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Add the Inverse-Sinc low-pass filter for P25 TX.

This commit is contained in:
Jonathan Naylor 2016-09-15 18:08:40 +01:00
parent 55db02461c
commit e9c2388d32
2 changed files with 21 additions and 2 deletions
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21
P25TX.cpp
View File

@ -37,6 +37,15 @@ static q15_t P25_C4FSK_FILTER[] = {401, 104, -340, -731, -847, -553, 112, 909, 1
const uint16_t P25_C4FSK_FILTER_LEN = 42U;
#endif
// Generated in MATLAB using the following commands, and then normalised for unity gain
// shape2 = 'Inverse-sinc Lowpass';
// d2 = fdesign.interpolator(2, shape2);
// h2 = design(d2, 'SystemObject', true);
static q15_t P25_LP_FILTER[] = {170, 401, 340, -203, -715, -478, 281, 419, -440, -1002, -103, 1114, 528, -1389, -1520, 1108, 2674, -388, -4662,
-2132, 9168, 20241, 20241, 9168, -2132, -4662, -388, 2674, 1108, -1520, -1389, 528, 1114, -103, -1002, -440, 419,
281, -478, -715, -203, 340, 401, 170};
const uint16_t P25_LP_FILTER_LEN = 44U;
// XXX These will need setting correctly
const q15_t P25_LEVELA[] = { 539, 539, 539, 539, 539};
const q15_t P25_LEVELB[] = { 179, 179, 179, 179, 179};
@ -48,7 +57,9 @@ const uint8_t P25_START_SYNC = 0x77U;
CP25TX::CP25TX() :
m_buffer(1500U),
m_modFilter(),
m_lpFilter(),
m_modState(),
m_lpState(),
m_poBuffer(),
m_poLen(0U),
m_poPtr(0U),
@ -56,10 +67,15 @@ m_txDelay(240U), // 200ms
m_count(0U)
{
::memset(m_modState, 0x00U, 70U * sizeof(q15_t));
::memset(m_lpState, 0x00U, 70U * sizeof(q15_t));
m_modFilter.numTaps = P25_C4FSK_FILTER_LEN;
m_modFilter.pState = m_modState;
m_modFilter.pCoeffs = P25_C4FSK_FILTER;
m_lpFilter.numTaps = P25_LP_FILTER_LEN;
m_lpFilter.pState = m_lpState;
m_lpFilter.pCoeffs = P25_LP_FILTER;
}
void CP25TX::process()
@ -121,6 +137,7 @@ uint8_t CP25TX::writeData(const uint8_t* data, uint8_t length)
void CP25TX::writeByte(uint8_t c)
{
q15_t inBuffer[P25_RADIO_SYMBOL_LENGTH * 4U + 1U];
q15_t intBuffer[P25_RADIO_SYMBOL_LENGTH * 4U + 1U];
q15_t outBuffer[P25_RADIO_SYMBOL_LENGTH * 4U + 1U];
const uint8_t MASK = 0xC0U;
@ -161,9 +178,9 @@ void CP25TX::writeByte(uint8_t c)
}
}
::arm_fir_fast_q15(&m_modFilter, inBuffer, outBuffer, blockSize);
::arm_fir_fast_q15(&m_modFilter, inBuffer, intBuffer, blockSize);
// Inverse sinc lowpass filter needed here
::arm_fir_fast_q15(&m_lpFilter, intBuffer, outBuffer, blockSize);
io.write(STATE_P25, outBuffer, blockSize);
}

2
P25TX.h
View File

@ -38,7 +38,9 @@ public:
private:
CSerialRB m_buffer;
arm_fir_instance_q15 m_modFilter;
arm_fir_instance_q15 m_lpFilter;
q15_t m_modState[70U]; // NoTaps + BlockSize - 1, 42 + 20 - 1 plus some spare
q15_t m_lpState[70U]; // NoTaps + BlockSize - 1, 44 + 20 - 1 plus some spare
uint8_t m_poBuffer[920U];
uint16_t m_poLen;
uint16_t m_poPtr;