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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
Change the I/Q data buffer pointet to a uint16_t *
This commit is contained in:
Malcolm Robb 2013-04-09 00:58:32 +01:00
parent 3113223029
commit ed80552ca1
1 changed files with 14 additions and 17 deletions
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dump1090.c
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@ -133,7 +133,7 @@ struct {
pthread_t reader_thread; pthread_t reader_thread;
pthread_mutex_t data_mutex; /* Mutex to synchronize buffer access. */ pthread_mutex_t data_mutex; /* Mutex to synchronize buffer access. */
pthread_cond_t data_cond; /* Conditional variable associated. */ 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 */ uint16_t *magnitude; /* Magnitude vector */
long long timestampBlk; /* Timestamp of the start of the current block */ long long timestampBlk; /* Timestamp of the start of the current block */
int fd; /* --ifile option file descriptor. */ 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 * 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, * every different I/Q pair will result in a different magnitude value,
* not losing any resolution. */ * not losing any resolution. */
Modes.maglut = malloc(129*129*2); Modes.maglut = malloc(256*256*2);
for (i = 0; i <= 128; i++) { for (i = 0; i <= 255; i++) {
for (q = 0; q <= 128; q++) { int mag_i = i-127;
Modes.maglut[i*129+q] = round(sqrt(i*i+q*q)*360); 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);
} }
} }
@ -443,7 +445,7 @@ void readDataFromFile(void) {
} }
toread = MODES_ASYNC_BUF_SIZE; toread = MODES_ASYNC_BUF_SIZE;
p = Modes.data; p = (unsigned char *) Modes.data;
while(toread) { while(toread) {
nread = read(Modes.fd, p, toread); nread = read(Modes.fd, p, toread);
if (nread <= 0) { if (nread <= 0) {
@ -1236,7 +1238,7 @@ void displayModesMessage(struct modesMessage *mm) {
* pointed by Modes.magnitude. */ * pointed by Modes.magnitude. */
void computeMagnitudeVector(void) { void computeMagnitudeVector(void) {
uint16_t *m = &Modes.magnitude[MODES_PREAMBLE_SAMPLES+MODES_LONG_MSG_SAMPLES]; 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; uint32_t j;
memcpy(Modes.magnitude,&Modes.magnitude[MODES_ASYNC_BUF_SAMPLES], MODES_PREAMBLE_SIZE+MODES_LONG_MSG_SIZE); 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 /* 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. */ * we rescale to the 0-255 range to exploit the full resolution. */
for (j = 0; j < MODES_ASYNC_BUF_SAMPLES; j ++) { for (j = 0; j < MODES_ASYNC_BUF_SAMPLES; j ++) {
int i = (*p++)-127; *m++ = Modes.maglut[*p++];
int q = (*p++)-127;
if (i < 0) i = -i;
if (q < 0) q = -q;
*m++ = Modes.maglut[i*129+q];
} }
} }