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Use a lookup table for SNR calculation.

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This commit is contained in:
Oliver Jowett 2014-09-22 14:53:06 +01:00
parent 2e45a59986
commit 76c958b03e
4 changed files with 30 additions and 12 deletions
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8
dump1090.c
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@ -98,6 +98,7 @@ void modesInit(void) {
((Modes.pFileData = (uint16_t *) malloc(MODES_ASYNC_BUF_SIZE) ) == NULL) ||
((Modes.magnitude = (uint16_t *) malloc(MODES_ASYNC_BUF_SIZE+MODES_PREAMBLE_SIZE+MODES_LONG_MSG_SIZE) ) == NULL) ||
((Modes.maglut = (uint16_t *) malloc(sizeof(uint16_t) * 256 * 256) ) == NULL) ||
((Modes.log10lut = (uint16_t *) malloc(sizeof(uint16_t) * 256 * 256) ) == NULL) ||
((Modes.beastOut = (char *) malloc(MODES_RAWOUT_BUF_SIZE) ) == NULL) ||
((Modes.rawOut = (char *) malloc(MODES_RAWOUT_BUF_SIZE) ) == NULL) )
{
@ -185,6 +186,13 @@ void modesInit(void) {
}
}
// Prepare the log10 lookup table.
// This maps from a magnitude value x (scaled as above) to 100log10(x)
for (i = 0; i <= 65535; i++) {
int l10 = (int) round(100 * log10( (i + 365.4798) * 258.433254) );
Modes.log10lut[i] = (uint16_t) ((l10 < 65535 ? l10 : 65535));
}
// Prepare error correction tables
modesInitErrorInfo();
}

3
dump1090.h
View file

@ -88,7 +88,7 @@
#define MODES_ASYNC_BUF_SAMPLES (MODES_ASYNC_BUF_SIZE / 2) // Each sample is 2 bytes
#define MODES_AUTO_GAIN -100 // Use automatic gain
#define MODES_MAX_GAIN 999999 // Use max available gain
#define MODES_MSG_SQUELCH_FACTOR 16 // Min SNR expressed as an amplitude ratio, scaled by 10. 20log(16/10) = 4.1dB
#define MODES_MSG_SQUELCH_DB 4.0 // Minimum SNR, in dB
#define MODES_MSG_ENCODER_ERRS 3 // Maximum number of encoding errors
// When changing, change also fixBitErrors() and modesInitErrorTable() !!
@ -257,6 +257,7 @@ struct { // Internal state
int fd; // --ifile option file descriptor
uint32_t *icao_cache; // Recently seen ICAO addresses cache
uint16_t *maglut; // I/Q -> Magnitude lookup table
uint16_t *log10lut; // Magnitude -> log10 lookup table
int exit; // Exit from the main loop when true
// RTLSDR

12
mode_ac.c
View file

@ -144,7 +144,7 @@ int detectModeA(uint16_t *m, struct modesMessage *mm)
int F1_sig, F1_noise;
int F2_sig, F2_noise;
int fSig, fNoise, fLevel, fLoLo;
float snr;
int snr;
// m[0] contains the energy from 0 -> 499 nS
// m[1] contains the energy from 500 -> 999 nS
@ -307,8 +307,14 @@ int detectModeA(uint16_t *m, struct modesMessage *mm)
if ((ModeABits < 3) || (ModeABits & 0xFFFF8808) || (ModeAErrs) )
{return (ModeABits = 0);}
snr = 5 * 20.0 * log10f((float)(fSig + fNoise + 365) / (fNoise + 365)); // 365 to adjust for magnitude value offset
mm->signalLevel = ((snr < 255) ? (uint8_t)round(snr) : 255);
// snr = 5 * 20log10(fSig / (fSig+fNoise)) (in units of 0.2dB)
// = 100log10(fSig) - 100log10(fSig+fNoise)
while (fSig > 65535 || (fSig + fNoise) > 65535) {
fSig >>= 1;
fNoise >>= 1;
}
snr = Modes.log10lut[fSig] - Modes.log10lut[fSig + fNoise];
mm->signalLevel = ((snr < 255) ? (uint8_t)snr : 255);
return ModeABits;
}

19
mode_s.c
View file

@ -1575,6 +1575,7 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
uint8_t theByte, theErrs;
int msglen, scanlen;
uint32_t sigLevel, noiseLevel;
uint16_t snr;
pPreamble = &m[j];
pPayload = &m[j+MODES_PREAMBLE_SAMPLES];
@ -1779,22 +1780,24 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
}
}
// adjust for magnitude zero offset
sigLevel += 365*56;
noiseLevel += 365*56;
// snr = 5 * 20log10(sigLevel / noiseLevel) (in units of 0.2dB)
// = 100log10(sigLevel) - 100log10(noiseLevel)
while (sigLevel > 65535 || noiseLevel > 65535) {
sigLevel >>= 1;
noiseLevel >>= 1;
}
snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel];
// When we reach this point, if error is small, and the signal strength is large enough
// we may have a Mode S message on our hands. It may still be broken and the CRC may not
// be correct, but this can be handled by the next layer.
if ( (msglen)
&& ((sigLevel * 10) > (noiseLevel * MODES_MSG_SQUELCH_FACTOR)) // (sigLevel/noiseLevel) > (MODES_MSG_SQUELCH_FACTOR/10)
&& ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
float snr;
// Set initial mm structure details
mm.timestampMsg = Modes.timestampBlk + (j*6);
snr = 5.0 * 20.0 * log10f( (float)sigLevel / noiseLevel ); // sig/noise levels are amplitudes, so square them when computing SNR
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)round(snr));
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
mm.phase_corrected = use_correction;
// Decode the received message