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NTSC chroma-encoder filter changes #760

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merged 2 commits into from
Jun 22, 2022
Merged

NTSC chroma-encoder filter changes #760

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17debb0
chroma-encoder: Add NTSC narrowband Q encoding
ifb Jun 21, 2022
9054e8e
chroma-encoder: Add NTSC-specific chroma component filter
ifb Jun 22, 2022
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4 changes: 3 additions & 1 deletion tools/ld-chroma-decoder/encoder/main.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -77,7 +77,7 @@ int main(int argc, char *argv[])

// Option to select chroma mode (--chroma-mode)
QCommandLineOption chromaOption(QStringList() << "chroma-mode",
QCoreApplication::translate("main", "NTSC only. Chroma encoder mode to use (wideband-yuv, wideband-yiq; default: wideband-yuv)"),
QCoreApplication::translate("main", "NTSC only. Chroma encoder mode to use (wideband-yuv, wideband-yiq, narrowband-q; default: wideband-yuv)"),
QCoreApplication::translate("main", "chroma-mode"));
parser.addOption(chromaOption);

Expand Down Expand Up @@ -142,6 +142,8 @@ int main(int argc, char *argv[])
chromaName = parser.value(chromaOption);
if (chromaName == "wideband-yiq") {
chromaMode = WIDEBAND_YIQ;
} else if (chromaName == "narrowband-q") {
chromaMode = NARROWBAND_Q;
} else if (chromaName == "wideband-yuv") {
chromaMode = WIDEBAND_YUV;
} else {
Expand Down
30 changes: 21 additions & 9 deletions tools/ld-chroma-decoder/encoder/ntscencoder.cpp
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Expand Up @@ -268,18 +268,26 @@ static double syncPulseGate(double t, double startTime, SyncPulseType type)
return raisedCosineGate(t, startTime, startTime + length, 200.0e-9 / 2.0);
}

// 1.3 MHz low-pass Gaussian filter, as used in pyctools-pal's coder.
// Generated by: c = scipy.signal.gaussian(13, 1.49); c / sum(c)
// 1.3 MHz low-pass filter
//
// The UV filter should be 0 dB at 0 Hz, >= -3 dB at 1.3 MHz, <= -20 dB at
// 4.0 MHz. [Clarke p8]
static constexpr std::array<double, 13> uvFilterCoeffs {
8.06454142158873e-05, 0.0009604748783110286, 0.007290763490157312, 0.035272860169480155, 0.10876496139131472,
0.21375585039760908, 0.2677488885178237, 0.21375585039760908, 0.10876496139131472, 0.035272860169480155,
0.007290763490157312, 0.0009604748783110286, 8.06454142158873e-05
// The filter should be 0 dB at 0 Hz, >= -2 dB at 1.3 MHz, < -20 dB at
// 3.6 MHz. [Clarke p15]
static constexpr std::array<double, 9> uvFilterCoeffs {
0.0021, 0.0191, 0.0903, 0.2308, 0.3153,
0.2308, 0.0903, 0.0191, 0.0021
};
static constexpr auto uvFilter = makeFIRFilter(uvFilterCoeffs);

// 0.6 MHz low-pass filter
//
// The filter should be 0 dB at 0 Hz, >= -2 dB at 0.4 MHz, >= -6 dB at
// 0.5 MHz, <= -6 dB at 0.6 MHz. [Clarke p15]
static constexpr std::array<double, 23> qFilterCoeffs {
0.0002, 0.0027, 0.0085, 0.0171, 0.0278, 0.0398, 0.0522, 0.0639, 0.0742, 0.0821, 0.0872, 0.0889,
0.0872, 0.0821, 0.0742, 0.0639, 0.0522, 0.0398, 0.0278, 0.0171, 0.0085, 0.0027, 0.0002
};
static constexpr auto qFilter = makeFIRFilter(qFilterCoeffs);

void NTSCEncoder::encodeLine(qint32 fieldNo, qint32 frameLine, const quint16 *rgbData, QVector<quint16> &outputLine)
{
// Resize the output line and fill with blanking
Expand Down Expand Up @@ -394,7 +402,11 @@ void NTSCEncoder::encodeLine(qint32 fieldNo, qint32 frameLine, const quint16 *rg

// Low-pass filter chroma components to 1.3 MHz [Poynton p342]
uvFilter.apply(C1);
uvFilter.apply(C2);
if (chromaMode == NARROWBAND_Q) {
qFilter.apply(C2);
} else {
uvFilter.apply(C2);
}
}

for (qint32 x = 0; x < outputLine.size(); x++) {
Expand Down
1 change: 1 addition & 0 deletions tools/ld-chroma-decoder/encoder/ntscencoder.h
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Expand Up @@ -35,6 +35,7 @@
enum ChromaMode {
WIDEBAND_YUV = 0, // Y'UV
WIDEBAND_YIQ, // Y'IQ
NARROWBAND_Q // Y'IQ with Q low-passed
};

class NTSCEncoder
Expand Down