more work on DAB decoding
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This commit is contained in:
AlexandreRouma 2024-09-23 00:51:06 +02:00
parent 064f25ee73
commit f8078ac3f0
6 changed files with 728 additions and 218 deletions

View File

@ -5,139 +5,139 @@
#include "dab_phase_sym.h"
namespace dab {
class CyclicSync : public dsp::Processor<dsp::complex_t, dsp::complex_t> {
using base_type = dsp::Processor<dsp::complex_t, dsp::complex_t>;
public:
CyclicSync() {}
// class CyclicSync : public dsp::Processor<dsp::complex_t, dsp::complex_t> {
// using base_type = dsp::Processor<dsp::complex_t, dsp::complex_t>;
// public:
// CyclicSync() {}
// TODO: The default AGC rate is probably way too fast, plot out the avgCorr to see how much it moves
CyclicSync(dsp::stream<dsp::complex_t>* in, double symbolLength, double cyclicPrefixLength, double samplerate, float agcRate = 1e-3) { init(in, symbolLength, cyclicPrefixLength, samplerate, agcRate); }
// // TODO: The default AGC rate is probably way too fast, plot out the avgCorr to see how much it moves
// CyclicSync(dsp::stream<dsp::complex_t>* in, double symbolLength, double cyclicPrefixLength, double samplerate, float agcRate = 1e-3) { init(in, symbolLength, cyclicPrefixLength, samplerate, agcRate); }
void init(dsp::stream<dsp::complex_t>* in, double symbolLength, double cyclicPrefixLength, double samplerate, float agcRate = 1e-3) {
// Computer the number of samples for the symbol and its cyclic prefix
symbolSamps = round(samplerate * symbolLength);
prefixSamps = round(samplerate * cyclicPrefixLength);
// void init(dsp::stream<dsp::complex_t>* in, double symbolLength, double cyclicPrefixLength, double samplerate, float agcRate = 1e-3) {
// // Computer the number of samples for the symbol and its cyclic prefix
// symbolSamps = round(samplerate * symbolLength);
// prefixSamps = round(samplerate * cyclicPrefixLength);
// Allocate and clear the delay buffer
delayBuf = dsp::buffer::alloc<dsp::complex_t>(STREAM_BUFFER_SIZE + 64000);
dsp::buffer::clear(delayBuf, symbolSamps);
// // Allocate and clear the delay buffer
// delayBuf = dsp::buffer::alloc<dsp::complex_t>(STREAM_BUFFER_SIZE + 64000);
// dsp::buffer::clear(delayBuf, symbolSamps);
// Allocate and clear the history buffer
histBuf = dsp::buffer::alloc<dsp::complex_t>(prefixSamps);
dsp::buffer::clear(histBuf, prefixSamps);
// // Allocate and clear the history buffer
// histBuf = dsp::buffer::alloc<dsp::complex_t>(prefixSamps);
// dsp::buffer::clear(histBuf, prefixSamps);
// Compute the delay input addresses
delayBufInput = &delayBuf[symbolSamps];
// // Compute the delay input addresses
// delayBufInput = &delayBuf[symbolSamps];
// Compute the correlation AGC configuration
this->agcRate = agcRate;
agcRateInv = 1.0f - agcRate;
// // Compute the correlation AGC configuration
// this->agcRate = agcRate;
// agcRateInv = 1.0f - agcRate;
base_type::init(in);
}
// base_type::init(in);
// }
void reset() {
assert(base_type::_block_init);
std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
base_type::tempStop();
// void reset() {
// assert(base_type::_block_init);
// std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
// base_type::tempStop();
base_type::tempStart();
}
// base_type::tempStart();
// }
int run() {
int count = base_type::_in->read();
if (count < 0) { return -1; }
// int run() {
// int count = base_type::_in->read();
// if (count < 0) { return -1; }
// Copy the data into the normal delay buffer
memcpy(delayBufInput, base_type::_in->readBuf, count * sizeof(dsp::complex_t));
// // Copy the data into the normal delay buffer
// memcpy(delayBufInput, base_type::_in->readBuf, count * sizeof(dsp::complex_t));
// Flush the input stream
base_type::_in->flush();
// // Flush the input stream
// base_type::_in->flush();
// Do cross-correlation
for (int i = 0; i < count; i++) {
// Get the current history slot
dsp::complex_t* slot = &histBuf[histId++];
// // Do cross-correlation
// for (int i = 0; i < count; i++) {
// // Get the current history slot
// dsp::complex_t* slot = &histBuf[histId++];
// Wrap around the history slot index (TODO: Check that the history buffer's length is correct)
histId %= prefixSamps;
// // Wrap around the history slot index (TODO: Check that the history buffer's length is correct)
// histId %= prefixSamps;
// Kick out last value from the correlation
corr -= *slot;
// // Kick out last value from the correlation
// corr -= *slot;
// Save input value and compute the new prodct
dsp::complex_t val = delayBuf[i];
dsp::complex_t prod = val.conj()*delayBuf[i+symbolSamps];
// // Save input value and compute the new prodct
// dsp::complex_t val = delayBuf[i];
// dsp::complex_t prod = val.conj()*delayBuf[i+symbolSamps];
// Add the new value to the correlation
*slot = prod;
// // Add the new value to the correlation
// *slot = prod;
// Add the new value to the history buffer
corr += prod;
// // Add the new value to the history buffer
// corr += prod;
// Compute sample amplitude
float rcorr = corr.amplitude();
// // Compute sample amplitude
// float rcorr = corr.amplitude();
// If a high enough peak is reached, reset the symbol counter
if (rcorr > avgCorr && rcorr > peakCorr) { // Note keeping an average level might not be needed
peakCorr = rcorr;
peakLCorr = lastCorr;
samplesSincePeak = 0;
}
// // If a high enough peak is reached, reset the symbol counter
// if (rcorr > avgCorr && rcorr > peakCorr) { // Note keeping an average level might not be needed
// peakCorr = rcorr;
// peakLCorr = lastCorr;
// samplesSincePeak = 0;
// }
// If this is the sample right after the peak, save it
if (samplesSincePeak == 1) {
peakRCorr = rcorr;
}
// // If this is the sample right after the peak, save it
// if (samplesSincePeak == 1) {
// peakRCorr = rcorr;
// }
// Write the sample to the output
out.writeBuf[samplesSincePeak++] = val;
// // Write the sample to the output
// out.writeBuf[samplesSincePeak++] = val;
// If the end of the symbol is reached, send it off
if (samplesSincePeak >= symbolSamps) {
if (!out.swap(symbolSamps)) {
return -1;
}
samplesSincePeak = 0;
peakCorr = 0;
}
// // If the end of the symbol is reached, send it off
// if (samplesSincePeak >= symbolSamps) {
// if (!out.swap(symbolSamps)) {
// return -1;
// }
// samplesSincePeak = 0;
// peakCorr = 0;
// }
// Update the average correlation
lastCorr = rcorr;
// // Update the average correlation
// lastCorr = rcorr;
// Update the average correlation value
avgCorr = agcRate*rcorr + agcRateInv*avgCorr;
}
// // Update the average correlation value
// avgCorr = agcRate*rcorr + agcRateInv*avgCorr;
// }
// Move unused data
memmove(delayBuf, &delayBuf[count], symbolSamps * sizeof(dsp::complex_t));
// // Move unused data
// memmove(delayBuf, &delayBuf[count], symbolSamps * sizeof(dsp::complex_t));
return count;
}
// return count;
// }
protected:
int symbolSamps;
int prefixSamps;
// protected:
// int symbolSamps;
// int prefixSamps;
int histId = 0;
dsp::complex_t* histBuf;
// int histId = 0;
// dsp::complex_t* histBuf;
dsp::complex_t* delayBuf;
dsp::complex_t* delayBufInput;
// dsp::complex_t* delayBuf;
// dsp::complex_t* delayBufInput;
dsp::complex_t corr = { 0.0f, 0.0f };
// dsp::complex_t corr = { 0.0f, 0.0f };
int samplesSincePeak = 0;
float lastCorr = 0.0f;
float peakCorr = 0.0f;
float peakLCorr = 0.0f;
float peakRCorr = 0.0f;
// int samplesSincePeak = 0;
// float lastCorr = 0.0f;
// float peakCorr = 0.0f;
// float peakLCorr = 0.0f;
// float peakRCorr = 0.0f;
// Note only required for DAB
float avgCorr = 0.0f;
float agcRate;
float agcRateInv;
};
// // Note only required for DAB
// float avgCorr = 0.0f;
// float agcRate;
// float agcRateInv;
// };
class FrameFreqSync : public dsp::Processor<dsp::complex_t, dsp::complex_t> {
using base_type = dsp::Processor<dsp::complex_t, dsp::complex_t>;

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@ -14,6 +14,7 @@
#include <chrono>
#include "dab_dsp.h"
#include <gui/widgets/constellation_diagram.h>
#include "ofdm.h"
#define CONCAT(a, b) ((std::string(a) + b).c_str())
@ -35,7 +36,7 @@ public:
M17DecoderModule(std::string name) {
this->name = name;
file = std::ofstream("sync4.f32", std::ios::out | std::ios::binary);
file = std::ofstream("sync5.f32", std::ios::out | std::ios::binary);
// Load config
config.acquire();
@ -47,7 +48,7 @@ public:
vfo->setSnapInterval(250);
// Initialize DSP here
csync.init(vfo->output, 1e-3, 246e-6, INPUT_SAMPLE_RATE);
csync.init(vfo->output, 2048, 504, 1e-3, INPUT_SAMPLE_RATE, 1e-6, 0.01, 0.005);
ffsync.init(&csync.out);
ns.init(&ffsync.out, handler, this);
@ -131,8 +132,9 @@ private:
std::string name;
bool enabled = true;
dab::CyclicSync csync;
//dab::CyclicSync csync;
dab::FrameFreqSync ffsync;
dsp::ofdm::CyclicTimeSync csync;
dsp::sink::Handler<dsp::complex_t> ns;
ImGui::ConstellationDiagram constDiagram;

View File

@ -11,67 +11,93 @@ namespace dsp::ofdm {
public:
CyclicTimeSync() {}
CyclicTimeSync(stream<complex_t>* in, int fftSize, double usefulSymbolTime, double cyclicPrefixRatio, double samplerate,
CyclicTimeSync(stream<complex_t>* in, int fftSize, int cpSize, double usefulSymbolTime, double samplerate,
double omegaGain, double muGain, double omegaRelLimit, int interpPhaseCount = 128, int interpTapCount = 8) {
init(in, fftSize, usefulSymbolTime, cyclicPrefixRatio, samplerate, omegaGain, muGain, omegaRelLimit, interpPhaseCount, interpTapCount);
init(in, fftSize, cpSize, usefulSymbolTime, samplerate, omegaGain, muGain, omegaRelLimit, interpPhaseCount, interpTapCount);
}
~CyclicTimeSync() {
if (!base_type::_block_init) { return; }
base_type::stop();
dsp::multirate::freePolyphaseBank(interpBank);
buffer::free(corrSampCache);
buffer::free(corrProdCache);
buffer::free(buffer);
}
void init(stream<complex_t>* in, int fftSize, double usefulSymbolTime, double cyclicPrefixRatio, double samplerate,
void init(stream<complex_t>* in, int fftSize, int cpSize, double usefulSymbolTime, double samplerate,
double omegaGain, double muGain, double omegaRelLimit, int interpPhaseCount = 128, int interpTapCount = 8) {
omega = 0; // TODO
_omegaGain = omegaGain;
_muGain = muGain;
_omegaRelLimit = omegaRelLimit;
_interpPhaseCount = interpPhaseCount;
_interpTapCount = interpTapCount;
// Save parameters
this->fftSize = fftSize;
this->cpSize = cpSize;
period = fftSize + cpSize;
// Compute the interpolator settings
omega = (usefulSymbolTime * samplerate) / (double)fftSize;
this->omegaGain = omegaGain;
this->muGain = muGain;
this->omegaRelLimit = omegaRelLimit;
this->interpPhaseCount = interpPhaseCount;
this->interpTapCount = interpTapCount;
pcl.init(_muGain, _omegaGain, 0.0, 0.0, 1.0, omega, omega * (1.0 - omegaRelLimit), omega * (1.0 + omegaRelLimit));
// Compute the correlator AGC settings
// TODO: Compute it using he FFT and CP sizes
this->corrAgcRate = 1e-4;
corrAgcInvRate = 1.0f - corrAgcRate;
// Initialize the control loop
pcl.init(muGain, omegaGain, 0.0, 0.0, 1.0, omega, omega * (1.0 - omegaRelLimit), omega * (1.0 + omegaRelLimit));
// Generate the interpolation taps
generateInterpTaps();
buffer = buffer::alloc<complex_t>(STREAM_BUFFER_SIZE + _interpTapCount);
bufStart = &buffer[_interpTapCount - 1];
// Allocate the buffers
corrSampCache = buffer::alloc<complex_t>(fftSize);
corrProdCache = buffer::alloc<complex_t>(cpSize);
buffer = buffer::alloc<complex_t>(STREAM_BUFFER_SIZE + interpTapCount);
bufStart = &buffer[interpTapCount - 1];
// Clear the buffers
buffer::clear(corrSampCache, fftSize);
buffer::clear(corrProdCache, cpSize);
buffer::clear(buffer, interpTapCount - 1);
base_type::init(in);
}
void setOmegaGain(double omegaGain) {
assert(base_type::_block_init);
std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
_omegaGain = omegaGain;
pcl.setCoefficients(_muGain, _omegaGain);
this->omegaGain = omegaGain;
pcl.setCoefficients(muGain, omegaGain);
}
void setMuGain(double muGain) {
assert(base_type::_block_init);
std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
_muGain = muGain;
pcl.setCoefficients(_muGain, _omegaGain);
this->muGain = muGain;
pcl.setCoefficients(muGain, omegaGain);
}
void setOmegaRelLimit(double omegaRelLimit) {
assert(base_type::_block_init);
std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
_omegaRelLimit = omegaRelLimit;
pcl.setFreqLimits(omega * (1.0 - _omegaRelLimit), omega * (1.0 + _omegaRelLimit));
this->omegaRelLimit = omegaRelLimit;
pcl.setFreqLimits(omega * (1.0 - omegaRelLimit), omega * (1.0 + omegaRelLimit));
}
void setInterpParams(int interpPhaseCount, int interpTapCount) {
assert(base_type::_block_init);
std::lock_guard<std::recursive_mutex> lck(base_type::ctrlMtx);
base_type::tempStop();
_interpPhaseCount = interpPhaseCount;
_interpTapCount = interpTapCount;
this->interpPhaseCount = interpPhaseCount;
this->interpTapCount = interpTapCount;
dsp::multirate::freePolyphaseBank(interpBank);
buffer::free(buffer);
generateInterpTaps();
buffer = buffer::alloc<complex_t>(STREAM_BUFFER_SIZE + _interpTapCount);
bufStart = &buffer[_interpTapCount - 1];
buffer = buffer::alloc<complex_t>(STREAM_BUFFER_SIZE + interpTapCount);
bufStart = &buffer[interpTapCount - 1];
base_type::tempStart();
}
@ -82,59 +108,139 @@ namespace dsp::ofdm {
offset = 0;
pcl.phase = 0.0f;
pcl.freq = omega;
// TODO: The rest
base_type::tempStart();
}
inline int process(int count, const complex_t* in, complex_t* out) {
int run() {
int count = base_type::_in->read();
if (count < 0) { return -1; }
// Copy data to work buffer
memcpy(bufStart, in, count * sizeof(complex_t));
memcpy(bufStart, base_type::_in->readBuf, count * sizeof(complex_t));
// Process all samples
int outCount = 0;
while (offset < count) {
// Get the cache slots
complex_t* sampSlot = &corrSampCache[corrSampCacheId++];
complex_t* prodSlot = &corrProdCache[corrProdCacheId++];
corrSampCacheId %= fftSize;
corrProdCacheId %= cpSize;
// Compute the interpolated sample
complex_t sample;
int phase = std::clamp<int>(floorf(pcl.phase * (float)_interpPhaseCount), 0, _interpPhaseCount - 1);
volk_32fc_32f_dot_prod_32fc((lv_32fc_t*)&sample, (lv_32fc_t*)&buffer[offset], interpBank.phases[phase], _interpTapCount);
int phase = std::clamp<int>(floorf(pcl.phase * (float)interpPhaseCount), 0, interpPhaseCount - 1);
volk_32fc_32f_dot_prod_32fc((lv_32fc_t*)&sample, (lv_32fc_t*)&buffer[offset], interpBank.phases[phase], interpTapCount);
// Update autocorrelation
// Write the sample to the output
if (outCount >= cpSize) {
out.writeBuf[outCount - cpSize] = sample;
}
// Send out a symbol when it's fully received
if ((++outCount) >= fftSize+cpSize) {
if (!out.swap(outCount)) { break; }
outCount = 0;
}
// Run autocorrelation
complex_t prod = sample.conj()*(*sampSlot);
corr += prod;
corr -= *prodSlot;
// Write back the new sample and product value to the cache
*sampSlot = sample;
*prodSlot = prod;
// Compute the correlation level
float corrLvl = corr.amplitude();
// Detect peak in autocorrelation
if (0/*TODO*/) {
// Detect peak in autocorrelation (TODO: level check maybe not needed now that corrPeak is reset to corrLvl)
if (corrLvl > corrAvg && corrLvl > corrPeak) {
// Save the current correlation as the peak
corrPeak = corrLvl;
// Save the value of the previous correlation as the left side of the peak
corrPeakL = corrLastLvl;
corrPeakL = corrLast;
// Save the symbol period
measuredSymbolPeriod = sampCount;
// Reset the sample count
sampCount = 0;
// TODO: Maybe save the error to apply it at the end of the frame? (will cause issues with the longer null symbol in DAB)
// Reset the peak distance counter
sincePeak = 0;
}
// Write the sample to the frame if within it
if (sampCount < symbolSize) {
symbol[sampCount++] = sample;
// The first sample after a peak is the right-side sample
if (sincePeak == 1) {
corrPeakR = corrLvl;
}
// When the end of the symbol is reached
if (sampCount == symbolSize) {
// Send out the symbol
// TODO
else if (sincePeak == cpSize) {
// Start the useful symbol counter
sinceCp = 0;
// Compute the fractional error (TODO: Probably very inaccurate with noise, use real slopes instead)
if (corrPeakL > corrPeakR) {
float maxSlope = corrPeakR - corrPeak;
float slope = corrPeak - corrPeakL;
fracErr = std::clamp<float>(0.5f * (1.0f + slope / maxSlope), -0.5f, 0.5f);
}
else {
float maxSlope = corrPeak - corrPeakL;
float slope = corrPeakR - corrPeak;
fracErr = std::clamp<float>(-0.5f * (1.0f + slope / maxSlope), -0.5f, 0.5f);
}
}
else if (sincePeak == fftSize) {
// Reset the peak detector
corrPeak = corrAvg;
}
// NOTE: THIS IS ONLY NEEDED FOR DAB
// Detect a wider-than-normal distance to adapt the output counter
else if (sincePeak == 2656) {
// Reset the output counter
outCount = 50;
}
// Last sample of useful symbol
if (sinceCp == fftSize) {
// If the fractional error is valid, run closed-loop
float err = 0.0f;
if (!std::isnan(fracErr)) {
// Compute the measured period using the distance to the last symbol
float measuredPeriod = (float)sinceLastSym - fracErr;
// NOTE: THIS IS ONLY NEEDED FOR DAB
if (measuredPeriod > 3828.0f) {
// Null symbol
err = measuredPeriod - (2552.0f+2656.0f);
}
else {
// Regular symbol
err = measuredPeriod - period;
}
// TODO: limit how much the sample count can grow otherwise otherflows will trigger a false frame detection
err = std::clamp<float>(err, -10.0f, 10.0f);
// Run the control loop
//pcl.advance(error); // TODO
pcl.advancePhase();
// Run the control loop in closed-loop mode
pcl.advance(err);
}
else {
// Otherwise, run open-loop
pcl.advancePhase();
}
// printf("%d\n", outCount);
// Nudge the symbol window if it's too out of sync
if (outCount > 100) {
// TODO: MOVE THE LAST SAMPLES OR THE SYMBOL WILL BE CORRUPTED!
outCount = 50;
flog::debug("NUDGE!");
}
// Reset the period counter
sinceLastSym = 0;
}
else {
// Run the control loop in open-loop mode
pcl.advancePhase();
}
// Update the offset and phase
float delta = floorf(pcl.phase);
@ -142,44 +248,45 @@ namespace dsp::ofdm {
pcl.phase -= delta;
// Update the last correlation level
corrLastLvl = corrLvl;
corrLast = corrLvl;
// Update correlation AGC
corrAvg = corrAvg*corrAgcInvRate + corrLvl*corrAgcRate;
// Increment the distance counters (TODO: Check if they happen at the right point, eg. after being reset to zero)
sincePeak++;
sinceLastSym++;
sinceCp++;
}
// Prepare offset for next buffer of samples
offset -= count;
// Update delay buffer
memmove(buffer, &buffer[count], (_interpTapCount - 1) * sizeof(complex_t));
return outCount;
}
int run() {
int count = base_type::_in->read();
if (count < 0) { return -1; }
int outCount = process(count, base_type::_in->readBuf, base_type::out.writeBuf);
memmove(buffer, &buffer[count], (interpTapCount - 1) * sizeof(complex_t));
// Swap if some data was generated
base_type::_in->flush();
if (outCount) {
if (!base_type::out.swap(outCount)) { return -1; }
}
return outCount;
return count;
}
protected:
void generateInterpTaps() {
double bw = 0.5 / (double)_interpPhaseCount;
dsp::tap<float> lp = dsp::taps::windowedSinc<float>(_interpPhaseCount * _interpTapCount, dsp::math::hzToRads(bw, 1.0), dsp::window::nuttall, _interpPhaseCount);
interpBank = dsp::multirate::buildPolyphaseBank<float>(_interpPhaseCount, lp);
double bw = 0.5 / (double)interpPhaseCount;
dsp::tap<float> lp = dsp::taps::windowedSinc<float>(interpPhaseCount * interpTapCount, dsp::math::hzToRads(bw, 1.0), dsp::window::nuttall, interpPhaseCount);
interpBank = dsp::multirate::buildPolyphaseBank<float>(interpPhaseCount, lp);
taps::free(lp);
}
// OFDM Configuration
int fftSize;
int cpSize;
float period;
// Interpolator
dsp::multirate::PolyphaseBank<float> interpBank;
int _interpPhaseCount;
int _interpTapCount;
int interpPhaseCount;
int interpTapCount;
int offset = 0;
complex_t* buffer = NULL;
complex_t* bufStart;
@ -187,24 +294,31 @@ namespace dsp::ofdm {
// Control loop
loop::PhaseControlLoop<float, false> pcl;
double omega;
double _omegaGain;
double _muGain;
double _omegaRelLimit;
double omegaGain;
double muGain;
double omegaRelLimit;
float fracErr = 0.0f;
// Autocorrelator
complex_t corr;
complex_t* corrProducts = NULL;
complex_t corr = {0.0f, 0.0f};
complex_t* corrSampCache = NULL;
complex_t* corrProdCache = NULL;
int corrSampCacheId = 0;
int corrProdCacheId = 0;
float corrAgcRate;
float corrAgcInvRate;
float corrLastLvl = 0;
float corrAvg = 0;
float corrLast = 0;
float corrPeakR = 0;
float corrPeak = 0;
float corrPeakL = 0;
// Symbol
complex_t* symbol; // TODO: Will use output stream buffer instead
int symbolSize;
int sampCount = 0;
int measuredSymbolPeriod = 0;
// Peak detection
int sincePeak = 0;
int sinceLastSym = 0;
int sinceCp = 0;
// Other shit to categorize
int outCount = 0;
};
};

View File

@ -1,34 +0,0 @@
0123456789
--- ---
0*4
1*5
2*6
1*5
2*6 = L + 3*7 - 0*4
3*7
2*6
3*7 = L + 4*8 - 1*5
4*8
3*7
4*8 = L + 5*9 - 2*6
5*9
0*5
1*6
2*7
1*6
2*7
3*8
2*7
3*8
4*9
=> Use same technique to cache the interpolation results

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@ -0,0 +1,31 @@
cyclicLen = 4
usefulLen = 12
A = 0*12 + 1*13 + 2*14 + 3*15
B = 1*13 + 2*14 + 3*15 + 4*16 = A - 0*12 + 4*16
C = 2*14 + 3*15 + 4*16 + 5*17 = B - 1*13 + 5*17
D = 3*15 + 4*16 + 5*17 + 6*18 = C - 2*14 + 6*18
E = 4*16 + 5*17 + 6*18 + 7*19 = D - 3*15 + 7*19
F = 5*17 + 6*18 + 7*19 + 8*20 = E - 4*16 + 8*20
G = 6*18 + 7*19 + 8*20 + 9*21 = F - 5*17 + 9*21
H = 7*19 + 8*20 + 9*21 + 10*22 = G - 6*18 + 10*22
I = 8*20 + 9*21 + 10*22 + 11*23 = H - 7*19 + 11*23
J = 9*21 + 10*22 + 11*23 + 12*24 = I - 8*20 + 12*24
K = 10*22 + 11*23 + 12*24 + 13*25 = J - 9*21 + 13*25
L = 11*23 + 12*24 + 13*25 + 14*26 = K - 10*22 + 14*26
M = 12*24 + 13*25 + 14*26 + 15*27 = L - 11*23 + 15*27
N = 13*25 + 14*26 + 15*27 + 16*28 = M - 12*24 + 16*28
O = 14*26 + 15*27 + 16*28 + 17*29 = N - 13*25 + 17*29
P = 15*27 + 16*28 + 17*29 + 18*30 = O - 14*26 + 18*30
Q = 16*28 + 17*29 + 18*30 + 19*31 = P - 15*27 + 19*31
R = 17*29 + 18*30 + 19*31 + 20*32 = Q - 16*28 + 20*32
S = 18*30 + 19*31 + 20*32 + 21*33 = R - 17*29 + 21*33
T = 19*31 + 20*32 + 21*33 + 22*34 = S - 18*30 + 22*34
U = 20*32 + 21*33 + 22*34 + 23*35 = T - 19*31 + 23*35
Conclusion:
sampCacheLen = usefulLen
prodCacheLen = cyclicLen
Peak correlation occurs when the current interpolated value is the last FFT sample

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