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79 lines
2.0 KiB
C++
79 lines
2.0 KiB
C++
// Copyright 2021 Mobilinkd LLC.
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#pragma once
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#include "SlidingDFT.h"
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#include <array>
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#include <complex>
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#include <cstddef>
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namespace modemm17 {
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/**
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* Data carrier detection using the difference of two DFTs, one in-band and
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* one out-of-band. The first frequency is the in-band frequency and the
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* second one is the out-of-band Frequency. The second frequency must be
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* within the normal passband of the receiver, but beyond the normal roll-off
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* frequency of the data carrier.
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*
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* This version uses the NSlidingDFT implementation to reduce the memory
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* footprint.
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*
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* As an example, the cut-off for 4.8k symbol/sec 4-FSK is 2400Hz, so 3000Hz
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* is a reasonable out-of-band frequency to use.
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*
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* Note: the input to this DCD must be unfiltered (raw) baseband input.
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*/
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template <size_t SampleRate, size_t Accuracy = 1000>
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struct DataCarrierDetect
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{
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NSlidingDFT<SampleRate, SampleRate / Accuracy, 2> dft_;
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float ltrigger_;
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float htrigger_;
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float level_1 = 0.0;
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float level_2 = 0.0;
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float level_ = 0.0;
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bool triggered_ = false;
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DataCarrierDetect(
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size_t freq1,
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size_t freq2,
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float ltrigger = 2.0,
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float htrigger = 5.0
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) :
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dft_({freq1, freq2}),
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ltrigger_(ltrigger),
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htrigger_(htrigger)
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{
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}
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/**
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* Accept unfiltered baseband input and output a decision on whether
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* a carrier has been detected after every @tparam BlockSize inputs.
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*/
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void operator()(float sample)
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{
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auto result = dft_(sample);
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level_1 += std::norm(result[0]);
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level_2 += std::norm(result[1]);
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}
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/**
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* Update the data carrier detection level.
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*/
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void update()
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{
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level_ = level_ * 0.8 + 0.2 * (level_1 / level_2);
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level_1 = 0.0;
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level_2 = 0.0;
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triggered_ = triggered_ ? level_ > ltrigger_ : level_ > htrigger_;
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}
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float level() const { return level_; }
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bool dcd() const { return triggered_; }
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};
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} // modemm17
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