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Speed up the I/Q to magnitude calculation

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Increase the speed of the I/Q to magnitude calculation lookup by
expanding the table to 65536 entries (256*256*2 bytes). At runtime, this
allows us to pick up raw I/Q bytes as a 16 bit value and index into the
magnitude table to get a 16 bit result. This removes the need for
subtracting 127, and then correcting for -ve numbers, so should be
faster, at the expense of a larger data table.

Change the maglut lookup table from 129*129 to 256*256
Initialise the maglut buffer accordingly
Change the data->maglut lookup to use the new maglut buffer
This commit is contained in:
Malcolm Robb 2013-04-08 17:55:17 +01:00
parent 43c648448f
commit 192cb80c08
1 changed files with 13 additions and 16 deletions
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21
dump1090.c
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@ -133,7 +133,7 @@ struct {
pthread_t reader_thread;
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 */
uint16_t *data; /* Raw IQ samples buffer */
uint16_t *magnitude; /* Magnitude vector */
long long timestampBlk; /* Timestamp of the start of the current block */
int fd; /* --ifile option file descriptor. */
@ -328,10 +328,12 @@ 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*2);
for (i = 0; i <= 128; i++) {
for (q = 0; q <= 128; q++) {
Modes.maglut[i*129+q] = round(sqrt(i*i+q*q)*360);
Modes.maglut = malloc(256*256*2);
for (i = 0; i <= 255; i++) {
int mag_i = i-127;
for (q = 0; q <= 255; q++) {
int mag_q = q - 127;
Modes.maglut[i*256+q] = round(sqrt((mag_i*mag_i)+(mag_q*mag_q))*360);
}
}
@ -1236,7 +1238,7 @@ void displayModesMessage(struct modesMessage *mm) {
* pointed by Modes.magnitude. */
void computeMagnitudeVector(void) {
uint16_t *m = &Modes.magnitude[MODES_PREAMBLE_SAMPLES+MODES_LONG_MSG_SAMPLES];
unsigned char *p = Modes.data;
uint16_t *p = Modes.data;
uint32_t j;
memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], MODES_PREAMBLE_SIZE+MODES_LONG_MSG_SIZE);
@ -1244,12 +1246,7 @@ void computeMagnitudeVector(void) {
/* Compute the magnitudo vector. It's just SQRT(I^2 + Q^2), but
* we rescale to the 0-255 range to exploit the full resolution. */
for (j = 0; j < MODES_ASYNC_BUF_SAMPLES; j ++) {
int i = (*p++)-127;
int q = (*p++)-127;
if (i < 0) i = -i;
if (q < 0) q = -q;
*m++ = Modes.maglut[i*129+q];
*m++ = Modes.maglut[*p++];
}
}