j+12] >= high ||
m[j+13] >= high ||
m[j+14] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage(
"Too high level in samples between 10 and 15",
msg, m, j);
continue;
}
Modes.stat_valid_preamble++;