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POCSAG2 Revised bit extractor (#1439)

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* Better bit extraction WIP
* Parallel clock signal detection for bit extraction
* Relax clock detection accuracy.
* Reset RateInfo state. TODOs
This commit is contained in:
Kyle Reed 2023-09-09 06:18:42 -07:00 committed by GitHub
parent b3312a704a
commit 4926cf8df5
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 77 additions and 152 deletions
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177
firmware/baseband/proc_pocsag2.cpp
View File

@ -36,7 +36,7 @@ using namespace std;
namespace {
/* Count of bits that differ between the two values. */
uint8_t differ_bit_count(uint32_t left, uint32_t right) {
uint8_t diff_bit_count(uint32_t left, uint32_t right) {
uint32_t diff = left ^ right;
uint8_t count = 0;
for (size_t i = 0; i < sizeof(diff) * 8; ++i) {
@ -127,150 +127,89 @@ uint32_t BitQueue::data() const {
void BitExtractor::extract_bits(const buffer_f32_t& audio) {
// Assumes input has been normalized +/- 1.0f.
for (size_t i = 0; i < audio.count; ++i) {
sample_ = audio.p[i];
++sample_index_;
auto sample = audio.p[i];
samples_until_next_ -= 1;
// There's a transition when both sides of the XOR are the
// same which will result in a the overall value being 0.
bool is_transition = ((last_sample_ < 0) ^ (sample_ >= 0)) == 0;
if (is_transition) {
if (handle_transition())
bad_transitions_ = 0;
else
++bad_transitions_;
// Too many bad transitions? Reset.
if (bad_transitions_ > bad_transition_reset_threshold)
reset();
if (!current_rate_) {
// Feed the known rate queues for clock detection.
for (auto& rate : known_rates_) {
if (handle_sample(rate, sample) &&
diff_bit_count(rate.bits.data(), clock_magic_number) <= 2) {
// Clock detected.
// NB: This block should only happen on the second sample of a pulse.
// samples_until_next_ to start sampling the *next* pulse.
current_rate_ = &rate;
samples_until_next_ = rate.sample_interval;
ready_to_send_ = false;
}
}
}
// Time to push the next bit?
if (sample_index_ >= next_bit_center_) {
// Use the two most recent samples for the bit value.
auto val = (sample_ + last_sample_) / 2.0;
bits_.push(val < 0); // NB: '1' is negative.
// Have a clock rate and it's time to process the next sample.
if (current_rate_ && samples_until_next_ <= 0) {
// TODO: It seems like it would be possible to combine this
// code with handle_sample. Nearly the same work.
if (current_rate_)
next_bit_center_ += current_rate_->bit_length;
// Only send on the second sample of a bit.
// Sampling twice helps mitigate noisy audio data.
if (ready_to_send_) {
auto value = (prev_sample_ + sample) / 2;
bits_.push(signbit(value)); // NB: negative == '1'
}
ready_to_send_ = !ready_to_send_;
prev_sample_ = sample;
samples_until_next_ += current_rate_->sample_interval;
}
last_sample_ = sample_;
}
}
void BitExtractor::configure(uint32_t sample_rate) {
sample_rate_ = sample_rate;
min_valid_length_ = UINT16_MAX;
// Build the baud rate info table based on the sample rate.
for (auto& info : known_rates_) {
info.bit_length = sample_rate / info.baud_rate;
// Allow for 20% deviation.
info.min_bit_length = 0.80 * info.bit_length;
info.max_bit_length = 1.20 * info.bit_length;
if (info.min_bit_length < min_valid_length_)
min_valid_length_ = info.min_bit_length;
}
reset();
// Sampling at 2x the baud rate to synchronize to bit transitions
// without needing to know exact transition boundaries.
for (auto& rate : known_rates_)
rate.sample_interval = sample_rate / (2.0 * rate.baud_rate);
}
void BitExtractor::reset() {
current_rate_ = nullptr;
rate_misses_ = 0;
samples_until_next_ = 0.0;
prev_sample_ = 0.0;
ready_to_send_ = false;
sample_ = 0.0;
last_sample_ = 0.0;
next_bit_center_ = 0.0;
sample_index_ = 0;
last_transition_index_ = 0;
bad_transitions_ = 0;
for (auto& rate : known_rates_) {
rate.samples_until_next = 0.0;
rate.last_sample = 0.0;
rate.bits.reset();
}
}
uint16_t BitExtractor::baud_rate() const {
return current_rate_ ? current_rate_->baud_rate : 0;
}
bool BitExtractor::handle_transition() {
auto length = sample_index_ - last_transition_index_;
last_transition_index_ = sample_index_;
bool BitExtractor::handle_sample(RateInfo& rate, float sample) {
// TODO: Still getting some clock misses at the start of messages.
rate.samples_until_next -= 1;
// Length is too short, ignore this.
if (length <= min_valid_length_) return false;
// Not time to process a sample yet.
if (rate.samples_until_next > 0)
return false;
// TODO: should the following be "bad" or "rate misses"?
// Is length a multiple of the current rate's bit length?
uint16_t bit_count = 0;
if (!count_bits(length, bit_count)) return false;
// Sample signs are the same, both samples are in the same bit pulse.
auto has_new_bit = signbit(sample) == signbit(rate.last_sample);
if (has_new_bit)
rate.bits.push(signbit(sample)); // NB: negative == '1'
// Does the bit length correspond to a known rate?
auto bit_length = length / static_cast<float>(bit_count);
auto rate = get_baud_info(bit_length);
if (!rate) return false;
// How long until the next sample?
rate.samples_until_next += rate.sample_interval;
rate.last_sample = sample;
// Set current rate if it hasn't been set yet.
if (!current_rate_)
current_rate_ = rate;
// Maybe current rate isn't the best rate?
auto rate_miss = rate != current_rate_;
if (rate_miss) {
++rate_misses_;
// Lots of rate misses, try another rate.
if (rate_misses_ > rate_miss_reset_threshold) {
current_rate_ = rate;
rate_misses_ = 0;
}
} else {
// Transition is aligned with the current rate, predict next bit.
auto half_bit = current_rate_->bit_length / 2.0;
next_bit_center_ = sample_index_ + half_bit;
}
return true;
}
bool BitExtractor::count_bits(uint32_t length, uint16_t& bit_count) {
bit_count = 0;
// No rate yet, assume one valid bit. Downstream will deal with it.
if (!current_rate_) {
bit_count = 1;
return true;
}
// How many bits span the specified length?
float exact_bits = length / current_rate_->bit_length;
// < 1 bit, current rate is probably too low.
if (exact_bits < 0.80) return false;
// Round to the nearest # of bits and determine how
// well the current rate fits the data.
float round_bits = std::round(exact_bits);
float error = std::abs(exact_bits - round_bits) / exact_bits;
// Good transition are w/in 15% of current rate estimate.
bit_count = round_bits;
return error < 0.15;
}
const BitExtractor::BaudInfo* BitExtractor::get_baud_info(float bit_length) const {
// NB: This assumes known_rates_ are ordered slowest first.
for (const auto& info : known_rates_) {
if (bit_length >= info.min_bit_length &&
bit_length <= info.max_bit_length) {
return &info;
}
}
return nullptr;
return has_new_bit;
}
/* CodewordExtractor *************************************/
@ -286,9 +225,9 @@ void CodewordExtractor::process_bits() {
// Wait for the sync frame.
if (!has_sync_) {
if (differ_bit_count(data_, sync_codeword) <= 2)
if (diff_bit_count(data_, sync_codeword) <= 2)
handle_sync(/*inverted=*/false);
else if (differ_bit_count(data_, ~sync_codeword) <= 2)
else if (diff_bit_count(data_, ~sync_codeword) <= 2)
handle_sync(/*inverted=*/true);
continue;
}

52
firmware/baseband/proc_pocsag2.hpp
View File

@ -84,52 +84,38 @@ class BitExtractor {
void extract_bits(const buffer_f32_t& audio);
void configure(uint32_t sample_rate);
void reset();
uint16_t baud_rate() const;
private:
/* Number of rate misses that would cause a rate update. */
static constexpr uint8_t rate_miss_reset_threshold = 5;
/* Clock signal detection magic number. */
static constexpr uint32_t clock_magic_number = 0xAAAAAAAA;
/* Number of rate misses that would cause a rate update. */
static constexpr uint8_t bad_transition_reset_threshold = 10;
struct RateInfo {
const int16_t baud_rate = 0;
float sample_interval = 0.0;
struct BaudInfo {
uint16_t baud_rate = 0;
float bit_length = 0.0;
float min_bit_length = 0.0;
float max_bit_length = 0.0;
float samples_until_next = 0.0;
float last_sample = 0.0;
BitQueue bits{};
};
/* Handle a transition, returns true if "good". */
bool handle_transition();
/* Updates a rate info with the given sample.
* Returns true if the rate info has a new bit in its queue. */
bool handle_sample(RateInfo& rate, float sample);
/* Count the number of bits the length represents.
* Returns true if valid given the current baud rate. */
bool count_bits(uint32_t length, uint16_t& bit_count);
/* Gets the best baud info associated with the specified bit length. */
const BaudInfo* get_baud_info(float bit_length) const;
std::array<BaudInfo, 3> known_rates_{
BaudInfo{512},
BaudInfo{1200},
BaudInfo{2400}};
std::array<RateInfo, 3> known_rates_{
RateInfo{512},
RateInfo{1200},
RateInfo{2400}};
BitQueue& bits_;
uint32_t sample_rate_ = 0;
uint16_t min_valid_length_ = 0;
const BaudInfo* current_rate_ = nullptr;
uint8_t rate_misses_ = 0;
RateInfo* current_rate_ = nullptr;
float sample_ = 0.0;
float last_sample_ = 0.0;
float next_bit_center_ = 0.0;
uint32_t sample_index_ = 0;
uint32_t last_transition_index_ = 0;
uint32_t bad_transitions_ = 0;
float samples_until_next_ = 0.0;
float prev_sample_ = 0.0;
bool ready_to_send_ = false;
};
/* Extracts codeword batches from the BitQueue. */