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dump1090/demod_2000.c

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Separate out 2MHz demodulator + support functions to a separate file.
2015-01-20 00:47:51 +01:00
// dump1090, a Mode S messages decoder for RTLSDR devices.
//
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
#include "dump1090.h"
// Mode S 2.0MHz demodulator
// ============================== Debugging =================================
//
// Helper function for dumpMagnitudeVector().
// It prints a single bar used to display raw signals.
//
// Since every magnitude sample is between 0-255, the function uses
// up to 63 characters for every bar. Every character represents
// a length of 4, 3, 2, 1, specifically:
//
// "O" is 4
// "o" is 3
// "-" is 2
// "." is 1
//
static void dumpMagnitudeBar(int index, int magnitude) {
char *set = " .-o";
char buf[256];
int div = magnitude / 256 / 4;
int rem = magnitude / 256 % 4;
memset(buf,'O',div);
buf[div] = set[rem];
buf[div+1] = '\0';
if (index >= 0)
printf("[%.3d] |%-66s 0x%04X\n", index, buf, magnitude);
else
printf("[%.2d] |%-66s 0x%04X\n", index, buf, magnitude);
}
//
//=========================================================================
//
// Display an ASCII-art alike graphical representation of the undecoded
// message as a magnitude signal.
//
// The message starts at the specified offset in the "m" buffer.
// The function will display enough data to cover a short 56 bit message.
//
// If possible a few samples before the start of the messsage are included
// for context.
//
static void dumpMagnitudeVector(uint16_t *m, uint32_t offset) {
uint32_t padding = 5; // Show a few samples before the actual start.
uint32_t start = (offset < padding) ? 0 : offset-padding;
uint32_t end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_SHORT_MSG_SAMPLES) - 1;
uint32_t j;
for (j = start; j <= end; j++) {
dumpMagnitudeBar(j-offset, m[j]);
}
}
//
//=========================================================================
//
// Produce a raw representation of the message as a Javascript file
// loadable by debug.html.
//
static void dumpRawMessageJS(char *descr, unsigned char *msg,
uint16_t *m, uint32_t offset, int fixable, char *bitpos)
{
int padding = 5; // Show a few samples before the actual start.
int start = offset - padding;
int end = offset + (MODES_PREAMBLE_SAMPLES)+(MODES_LONG_MSG_SAMPLES) - 1;
FILE *fp;
int j;
MODES_NOTUSED(fixable);
if ((fp = fopen("frames.js","a")) == NULL) {
fprintf(stderr, "Error opening frames.js: %s\n", strerror(errno));
exit(1);
}
fprintf(fp,"frames.push({\"descr\": \"%s\", \"mag\": [", descr);
for (j = start; j <= end; j++) {
fprintf(fp,"%d", j < 0 ? 0 : m[j]);
if (j != end) fprintf(fp,",");
}
fprintf(fp,"], \"fix1\": %d, \"fix2\": %d, \"bits\": %d, \"hex\": \"",
bitpos[0], bitpos[1] , modesMessageLenByType(msg[0]>>3));
for (j = 0; j < MODES_LONG_MSG_BYTES; j++)
fprintf(fp,"\\x%02x",msg[j]);
fprintf(fp,"\"});\n");
fclose(fp);
}
//
//=========================================================================
//
// This is a wrapper for dumpMagnitudeVector() that also show the message
// in hex format with an additional description.
//
// descr is the additional message to show to describe the dump.
// msg points to the decoded message
// m is the original magnitude vector
// offset is the offset where the message starts
//
// The function also produces the Javascript file used by debug.html to
// display packets in a graphical format if the Javascript output was
// enabled.
//
static void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset) {
int j;
int msgtype = msg[0] >> 3;
int fixable = 0;
char bitpos[MODES_MAX_BITERRORS];
for (j = 0; j < MODES_MAX_BITERRORS; j++) {
bitpos[j] = -1;
}
if (msgtype == 17) {
fixable = fixBitErrors(msg, MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, bitpos);
}
if (Modes.debug & MODES_DEBUG_JS) {
dumpRawMessageJS(descr, msg, m, offset, fixable, bitpos);
return;
}
printf("\n--- %s\n ", descr);
for (j = 0; j < MODES_LONG_MSG_BYTES; j++) {
printf("%02x",msg[j]);
if (j == MODES_SHORT_MSG_BYTES-1) printf(" ... ");
}
printf(" (DF %d, Fixable: %d)\n", msgtype, fixable);
dumpMagnitudeVector(m,offset);
printf("---\n\n");
}
//
//=========================================================================
//
// Return -1 if the message is out of fase left-side
// Return 1 if the message is out of fase right-size
// Return 0 if the message is not particularly out of phase.
//
// Note: this function will access pPreamble[-1], so the caller should make sure to
// call it only if we are not at the start of the current buffer
//
static int detectOutOfPhase(uint16_t *pPreamble) {
if (pPreamble[ 3] > pPreamble[2]/3) return 1;
if (pPreamble[10] > pPreamble[9]/3) return 1;
if (pPreamble[ 6] > pPreamble[7]/3) return -1;
if (pPreamble[-1] > pPreamble[1]/3) return -1;
return 0;
}
static uint16_t clamped_scale(uint16_t v, uint16_t scale) {
uint32_t scaled = (uint32_t)v * scale / 16384;
if (scaled > 65535) return 65535;
return (uint16_t) scaled;
}
// This function decides whether we are sampling early or late,
// and by approximately how much, by looking at the energy in
// preamble bits before and after the expected pulse locations.
//
// It then deals with one sample pair at a time, comparing samples
// to make a decision about the bit value. Based on this decision it
// modifies the sample value of the *adjacent* sample which will
// contain some of the energy from the bit we just inspected.
//
// pPayload[0] should be the start of the preamble,
// pPayload[-1 .. MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] should be accessible.
// pPayload[MODES_PREAMBLE_SAMPLES .. MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] will be updated.
static void applyPhaseCorrection(uint16_t *pPayload) {
int j;
// we expect 1 bits at 0, 2, 7, 9
// and 0 bits at -1, 1, 3, 4, 5, 6, 8, 10, 11, 12, 13, 14
// use bits -1,6 for early detection (bit 0/7 arrived a little early, our sample period starts after the bit phase so we include some of the next bit)
// use bits 3,10 for late detection (bit 2/9 arrived a little late, our sample period starts before the bit phase so we include some of the last bit)
uint32_t onTime = (pPayload[0] + pPayload[2] + pPayload[7] + pPayload[9]);
uint32_t early = (pPayload[-1] + pPayload[6]) << 1;
uint32_t late = (pPayload[3] + pPayload[10]) << 1;
if (onTime == 0 && early == 0 && late == 0) {
// Blah, can't do anything with this, avoid a divide-by-zero
return;
}
if (early > late) {
// Our sample period starts late and so includes some of the next bit.
uint16_t scaleUp = 16384 + 16384 * early / (early + onTime); // 1 + early / (early+onTime)
uint16_t scaleDown = 16384 - 16384 * early / (early + onTime); // 1 - early / (early+onTime)
// trailing bits are 0; final data sample will be a bit low.
pPayload[MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1] =
clamped_scale(pPayload[MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 1], scaleUp);
for (j = MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 2; j > MODES_PREAMBLE_SAMPLES; j -= 2) {
if (pPayload[j] > pPayload[j+1]) {
// x [1 0] y
// x overlapped with the "1" bit and is slightly high
pPayload[j-1] = clamped_scale(pPayload[j-1], scaleDown);
} else {
// x [0 1] y
// x overlapped with the "0" bit and is slightly low
pPayload[j-1] = clamped_scale(pPayload[j-1], scaleUp);
}
}
} else {
// Our sample period starts early and so includes some of the previous bit.
uint16_t scaleUp = 16384 + 16384 * late / (late + onTime); // 1 + late / (late+onTime)
uint16_t scaleDown = 16384 - 16384 * late / (late + onTime); // 1 - late / (late+onTime)
// leading bits are 0; first data sample will be a bit low.
pPayload[MODES_PREAMBLE_SAMPLES] = clamped_scale(pPayload[MODES_PREAMBLE_SAMPLES], scaleUp);
for (j = MODES_PREAMBLE_SAMPLES; j < MODES_PREAMBLE_SAMPLES + MODES_LONG_MSG_SAMPLES - 2; j += 2) {
if (pPayload[j] > pPayload[j+1]) {
// x [1 0] y
// y overlapped with the "0" bit and is slightly low
pPayload[j+2] = clamped_scale(pPayload[j+2], scaleUp);
} else {
// x [0 1] y
// y overlapped with the "1" bit and is slightly high
pPayload[j+2] = clamped_scale(pPayload[j+2], scaleDown);
}
}
}
}
//
//=========================================================================
//
// 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 demodulate2000(uint16_t *m, uint32_t mlen) {
struct modesMessage mm;
unsigned char msg[MODES_LONG_MSG_BYTES], *pMsg;
uint16_t aux[MODES_PREAMBLE_SAMPLES+MODES_LONG_MSG_SAMPLES+1];
uint32_t j;
int use_correction = 0;
memset(&mm, 0, sizeof(mm));
// The Mode S preamble is made of impulses of 0.5 microseconds at
// the following time offsets:
//
// 0 - 0.5 usec: first impulse.
// 1.0 - 1.5 usec: second impulse.
// 3.5 - 4 usec: third impulse.
// 4.5 - 5 usec: last impulse.
//
// Since we are sampling at 2 Mhz every sample in our magnitude vector
// is 0.5 usec, so the preamble will look like this, assuming there is
// an impulse at offset 0 in the array:
//
// 0 -----------------
// 1 -
// 2 ------------------
// 3 --
// 4 -
// 5 --
// 6 -
// 7 ------------------
// 8 --
// 9 -------------------
//
for (j = 0; j < mlen; j++) {
int high, i, errors, errors56, errorsTy;
uint16_t *pPreamble, *pPayload, *pPtr;
uint8_t theByte, theErrs;
int msglen, scanlen;
uint32_t sigLevel, noiseLevel;
uint16_t snr;
pPreamble = &m[j];
pPayload = &m[j+MODES_PREAMBLE_SAMPLES];
// Rather than clear the whole mm structure, just clear the parts which are required. The clear
// is required for every bit of the input stream, and we don't want to be memset-ing the whole
// modesMessage structure two million times per second if we don't have to..
mm.bFlags =
mm.crcok =
mm.correctedbits = 0;
if (!use_correction) // This is not a re-try with phase correction
{ // so try to find a new preamble
if (Modes.mode_ac)
{
int ModeA = detectModeA(pPreamble, &mm);
if (ModeA) // We have found a valid ModeA/C in the data
{
mm.timestampMsg = Modes.timestampBlk + ((j+1) * 6);
// Decode the received message
decodeModeAMessage(&mm, ModeA);
// Pass data to the next layer
useModesMessage(&mm);
j += MODEAC_MSG_SAMPLES;
Modes.stat_ModeAC++;
continue;
}
}
// First check of relations between the first 10 samples
// representing a valid preamble. We don't even investigate further
// if this simple test is not passed
if (!(pPreamble[0] > pPreamble[1] &&
pPreamble[1] < pPreamble[2] &&
pPreamble[2] > pPreamble[3] &&
pPreamble[3] < pPreamble[0] &&
pPreamble[4] < pPreamble[0] &&
pPreamble[5] < pPreamble[0] &&
pPreamble[6] < pPreamble[0] &&
pPreamble[7] > pPreamble[8] &&
pPreamble[8] < pPreamble[9] &&
pPreamble[9] > pPreamble[6]))
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
*pPreamble > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage("Unexpected ratio among first 10 samples", 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 = (pPreamble[0] + pPreamble[2] + pPreamble[7] + pPreamble[9]) / 6;
if (pPreamble[4] >= high ||
pPreamble[5] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
*pPreamble > 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 (pPreamble[11] >= high ||
pPreamble[12] >= high ||
pPreamble[13] >= high ||
pPreamble[14] >= high)
{
if (Modes.debug & MODES_DEBUG_NOPREAMBLE &&
*pPreamble > MODES_DEBUG_NOPREAMBLE_LEVEL)
dumpRawMessage("Too high level in samples between 10 and 15", msg, m, j);
continue;
}
Modes.stat_valid_preamble++;
}
else {
// If the previous attempt with this message failed, retry using
// magnitude correction
// Make a copy of the Payload, and phase correct the copy
memcpy(aux, &pPreamble[-1], sizeof(aux));
applyPhaseCorrection(&aux[1]);
Modes.stat_out_of_phase++;
pPayload = &aux[1 + MODES_PREAMBLE_SAMPLES];
// TODO ... apply other kind of corrections
}
// Decode all the next 112 bits, regardless of the actual message
// size. We'll check the actual message type later
pMsg = &msg[0];
pPtr = pPayload;
theByte = 0;
theErrs = 0; errorsTy = 0;
errors = 0; errors56 = 0;
// We should have 4 'bits' of 0/1 and 1/0 samples in the preamble,
// so include these in the signal strength
sigLevel = pPreamble[0] + pPreamble[2] + pPreamble[7] + pPreamble[9];
noiseLevel = pPreamble[1] + pPreamble[3] + pPreamble[4] + pPreamble[6] + pPreamble[8];
msglen = scanlen = MODES_LONG_MSG_BITS;
for (i = 0; i < scanlen; i++) {
uint32_t a = *pPtr++;
uint32_t b = *pPtr++;
if (a > b)
{theByte |= 1; if (i < 56) { sigLevel += a; noiseLevel += b; }}
else if (a < b)
{/*theByte |= 0;*/ if (i < 56) { sigLevel += b; noiseLevel += a; }}
else {
if (i < 56) { sigLevel += a; noiseLevel += a; }
if (i >= MODES_SHORT_MSG_BITS) //(a == b), and we're in the long part of a frame
{errors++; /*theByte |= 0;*/}
else if (i >= 5) //(a == b), and we're in the short part of a frame
{scanlen = MODES_LONG_MSG_BITS; errors56 = ++errors;/*theByte |= 0;*/}
else if (i) //(a == b), and we're in the message type part of a frame
{errorsTy = errors56 = ++errors; theErrs |= 1; /*theByte |= 0;*/}
else //(a == b), and we're in the first bit of the message type part of a frame
{errorsTy = errors56 = ++errors; theErrs |= 1; theByte |= 1;}
}
if ((i & 7) == 7)
{*pMsg++ = theByte;}
else if (i == 4) {
msglen = modesMessageLenByType(theByte);
if (errors == 0)
{scanlen = msglen;}
}
theByte = theByte << 1;
if (i < 7)
{theErrs = theErrs << 1;}
// If we've exceeded the permissible number of encoding errors, abandon ship now
if (errors > MODES_MSG_ENCODER_ERRS) {
if (i < MODES_SHORT_MSG_BITS) {
msglen = 0;
} else if ((errorsTy == 1) && (theErrs == 0x80)) {
// If we only saw one error in the first bit of the byte of the frame, then it's possible
// we guessed wrongly about the value of the bit. We may be able to correct it by guessing
// the other way.
//
// We guessed a '1' at bit 7, which is the DF length bit == 112 Bits.
// Inverting bit 7 will change the message type from a long to a short.
// Invert the bit, cross your fingers and carry on.
msglen = MODES_SHORT_MSG_BITS;
msg[0] ^= theErrs; errorsTy = 0;
errors = errors56; // revert to the number of errors prior to bit 56
Modes.stat_DF_Len_Corrected++;
} else if (i < MODES_LONG_MSG_BITS) {
msglen = MODES_SHORT_MSG_BITS;
errors = errors56;
} else {
msglen = MODES_LONG_MSG_BITS;
}
break;
}
}
// Ensure msglen is consistent with the DF type
if (msglen > 0) {
i = modesMessageLenByType(msg[0] >> 3);
if (msglen > i) {msglen = i;}
else if (msglen < i) {msglen = 0;}
}
//
// If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
// Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
// message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
// if the new value is ICAO defined, then update it in our message.
if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
// We guessed at one (and only one) of the message type bits. See if our guess is "likely"
// to be correct by comparing the DF against a list of known good DF's
int thisDF = ((theByte = msg[0]) >> 3) & 0x1f;
uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
uint32_t thisDFbit = (1 << thisDF);
if (0 == (validDFbits & thisDFbit)) {
// The current DF is not ICAO defined, so is probably an errors.
// Toggle the bit we guessed at and see if the resultant DF is more likely
theByte ^= theErrs;
thisDF = (theByte >> 3) & 0x1f;
thisDFbit = (1 << thisDF);
// if this DF any more likely?
if (validDFbits & thisDFbit) {
// Yep, more likely, so update the main message
msg[0] = theByte;
Modes.stat_DF_Type_Corrected++;
errors--; // decrease the error count so we attempt to use the modified DF.
}
}
}
// 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)
&& ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
// Set initial mm structure details
mm.timestampMsg = Modes.timestampBlk + (j*6);
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
mm.phase_corrected = use_correction;
// Decode the received message
decodeModesMessage(&mm, msg);
// Update statistics
if (Modes.stats) {
struct demod_stats *dstats = (use_correction ? &Modes.stat_demod_phasecorrected : &Modes.stat_demod);
switch (errors) {
case 0: dstats->demodulated0++; break;
case 1: dstats->demodulated1++; break;
case 2: dstats->demodulated2++; break;
default: dstats->demodulated3++; break;
}
if (mm.crcok) {
dstats->goodcrc++;
dstats->goodcrc_byphase[0]++;
} else if (mm.correctedbits > 0) {
dstats->badcrc++;
dstats->fixed++;
if (mm.correctedbits <= MODES_MAX_BITERRORS)
dstats->bit_fix[mm.correctedbits-1] += 1;
} else {
dstats->badcrc++;
}
}
// Output debug mode info if needed
if (use_correction) {
if (Modes.debug & MODES_DEBUG_DEMOD)
dumpRawMessage("Demodulated with 0 errors", msg, m, j);
else if (Modes.debug & MODES_DEBUG_BADCRC &&
mm.msgtype == 17 &&
(!mm.crcok || mm.correctedbits != 0))
dumpRawMessage("Decoded with bad CRC", msg, m, j);
else if (Modes.debug & MODES_DEBUG_GOODCRC && mm.crcok &&
mm.correctedbits == 0)
dumpRawMessage("Decoded with good CRC", msg, m, j);
}
// Skip this message if we are sure it's fine
if (mm.crcok || mm.correctedbits) {
j += (MODES_PREAMBLE_US+msglen)*2 - 1;
}
// Pass data to the next layer
useModesMessage(&mm);
} else {
if (Modes.debug & MODES_DEBUG_DEMODERR && use_correction) {
printf("The following message has %d demod errors\n", errors);
dumpRawMessage("Demodulated with errors", msg, m, j);
}
}
// Retry with phase correction if enabled, necessary and possible.
if (Modes.phase_enhance && !mm.crcok && !mm.correctedbits && !use_correction && j && detectOutOfPhase(pPreamble)) {
use_correction = 1; j--;
} else {
use_correction = 0;
}
}
}
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