mirror of
https://github.com/f4exb/sdrangel.git
synced 2024-10-31 15:07:12 -04:00
253 lines
6.7 KiB
C++
253 lines
6.7 KiB
C++
// Copyright 2021 Mobilinkd LLC.
|
|
|
|
#pragma once
|
|
|
|
#include <vector>
|
|
#include <cstddef>
|
|
#include <cstdint>
|
|
#include <numeric>
|
|
#include <cassert>
|
|
|
|
#include "export.h"
|
|
|
|
namespace modemm17
|
|
{
|
|
|
|
/**
|
|
* Calculate the phase estimates for each sample position.
|
|
*
|
|
* This performs a running calculation of the phase of each bit position.
|
|
* It is very noisy for individual samples, but quite accurate when
|
|
* averaged over an entire M17 frame.
|
|
*
|
|
* It is designed to be used to calculate the best bit position for each
|
|
* frame of data. Samples are collected and averaged. When update() is
|
|
* called, the best sample index and clock are estimated, and the counters
|
|
* reset for the next frame.
|
|
*
|
|
* It starts counting bit 0 as the first bit received after a reset.
|
|
*
|
|
* This is very efficient as it only uses addition and subtraction for
|
|
* each bit sample. And uses one multiply and divide per update (per
|
|
* frame).
|
|
*
|
|
* This will permit a clock error of up to 500ppm. This allows up to
|
|
* 250ppm error for both transmitter and receiver clocks. This is
|
|
* less than one sample per frame when the sample rate is 48000 SPS.
|
|
*
|
|
* @inv current_index_ is in the interval [0, SAMPLES_PER_SYMBOL).
|
|
* @inv sample_index_ is in the interval [0, SAMPLES_PER_SYMBOL).
|
|
* @inv clock_ is in the interval [0.9995, 1.0005]
|
|
*/
|
|
class MODEMM17_API ClockRecovery
|
|
{
|
|
std::vector<float> estimates_;
|
|
size_t sample_count_ = 0;
|
|
uint16_t frame_count_ = 0;
|
|
uint8_t sample_index_ = 0;
|
|
uint8_t prev_sample_index_ = 0;
|
|
uint8_t index_ = 0;
|
|
float offset_ = 0.0;
|
|
float clock_ = 1.0;
|
|
float prev_sample_ = 0.0;
|
|
|
|
/**
|
|
* Find the sample index.
|
|
*
|
|
* There are @p SAMPLES_PER_INDEX bins. It is expected that half are
|
|
* positive values and half are negative. The positive and negative
|
|
* bins will be grouped together such that there is a single transition
|
|
* from positive values to negative values.
|
|
*
|
|
* The best bit position is always the position with the positive value
|
|
* at that transition point. It will be the bit index with the highest
|
|
* energy.
|
|
*
|
|
* @post sample_index_ contains the best sample point.
|
|
*/
|
|
void update_sample_index_()
|
|
{
|
|
uint8_t index = 0;
|
|
|
|
// Find falling edge.
|
|
bool is_positive = false;
|
|
for (size_t i = 0; i != samples_per_symbol_; ++i)
|
|
{
|
|
float phase = estimates_[i];
|
|
|
|
if (!is_positive && phase > 0)
|
|
{
|
|
is_positive = true;
|
|
}
|
|
else if (is_positive && phase < 0)
|
|
{
|
|
index = i;
|
|
break;
|
|
}
|
|
}
|
|
|
|
sample_index_ = index == 0 ? samples_per_symbol_ - 1 : index - 1;
|
|
}
|
|
|
|
/**
|
|
* Compute the drift in sample points from the last update.
|
|
*
|
|
* This should never be greater than one.
|
|
*/
|
|
float calc_offset_()
|
|
{
|
|
int8_t offset = sample_index_ - prev_sample_index_;
|
|
|
|
// When in spec, the clock should drift by less than 1 sample per frame.
|
|
if (offset >= max_offset_)
|
|
{
|
|
offset -= samples_per_symbol_;
|
|
}
|
|
else if (offset <= -max_offset_)
|
|
{
|
|
offset += samples_per_symbol_;
|
|
}
|
|
|
|
return offset;
|
|
}
|
|
|
|
void update_clock_()
|
|
{
|
|
// update_sample_index_() must be called first.
|
|
|
|
if (frame_count_ == 0)
|
|
{
|
|
prev_sample_index_ = sample_index_;
|
|
offset_ = 0.0;
|
|
clock_ = 1.0;
|
|
return;
|
|
}
|
|
|
|
offset_ += calc_offset_();
|
|
prev_sample_index_ = sample_index_;
|
|
clock_ = 1.0 + (offset_ / (frame_count_ * sample_count_));
|
|
clock_ = std::min(MAX_CLOCK, std::max(MIN_CLOCK, clock_));
|
|
}
|
|
|
|
public:
|
|
ClockRecovery(size_t sampleRate, size_t symbolRate) :
|
|
sampleRate_(sampleRate),
|
|
symbolRate_(symbolRate)
|
|
{
|
|
samples_per_symbol_ = sampleRate_ / symbolRate_;
|
|
max_offset_ = samples_per_symbol_ / 2;
|
|
dx_ = 1.0 / samples_per_symbol_;
|
|
estimates_.resize(samples_per_symbol_);
|
|
std::fill(estimates_.begin(), estimates_.end(), 0);
|
|
}
|
|
|
|
/**
|
|
* Update clock recovery with the given sample. This will advance the
|
|
* current sample index by 1.
|
|
*/
|
|
void operator()(float sample)
|
|
{
|
|
float dy = (sample - prev_sample_);
|
|
|
|
if (sample + prev_sample_ < 0)
|
|
{
|
|
// Invert the phase estimate when sample midpoint is less than 0.
|
|
dy = -dy;
|
|
}
|
|
|
|
prev_sample_ = sample;
|
|
|
|
estimates_[index_] += dy;
|
|
index_ += 1;
|
|
if (index_ == samples_per_symbol_)
|
|
{
|
|
index_ = 0;
|
|
}
|
|
sample_count_ += 1;
|
|
}
|
|
|
|
/**
|
|
* Reset the state of the clock recovery system. This should be called
|
|
* when a new transmission is detected.
|
|
*/
|
|
void reset()
|
|
{
|
|
sample_count_ = 0;
|
|
frame_count_ = 0;
|
|
index_ = 0;
|
|
sample_index_ = 0;
|
|
std::fill(estimates_.begin(), estimates_.end(), 0);
|
|
}
|
|
|
|
/**
|
|
* Return the current sample index. This will always be in the range of
|
|
* [0..SAMPLES_PER_SYMBOL).
|
|
*/
|
|
uint8_t current_index() const
|
|
{
|
|
return index_;
|
|
}
|
|
|
|
/**
|
|
* Return the estimated sample clock increment based on the last update.
|
|
*
|
|
* The value is only valid after samples have been collected and update()
|
|
* has been called.
|
|
*/
|
|
float clock_estimate() const
|
|
{
|
|
return clock_;
|
|
}
|
|
|
|
/**
|
|
* Return the estimated "best sample index" based on the last update.
|
|
*
|
|
* The value is only valid after samples have been collected and update()
|
|
* has been called.
|
|
*/
|
|
uint8_t sample_index() const
|
|
{
|
|
return sample_index_;
|
|
}
|
|
|
|
/**
|
|
* Update the sample index and clock estimates, and reset the state for
|
|
* the next frame of data.
|
|
*
|
|
* @pre index_ = 0
|
|
* @pre sample_count_ > 0
|
|
*
|
|
* After this is called, sample_index() and clock_estimate() will have
|
|
* valid, updated results.
|
|
*
|
|
* The more samples between calls to update, the more accurate the
|
|
* estimates will be.
|
|
*
|
|
* @return true if the preconditions are met and the update has been
|
|
* performed, otherwise false.
|
|
*/
|
|
bool update()
|
|
{
|
|
if (!(sample_count_ != 0 && index_ == 0)) return false;
|
|
|
|
update_sample_index_();
|
|
update_clock_();
|
|
|
|
frame_count_ = std::min(0x1000, 1 + frame_count_);
|
|
sample_count_ = 0;
|
|
std::fill(estimates_.begin(), estimates_.end(), 0);
|
|
return true;
|
|
}
|
|
private:
|
|
size_t sampleRate_;
|
|
size_t symbolRate_;
|
|
size_t samples_per_symbol_;
|
|
int8_t max_offset_;
|
|
float dx_;
|
|
|
|
static const float MAX_CLOCK;
|
|
static const float MIN_CLOCK;
|
|
};
|
|
|
|
} // modemm17
|