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Cleanup dead code in the ADSB RX proc (#1572)

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* Cleanup dead code in the ADSB RX proc

* Fix comments

* Formatting
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Kyle Reed 2023-11-10 11:38:06 -08:00 committed by GitHub
parent f4f538f69b
commit f7f784c0f4
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2 changed files with 59 additions and 61 deletions
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94
firmware/baseband/proc_adsbrx.cpp
View File

@ -20,6 +20,8 @@
* Boston, MA 02110-1301, USA.
*/
// https://www.icao.int/SAM/Documents/2015-SEMAUTOM/Ses4%20Presentation%20CUBA_ADSB.pdf
#include "proc_adsbrx.hpp"
#include "portapack_shared_memory.hpp"
#include "sine_table_int8.hpp"
@ -31,29 +33,27 @@
using namespace adsb;
void ADSBRXProcessor::execute(const buffer_c8_t& buffer) {
int8_t re, im;
uint32_t mag;
uint32_t c;
uint8_t bit, byte{};
// This is called at 2M/2048 = 977Hz
// One pulse = 500ns = 2 samples
// One bit = 2 pulses = 1us = 4 samples
// Each sample is 500ns.
// One bit is 2 samples == 1us.
// Bit value is the transition between samples.
// i.e. lo->hi == 0, hi->lo == 1
if (!configured) return;
uint8_t bit = 0;
uint8_t byte = 0;
for (size_t i = 0; i < buffer.count; i++) {
// Compute sample's magnitude
re = (int32_t)buffer.p[i].real();
im = (int32_t)buffer.p[i].imag();
mag = ((uint32_t)(re * re) + (uint32_t)(im * im));
// Compute sample's magnitude.
int8_t re = buffer.p[i].real();
int8_t im = buffer.p[i].imag();
uint16_t mag = (re * re) + (im * im);
if (decoding) {
// Decode
// 1 bit lasts 2 samples
if (sample_count & 1) {
if (bit_count >= msgLen) {
// 1 bit == 2 samples, transition defines bit value.
if ((sample_count & 1) == 1) {
if (bit_count >= msg_len) {
const ADSBFrameMessage message(frame, amp);
shared_memory.application_queue.push(message);
decoding = false;
@ -65,40 +65,42 @@ void ADSBRXProcessor::execute(const buffer_c8_t& buffer) {
byte = bit | (byte << 1);
bit_count++;
// Perform checks at the end of the first byte
if (!(bit_count & 7)) {
// Store the byte
// Every 8th bit...
if ((bit_count & 0x7) == 0) {
// Store the byte.
frame.push_byte(byte);
// Check at the end of the first byte of the message
// Perform additional check on the first byte.
if (bit_count == 8) {
// Abandon all frames that aren't DF17 or DF18 extended squitters.
uint8_t df = (byte >> 3);
if ((bit_count == 8) && !(df & 0x10)) {
msgLen = 56; // DFs 16 or greater are long 112. DFs 15 or less are short 56.
}
// Abondon all frames that arent DF17 or DF18 extended squitters
if ((bit_count == 8) && !((df == 17) || (df == 18))) {
if (df != 17 && df != 18) {
decoding = false;
bit = (prev_mag > mag) ? 1 : 0;
frame.clear();
}
} // last bit of a byte
} // Second sample of each bit
}
}
}
sample_count++;
}
// Continue looking for preamble even if in a packet
// switch if new preamble is higher magnitude
// Continue looking for preamble, even if in a packet.
// Switch if new preamble is higher magnitude.
// Shift the preamble
for (c = 0; c < (ADSB_PREAMBLE_LENGTH); c++) {
// Shift the preamble.
for (uint8_t c = 0; c < ADSB_PREAMBLE_LENGTH; c++) {
shifter[c] = shifter[c + 1];
}
shifter[ADSB_PREAMBLE_LENGTH] = mag;
// 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
// First check of relations between the first 12 samples
// representing a valid preamble. We don't even investigate
// further if this simple test is not passed.
// Preamble is 8us - or 16 samples.
// 0123456789ABCDEF
// _-_-____-_-_____
if (shifter[0] < shifter[1] &&
shifter[1] > shifter[2] &&
shifter[2] < shifter[3] &&
@ -113,32 +115,30 @@ void ADSBRXProcessor::execute(const buffer_c8_t& buffer) {
// 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
int32_t thisAmp = (shifter[1] + shifter[3] + shifter[8] + shifter[10]);
uint32_t high = thisAmp / 9;
if (
shifter[5] < high &&
// energy can be in the near samples.
int32_t this_amp = (shifter[1] + shifter[3] + shifter[8] + shifter[10]);
uint32_t high = this_amp / 9; // TBD: Why 9?
if (shifter[5] < high &&
shifter[6] < high &&
// Similarly samples in the range 11-13 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
// bits too near to high levels, see above.
shifter[12] < high &&
shifter[13] < high &&
shifter[14] < high) {
if ((decoding == false) || // New preamble
((decoding == true) && (thisAmp > amp))) // Higher power than existing packet
((decoding == true) && (this_amp > amp))) // Higher power than existing packet
{
decoding = true;
msgLen = 112;
amp = thisAmp;
amp = this_amp;
sample_count = 0;
bit_count = 0;
frame.clear();
}
} // 4 & 5 low and 11-14 low
} // Check for preamble pattern
}
}
// Store mag for next time
// Store mag for next time.
prev_mag = mag;
}
}

18
firmware/baseband/proc_adsbrx.hpp
View File

@ -39,21 +39,19 @@ class ADSBRXProcessor : public BasebandProcessor {
void on_message(const Message* const message) override;
private:
static constexpr size_t baseband_fs = 2000000;
static constexpr size_t baseband_fs = 2'000'000;
static constexpr size_t msg_len = 112;
ADSBFrame frame{};
bool configured{false};
bool decoding{false};
uint32_t prev_mag{0};
size_t bit_count{0}, sample_count{0};
size_t msgLen{112};
uint32_t shifter[ADSB_PREAMBLE_LENGTH + 1];
bool decoding{};
bool preamble{}, active{};
uint16_t bit_pos{0};
uint8_t cur_bit{0};
uint32_t sample{0};
int32_t re{}, im{};
int32_t amp{0};
size_t bit_count{0};
size_t sample_count{0};
uint32_t shifter[ADSB_PREAMBLE_LENGTH + 1];
/* NB: Threads should be the last members in the class definition. */
BasebandThread baseband_thread{baseband_fs, this, baseband::Direction::Receive};