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POCSAG clock detection 🐋 improvement(?) (#1442)

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* WIP convergence
* Tighter code, allow for sample nudges during clock discovery.
This commit is contained in:
Kyle Reed 2023-09-09 08:49:22 -07:00 committed by GitHub
parent 4926cf8df5
commit b28283271b
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 66 additions and 56 deletions
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100
firmware/baseband/proc_pocsag2.cpp
View File

@ -127,41 +127,26 @@ uint32_t BitQueue::data() const {
void BitExtractor::extract_bits(const buffer_f32_t& audio) {
// Assumes input has been normalized +/- 1.0f.
// Positive == 0, Negative == 1.
for (size_t i = 0; i < audio.count; ++i) {
auto sample = audio.p[i];
samples_until_next_ -= 1;
if (!current_rate_) {
// Feed the known rate queues for clock detection.
if (current_rate_) {
if (current_rate_->handle_sample(sample)) {
auto value = (current_rate_->bits.data() & 1) == 1;
bits_.push(value);
}
} else {
// Feed sample to all known rates 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.
if (rate.handle_sample(sample) &&
diff_bit_count(rate.bits.data(), clock_magic_number) <= 3) {
// Clock detected, continue with this rate.
rate.is_stable = true;
current_rate_ = &rate;
samples_until_next_ = rate.sample_interval;
ready_to_send_ = false;
}
}
}
// 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.
// 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;
}
}
}
@ -177,39 +162,60 @@ void BitExtractor::configure(uint32_t sample_rate) {
void BitExtractor::reset() {
current_rate_ = nullptr;
samples_until_next_ = 0.0;
prev_sample_ = 0.0;
ready_to_send_ = false;
for (auto& rate : known_rates_) {
rate.samples_until_next = 0.0;
rate.last_sample = 0.0;
rate.bits.reset();
}
for (auto& rate : known_rates_)
rate.reset();
}
uint16_t BitExtractor::baud_rate() const {
return current_rate_ ? current_rate_->baud_rate : 0;
}
bool BitExtractor::handle_sample(RateInfo& rate, float sample) {
// TODO: Still getting some clock misses at the start of messages.
rate.samples_until_next -= 1;
bool BitExtractor::RateInfo::handle_sample(float sample) {
samples_until_next -= 1;
// Not time to process a sample yet.
if (rate.samples_until_next > 0)
// Time to process a sample?
if (samples_until_next > 0)
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'
bool value = signbit(sample); // NB: negative == '1'
bool bit_pushed = false;
switch (state) {
case State::WaitForSample:
// Just need to wait for the first sample of the bit.
state = State::ReadyToSend;
break;
case State::ReadyToSend:
if (!is_stable && prev_value != value) {
// Still looking for the clock signal but found a transition.
// Nudge the next sample a bit to try avoiding pulse edges.
samples_until_next += (sample_interval / 8.0);
} else {
// Either the clock has been found or both samples were
// (probably) in the same pulse. Send the bit.
// TODO: Wider/more samples for noise reduction?
state = State::WaitForSample;
bit_pushed = true;
bits.push(value);
}
break;
}
// How long until the next sample?
rate.samples_until_next += rate.sample_interval;
rate.last_sample = sample;
samples_until_next += sample_interval;
prev_value = value;
return has_new_bit;
return bit_pushed;
}
void BitExtractor::RateInfo::reset() {
state = State::WaitForSample;
samples_until_next = 0.0;
prev_value = false;
is_stable = false;
bits.reset();
}
/* CodewordExtractor *************************************/

22
firmware/baseband/proc_pocsag2.hpp
View File

@ -91,17 +91,25 @@ class BitExtractor {
static constexpr uint32_t clock_magic_number = 0xAAAAAAAA;
struct RateInfo {
enum class State : uint8_t {
WaitForSample,
ReadyToSend
};
const int16_t baud_rate = 0;
float sample_interval = 0.0;
State state = State::WaitForSample;
float samples_until_next = 0.0;
float last_sample = 0.0;
bool prev_value = false;
bool is_stable = false;
BitQueue bits{};
};
/* 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);
/* Updates a rate info with the given sample.
* Returns true if the rate info has a new bit in its queue. */
bool handle_sample(float sample);
void reset();
};
std::array<RateInfo, 3> known_rates_{
RateInfo{512},
@ -112,10 +120,6 @@ class BitExtractor {
uint32_t sample_rate_ = 0;
RateInfo* current_rate_ = nullptr;
float samples_until_next_ = 0.0;
float prev_sample_ = 0.0;
bool ready_to_send_ = false;
};
/* Extracts codeword batches from the BitQueue. */