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AX25Demodulator.cpp
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AX25Demodulator.cpp
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/*
* Copyright (C) 2020 by Jonathan Naylor G4KLX
* Copyright 2015-2019 Mobilinkd LLC <rob@mobilinkd.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include "Config.h"
#if defined(MODE_AX25)
#include "Globals.h"
#include "AX25Demodulator.h"
#include "AX25Defines.h"
const float32_t SAMPLE_RATE = 24000.0F;
const float32_t SYMBOL_RATE = 1200.0F;
const uint16_t DELAY_LEN = 11U;
const float32_t SAMPLES_PER_SYMBOL = SAMPLE_RATE / SYMBOL_RATE;
const float32_t PLL_LIMIT = SAMPLES_PER_SYMBOL / 2.0F;
const uint32_t LPF_FILTER_LEN = 48U;
q15_t LPF_FILTER_COEFFS[] = {
-2, -8, -17, -28, -40, -47, -47, -34,
-5, 46, 122, 224, 354, 510, 689, 885,
1092, 1302, 1506, 1693, 1856, 1987, 2077, 2124,
2124, 2077, 1987, 1856, 1693, 1506, 1302, 1092,
885, 689, 510, 354, 224, 122, 46, -5,
-34, -47, -47, -40, -28, -17, -8, -2
};
// Lock low-pass filter taps (80Hz Bessel)
// scipy.signal:
// b, a = bessel(4, [80.0/(1200/2)], 'lowpass')
//
const uint8_t PLL_IIR_SIZE = 5U;
const float32_t PLL_LOCK_B[] = {
1.077063e-03,4.308253e-03,6.462379e-03,4.308253e-03,1.077063e-03};
const float32_t PLL_LOCK_A[] = {
1.000000e+00,-2.774567e+00,2.962960e+00,-1.437990e+00,2.668296e-01};
// 64 Hz loop filter.
// scipy.signal:
// loop_coeffs = firwin(9, [64.0/(1200/2)], width = None,
// pass_zero = True, scale = True, window='hann')
//
const uint32_t PLL_FILTER_LEN = 7U;
float32_t PLL_FILTER_COEFFS[] = {3.196252e-02F, 1.204223e-01F, 2.176819e-01F, 2.598666e-01F, 2.176819e-01F, 1.204223e-01F, 3.196252e-02F};
CAX25Demodulator::CAX25Demodulator(int8_t n) :
m_frame(),
m_twist(n),
m_lpfFilter(),
m_lpfState(),
m_delayLine(NULL),
m_delayPos(0U),
m_nrziState(false),
m_pllFilter(),
m_pllState(),
m_pllLast(false),
m_pllBits(1U),
m_pllCount(0.0F),
m_pllJitter(0.0F),
m_pllDCD(false),
m_iirHistory(),
m_hdlcOnes(0U),
m_hdlcFlag(false),
m_hdlcBuffer(0U),
m_hdlcBits(0U),
m_hdlcState(AX25_IDLE)
{
m_delayLine = new bool[DELAY_LEN];
m_lpfFilter.numTaps = LPF_FILTER_LEN;
m_lpfFilter.pState = m_lpfState;
m_lpfFilter.pCoeffs = LPF_FILTER_COEFFS;
m_pllFilter.numTaps = PLL_FILTER_LEN;
m_pllFilter.pState = m_pllState;
m_pllFilter.pCoeffs = PLL_FILTER_COEFFS;
for (uint8_t i = 0U; i < PLL_IIR_SIZE; i++)
m_iirHistory[i] = 0.0F;
}
bool CAX25Demodulator::process(q15_t* samples, uint8_t length, CAX25Frame& frame)
{
bool result = false;
q15_t fa[RX_BLOCK_SIZE];
m_twist.process(samples, fa, RX_BLOCK_SIZE);
int16_t buffer[RX_BLOCK_SIZE];
for (uint8_t i = 0; i < length; i++) {
bool level = (fa[i] >= 0);
bool delayed = delay(level);
buffer[i] = (int16_t(level ^ delayed) << 1) - 1;
}
q15_t fc[RX_BLOCK_SIZE];
::arm_fir_fast_q15(&m_lpfFilter, buffer, fc, RX_BLOCK_SIZE);
for (uint8_t i = 0; i < length; i++) {
bool bit = fc[i] >= 0;
bool sample = PLL(bit);
if (sample) {
// We will only ever get one frame because there are
// not enough bits in a block for more than one.
if (result) {
HDLC(NRZI(bit));
} else {
result = HDLC(NRZI(bit));
if (result) {
// Copy the frame data.
::memcpy(frame.m_data, m_frame.m_data, AX25_MAX_PACKET_LEN);
frame.m_length = m_frame.m_length;
frame.m_fcs = m_frame.m_fcs;
m_frame.m_length = 0U;
}
}
}
}
return result;
}
bool CAX25Demodulator::delay(bool b)
{
bool r = m_delayLine[m_delayPos];
m_delayLine[m_delayPos++] = b;
if (m_delayPos >= DELAY_LEN)
m_delayPos = 0U;
return r;
}
bool CAX25Demodulator::NRZI(bool b)
{
bool result = (b == m_nrziState);
m_nrziState = b;
return result;
}
bool CAX25Demodulator::PLL(bool input)
{
bool sample = false;
if (input != m_pllLast || m_pllBits > 16U) {
// Record transition.
m_pllLast = input;
if (m_pllCount > PLL_LIMIT)
m_pllCount -= SAMPLES_PER_SYMBOL;
float32_t adjust = m_pllBits > 16U ? 5.0F : 0.0F;
float32_t offset = m_pllCount / float32_t(m_pllBits);
float32_t jitter;
::arm_fir_f32(&m_pllFilter, &offset, &jitter, 1U);
float32_t absOffset = adjust;
if (offset < 0.0F)
absOffset -= offset;
else
absOffset += offset;
m_pllJitter = iir(absOffset);
m_pllCount -= jitter / 2.0F;
m_pllBits = 1U;
} else {
if (m_pllCount > PLL_LIMIT) {
sample = true;
m_pllCount -= SAMPLES_PER_SYMBOL;
m_pllBits++;
}
}
m_pllCount += 1.0F;
return sample;
}
bool CAX25Demodulator::HDLC(bool b)
{
if (m_hdlcOnes == AX25_MAX_ONES) {
if (b) {
// flag byte
m_hdlcFlag = true;
} else {
// bit stuffing...
m_hdlcFlag = false;
m_hdlcOnes = 0U;
return false;
}
}
m_hdlcBuffer >>= 1;
m_hdlcBuffer |= b ? 128U : 0U;
m_hdlcBits++; // Free-running until Sync byte.
if (b)
m_hdlcOnes++;
else
m_hdlcOnes = 0U;
if (m_hdlcFlag) {
bool result = false;
switch (m_hdlcBuffer) {
case AX25_FRAME_END:
if (m_frame.m_length >= AX25_MIN_FRAME_LENGTH) {
result = m_frame.checkCRC();
if (!result)
m_frame.m_length = 0U;
} else {
m_frame.m_length = 0U;
}
m_hdlcState = AX25_SYNC;
m_hdlcFlag = false;
m_hdlcBits = 0U;
break;
case AX25_FRAME_ABORT:
// Frame aborted
m_frame.m_length = 0U;
m_hdlcState = AX25_IDLE;
m_hdlcFlag = false;
m_hdlcBits = 0U;
break;
default:
break;
}
return result;
}
switch (m_hdlcState) {
case AX25_IDLE:
break;
case AX25_SYNC:
if (m_hdlcBits == 8U) { // 8th bit.
// Start of frame data.
m_hdlcState = AX25_RECEIVE;
m_frame.append(m_hdlcBuffer);
m_hdlcBits = 0U;
}
break;
case AX25_RECEIVE:
if (m_hdlcBits == 8U) { // 8th bit.
m_frame.append(m_hdlcBuffer);
m_hdlcBits = 0U;
}
break;
default:
break;
}
return false;
}
void CAX25Demodulator::setTwist(int8_t n)
{
m_twist.setTwist(n);
}
bool CAX25Demodulator::isDCD()
{
if (m_pllJitter <= (SAMPLES_PER_SYMBOL * 0.03F))
m_pllDCD = true;
else if (m_pllJitter >= (SAMPLES_PER_SYMBOL * 0.15F))
m_pllDCD = false;
return m_pllDCD;
}
float32_t CAX25Demodulator::iir(float32_t input)
{
for (int8_t i = int8_t(PLL_IIR_SIZE) - 1; i != 0; i--)
m_iirHistory[i] = m_iirHistory[i - 1];
m_iirHistory[0] = input;
for (uint8_t i = 1U; i < PLL_IIR_SIZE; i++)
m_iirHistory[0] -= PLL_LOCK_A[i] * m_iirHistory[i];
float32_t result = 0.0F;
for (uint8_t i = 0U; i < PLL_IIR_SIZE; i++)
result += PLL_LOCK_B[i] * m_iirHistory[i];
return result;
}
#endif