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Use 16 bit for the magnitude vector to improve sensitivity.

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As @keenerd noted on ##rtlsdr, using an 8 bit magnitude vector is not
enough in order to distinguish every different I/Q pair.

With this commit a few more messages with good CRC are detected.
This commit is contained in:
antirez 2013-01-06 16:18:07 +01:00
parent 417cab5fda
commit b3945d1b4f
1 changed files with 18 additions and 15 deletions
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33
dump1090.c
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@ -95,13 +95,13 @@ struct {
pthread_mutex_t data_mutex; /* Mutex to synchronize buffer access. */
pthread_cond_t data_cond; /* Conditional variable associated. */
unsigned char *data; /* Raw IQ samples buffer */
unsigned char *magnitude; /* Magnitude vector */
uint16_t *magnitude; /* Magnitude vector */
uint32_t data_len; /* Buffer length. */
int fd; /* --ifile option file descriptor. */
int data_ready; /* Data ready to be processed. */
uint32_t icao_cache[MODES_ICAO_CACHE_LEN];/* Recently seen ICAO addresses */
int icao_cache_idx; /* icao_cache circular buf idx. */
unsigned char *maglut; /* I/Q -> Magnitude lookup table. */
uint16_t *maglut; /* I/Q -> Magnitude lookup table. */
int exit; /* Exit from the main loop when true. */
/* RTLSDR */
@ -220,7 +220,7 @@ void modesInit(void) {
Modes.aircrafts = NULL;
Modes.interactive_last_update = 0;
if ((Modes.data = malloc(Modes.data_len)) == NULL ||
(Modes.magnitude = malloc(Modes.data_len)) == NULL) {
(Modes.magnitude = malloc(Modes.data_len*2)) == NULL) {
fprintf(stderr, "Out of memory allocating data buffer.\n");
exit(1);
}
@ -232,10 +232,10 @@ void modesInit(void) {
* We scale to 0-255 range multiplying by 1.4 in order to ensure that
* every different I/Q pair will result in a different magnitude value,
* not losing any resolution. */
Modes.maglut = malloc(129*129);
Modes.maglut = malloc(129*129*2);
for (i = 0; i <= 128; i++) {
for (q = 0; q <= 128; q++) {
Modes.maglut[i*129+q] = round(sqrt(i*i+q*q)*1.408);
Modes.maglut[i*129+q] = round(sqrt(i*i+q*q)*360);
}
}
@ -398,14 +398,16 @@ void dumpMagnitudeBar(int index, int magnitude) {
* If possible a few samples before the start of the messsage are included
* for context. */
void dumpMagnitudeVector(unsigned char *m, uint32_t offset) {
void dumpMagnitudeVector(uint16_t *m, uint32_t offset) {
uint32_t padding = 5; /* Show 5 samples before the actual start. */
uint32_t start = (offset < padding) ? 0 : offset-padding;
uint32_t end = offset + (MODES_PREAMBLE_US*2)+(MODES_SHORT_MSG_BITS*2) - 1;
uint32_t j;
for (j = start; j <= end; j++)
dumpMagnitudeBar(j-offset, m[j]);
for (j = start; j <= end; j++) {
/* Scale magnitude to 0-255 from 0-65535 before printing. */
dumpMagnitudeBar(j-offset, m[j]/256);
}
}
/* This is a wrapper for dumpMagnitudeVector() that also show the message
@ -417,7 +419,7 @@ void dumpMagnitudeVector(unsigned char *m, uint32_t offset) {
* offset is the offset where the message starts
*/
void dumpRawMessage(char *descr, unsigned char *msg,
unsigned char *m, uint32_t offset)
uint16_t *m, uint32_t offset)
{
int j;
@ -935,7 +937,8 @@ void displayModesMessage(struct modesMessage *mm) {
/* Turn I/Q samples pointed by Modes.data into the magnitude vector
* pointed by Modes.magnitude. */
void computeMagnitudeVector(void) {
unsigned char *m = Modes.magnitude, *p = Modes.data;
uint16_t *m = Modes.magnitude;
unsigned char *p = Modes.data;
uint32_t j;
/* Compute the magnitudo vector. It's just SQRT(I^2 + Q^2), but
@ -953,7 +956,7 @@ void computeMagnitudeVector(void) {
/* Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of
* size 'mlen' bytes. Every detected Mode S message is convert it into a
* stream of bits and passed to the function to display it. */
void detectModeS(unsigned char *m, uint32_t mlen) {
void detectModeS(uint16_t *m, uint32_t mlen) {
unsigned char bits[MODES_LONG_MSG_BITS];
unsigned char msg[MODES_LONG_MSG_BITS/2];
uint32_t j;
@ -1082,10 +1085,10 @@ void detectModeS(unsigned char *m, uint32_t mlen) {
}
delta /= msglen*4;
/* A avg delta of three is small enough to let almost every kind of
* message to pass, but high enough to filter some random noise,
* especially when --no-crc-check is used. */
if (delta < 3) continue;
/* Filter for an average delta of three is small enough to let almost
* every kind of message to pass, but high enough to filter some
* random noise. */
if (delta < 10*255) continue;
/* If we reached this point, and error is zero, we are very likely
* with a Mode S message in our hands, but it may still be broken