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Add a smoothing filter to the POCSAG TX waveform.

This commit is contained in:
Jonathan Naylor 2018-06-25 18:00:11 +01:00
parent 88c530d9e6
commit dcd8142c51
2 changed files with 24 additions and 9 deletions
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21
POCSAGTX.cpp
View File

@ -29,15 +29,25 @@ const uint16_t POCSAG_RADIO_SYMBOL_LENGTH = 20U;
const q15_t POCSAG_LEVEL1[] = { 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700}; const q15_t POCSAG_LEVEL1[] = { 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700, 1700};
const q15_t POCSAG_LEVEL0[] = {-1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700}; const q15_t POCSAG_LEVEL0[] = {-1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700, -1700};
static q15_t SHAPING_FILTER[] = {5461, 5461, 5461, 5461, 5461, 5461};
const uint16_t SHAPING_FILTER_LEN = 6U;
const uint8_t POCSAG_SYNC = 0xAAU; const uint8_t POCSAG_SYNC = 0xAAU;
CPOCSAGTX::CPOCSAGTX() : CPOCSAGTX::CPOCSAGTX() :
m_buffer(4000U), m_buffer(4000U),
m_modFilter(),
m_modState(),
m_poBuffer(), m_poBuffer(),
m_poLen(0U), m_poLen(0U),
m_poPtr(0U), m_poPtr(0U),
m_txDelay(POCSAG_PREAMBLE_LENGTH_BYTES) m_txDelay(POCSAG_PREAMBLE_LENGTH_BYTES)
{ {
::memset(m_modState, 0x00U, 170U * sizeof(q15_t));
m_modFilter.numTaps = SHAPING_FILTER_LEN;
m_modFilter.pState = m_modState;
m_modFilter.pCoeffs = SHAPING_FILTER;
} }
void CPOCSAGTX::process() void CPOCSAGTX::process()
@ -94,7 +104,8 @@ uint8_t CPOCSAGTX::writeData(const uint8_t* data, uint8_t length)
void CPOCSAGTX::writeByte(uint8_t c) void CPOCSAGTX::writeByte(uint8_t c)
{ {
q15_t buffer[POCSAG_RADIO_SYMBOL_LENGTH * 8U]; q15_t inBuffer[POCSAG_RADIO_SYMBOL_LENGTH * 8U];
q15_t outBuffer[POCSAG_RADIO_SYMBOL_LENGTH * 8U];
const uint8_t MASK = 0x80U; const uint8_t MASK = 0x80U;
@ -102,15 +113,17 @@ void CPOCSAGTX::writeByte(uint8_t c)
for (uint8_t i = 0U; i < 8U; i++, c <<= 1, n += POCSAG_RADIO_SYMBOL_LENGTH) { for (uint8_t i = 0U; i < 8U; i++, c <<= 1, n += POCSAG_RADIO_SYMBOL_LENGTH) {
switch (c & MASK) { switch (c & MASK) {
case 0x80U: case 0x80U:
::memcpy(buffer + n, POCSAG_LEVEL1, POCSAG_RADIO_SYMBOL_LENGTH * sizeof(q15_t)); ::memcpy(inBuffer + n, POCSAG_LEVEL1, POCSAG_RADIO_SYMBOL_LENGTH * sizeof(q15_t));
break; break;
default: default:
::memcpy(buffer + n, POCSAG_LEVEL0, POCSAG_RADIO_SYMBOL_LENGTH * sizeof(q15_t)); ::memcpy(inBuffer + n, POCSAG_LEVEL0, POCSAG_RADIO_SYMBOL_LENGTH * sizeof(q15_t));
break; break;
} }
} }
io.write(STATE_POCSAG, buffer, POCSAG_RADIO_SYMBOL_LENGTH * 8U); ::arm_fir_fast_q15(&m_modFilter, inBuffer, outBuffer, POCSAG_RADIO_SYMBOL_LENGTH * 8U);
io.write(STATE_POCSAG, outBuffer, POCSAG_RADIO_SYMBOL_LENGTH * 8U);
} }
void CPOCSAGTX::setTXDelay(uint8_t delay) void CPOCSAGTX::setTXDelay(uint8_t delay)

2
POCSAGTX.h
View File

@ -37,6 +37,8 @@ public:
private: private:
CSerialRB m_buffer; CSerialRB m_buffer;
arm_fir_instance_q15 m_modFilter;
q15_t m_modState[170U]; // NoTaps + BlockSize - 1, 6 + 160 - 1 plus some spare
uint8_t m_poBuffer[1200U]; uint8_t m_poBuffer[1200U];
uint16_t m_poLen; uint16_t m_poLen;
uint16_t m_poPtr; uint16_t m_poPtr;