rdsdecoder.cpp

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// Copyright (C) 2015 F4EXB                                                      //
// written by Edouard Griffiths                                                  //
//                                                                               //
// 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 as version 3 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 V3 for more details.                               //
//                                                                               //
// You should have received a copy of the GNU General Public License             //
// along with this program. If not, see <http://www.gnu.org/licenses/>.          //

#include "rdsdecoder.h"

#include <QDebug>
#include <string.h>

const unsigned int RDSDecoder::offset_pos[5] = {0,1,2,3,2};
const unsigned int RDSDecoder::offset_word[5] = {252,408,360,436,848};
const unsigned int RDSDecoder::syndrome[5] = {383,14,303,663,748};

RDSDecoder::RDSDecoder()
{
    m_reg = 0;
    m_presync = false;
    m_lastseenOffsetCounter = 0;
    m_bitCounter = 0;
    m_lastseenOffset = 0;
    m_blocksCounter        = 0;
    m_blockBitCounter      = 0;
    m_blockNumber          = 0;
    m_groupAssemblyStarted = false;
    m_sync                 = SYNC;
    m_groupGoodBlocksCounter = 0;
    m_wrongBlocksCounter   = 0;
    m_goodBlock            = false;
    m_qua                  = 0.0f;
    memset(m_group, 0, 4*sizeof(int));
}

RDSDecoder::~RDSDecoder()
{
}

bool RDSDecoder::frameSync(bool bit)
{
    bool group_ready = false;
    unsigned int reg_syndrome;
    unsigned long bit_distance, block_distance;
    unsigned int block_calculated_crc, block_received_crc, checkword, dataword;

    m_reg = (m_reg<<1) | bit;

    switch (m_sync)
    {
    case NO_SYNC:
        reg_syndrome = calc_syndrome(m_reg,26);

        for (int j = 0; j < 5; j++)
        {
            if (reg_syndrome == syndrome[j])
            {
                if (!m_presync)
                {
                    m_lastseenOffset = j;
                    m_lastseenOffsetCounter = m_bitCounter;
                    m_presync=true;
                }
                else
                {
                    bit_distance = m_bitCounter - m_lastseenOffsetCounter;

                    if (offset_pos[m_lastseenOffset] >= offset_pos[j])
                    {
                        block_distance = offset_pos[j] + 4 - offset_pos[m_lastseenOffset];
                    }
                    else
                    {
                        block_distance = offset_pos[j] - offset_pos[m_lastseenOffset];
                    }

                    if ((block_distance*26)!=bit_distance)
                    {
                        m_presync = false;
                    }
                    else
                    {
                        //qDebug("RDSDecoder::frameSync: Sync State Detected");
                        enter_sync(j);
                    }
                }

                break; //syndrome found, no more cycles
            }
        }
        break;

    case SYNC:
        // wait until 26 bits enter the buffer
        if (m_blockBitCounter < 25)
        {
            m_blockBitCounter++;
        }
        else
        {
            m_goodBlock = false;
            dataword = (m_reg>>10) & 0xffff;
            block_calculated_crc = calc_syndrome(dataword, 16);
            checkword = m_reg & 0x3ff;

            // manage special case of C or C' offset word
            if (m_blockNumber == 2)
            {
                block_received_crc = checkword ^ offset_word[m_blockNumber];

                if (block_received_crc==block_calculated_crc)
                {
                    m_goodBlock = true;
                }
                else
                {
                    block_received_crc=checkword^offset_word[4];

                    if (block_received_crc==block_calculated_crc)
                    {
                        m_goodBlock = true;
                    }
                    else
                    {
                        m_wrongBlocksCounter++;
                        m_goodBlock = false;
                    }
                }
            }
            else
            {
                block_received_crc = checkword ^ offset_word[m_blockNumber];

                if (block_received_crc==block_calculated_crc)
                {
                    m_goodBlock = true;
                }
                else
                {
                    m_wrongBlocksCounter++;
                    m_goodBlock = false;
                }
            }

            //done checking CRC
            if (m_blockNumber == 0 && m_goodBlock)
            {
                m_groupAssemblyStarted = true;
                m_groupGoodBlocksCounter = 1;
            }

            if (m_groupAssemblyStarted)
            {
                if (!m_goodBlock)
                {
                    m_groupAssemblyStarted = false;
                }
                else
                {
                    m_group[m_blockNumber] = dataword;
                    m_groupGoodBlocksCounter++;
                }

                if (m_groupGoodBlocksCounter == 5)
                {
                    group_ready = true; //decode_group(group); pass on to the group parser
                }
            }

            m_blockBitCounter = 0;
            m_blockNumber = (m_blockNumber + 1) % 4;
            m_blocksCounter++;

            // 1187.5 bps / 104 bits = 11.4 groups/sec, or 45.7 blocks/sec
            if (m_blocksCounter == 50)
            {
                if (m_wrongBlocksCounter > 35)
                {
                    /*
                    qDebug() << "RDSDecoder::frameSync: Lost Sync (Got " << m_wrongBlocksCounter
                        << " bad blocks on " << m_blocksCounter
                        << " total)";*/
                    enter_no_sync();
                }
                else
                {
                    /*
                    qDebug() << "RDSDecoder::frameSync: Still Sync-ed (Got " << m_wrongBlocksCounter
                        << " bad blocks on " << m_blocksCounter
                        << " total)";*/
                }

                m_qua = 2.0 * (50 - m_wrongBlocksCounter);
                m_blocksCounter = 0;
                m_wrongBlocksCounter = 0;
            }
        }
        break;
    default:
        break;
    }

    m_bitCounter++;

    return group_ready;
}


void RDSDecoder::enter_sync(unsigned int sync_block_number)
{
    m_wrongBlocksCounter   = 0;
    m_blocksCounter        = 0;
    m_blockBitCounter      = 0;
    m_blockNumber          = (sync_block_number + 1) % 4;
    m_groupAssemblyStarted = false;
    m_sync                 = SYNC;
}

void RDSDecoder::enter_no_sync()
{
    m_presync = false;
    m_sync = NO_SYNC;
}

unsigned int RDSDecoder::calc_syndrome(unsigned long message, unsigned char mlen)
{
    unsigned long reg = 0;
    unsigned int i;
    const unsigned long poly = 0x5B9;
    const unsigned char plen = 10;

    for (i = mlen; i > 0; i--)
    {
        reg = (reg << 1) | ((message >> (i-1)) & 0x01);
        if (reg & (1 << plen)) reg = reg ^ poly;
    }

    for (i = plen; i > 0; i--)
    {
        reg = reg << 1;
        if (reg & (1<<plen)) {
            reg = reg ^ poly;
        }
    }

    return (reg & ((1<<plen)-1));   // select the bottom plen bits of reg
}