demod_flex.c

/*
 *      demod_flex.c
 *
 *      Copyright (C) 2015 Craig Shelley (craig@microtron.org.uk)
 *
 *      FLEX Radio Paging Decoder - Adapted from GNURadio for use with Multimon
 *
 *	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; either version 2 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 for more details.
 *
 *	You should have received a copy of the GNU General Public License
 *	along with this program; if not, write to the Free Software
 *	Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

/* ---------------------------------------------------------------------- */

#include "multimon.h"
#include "filter.h"
#include "BCHCode.h"
#include <math.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>

/* ---------------------------------------------------------------------- */

#define FREQ_SAMP            22050
#define FILTLEN              1


#define FLEX_SYNC_MARKER     0xA6C6AAAA    // Synchronisation code marker for FLEX
#define SLICE_THRESHOLD      0.667         // For 4 level code, levels 0 and 3 have 3 times the amplitude of levels 1 and 2, so quantise at 2/3
#define DC_OFFSET_FILTER     0.010         // DC Offset removal IIR filter response (seconds)
#define PHASE_LOCKED_RATE    0.010         // Correction factor for locked state
#define PHASE_UNLOCKED_RATE  0.050         // Correction factor for unlocked state
#define LOCK_LEN             24            // Number of symbols to check for phase locking (max 32)
#define IDLE_THRESHOLD       0             // Number of idle codewords allowed in data section


enum Flex_PageTypeEnum {
	FLEX_PAGETYPE_SECURE,
	FLEX_PAGETYPE_UNKNOWN,
	FLEX_PAGETYPE_TONE,
	FLEX_PAGETYPE_STANDARD_NUMERIC,
	FLEX_PAGETYPE_SPECIAL_NUMERIC,
	FLEX_PAGETYPE_ALPHANUMERIC,
	FLEX_PAGETYPE_BINARY,
	FLEX_PAGETYPE_NUMBERED_NUMERIC
};


enum Flex_StateEnum {
	FLEX_STATE_SYNC1,
	FLEX_STATE_FIW,
	FLEX_STATE_SYNC2,
	FLEX_STATE_DATA
};


struct Flex_Demodulator {
	unsigned int                sample_freq;
	double                      sample_last;
	int                         locked;
	int                         phase;
	unsigned int                sample_count;
	unsigned int                symbol_count;
	double                      envelope_sum;
	int                         envelope_count;
	unsigned long long          lock_buf;
	int                         symcount[4];
	int                         timeout;
	int                         nonconsec;
	unsigned int                baud;          // Current baud rate
};


struct Flex_Modulation {
	double                      symbol_rate;
	double                      envelope;
	double                      zero;
};


struct Flex_State {
	unsigned int                sync2_count;
	unsigned int                data_count;
	unsigned int                fiwcount;
	enum Flex_StateEnum         Current;
	enum Flex_StateEnum         Previous;
};


struct Flex_Sync {
	unsigned int                sync;          // Outer synchronization code
	unsigned int                baud;          // Baudrate of SYNC2 and DATA
	unsigned int                levels;        // FSK encoding of SYNC2 and DATA
	unsigned int                polarity;      // 0=Positive (Normal) 1=Negative (Inverted)
	unsigned long long          syncbuf;
};


struct Flex_FIW {
	unsigned int                rawdata;
	unsigned int                checksum;
	unsigned int                cycleno;
	unsigned int                frameno;
	unsigned int                fix3;
};


struct Flex_Phase {
	unsigned int                buf[88];
	int                         idle_count;
};


struct Flex_Data {
	int                         phase_toggle;
	unsigned int                data_bit_counter;
	struct Flex_Phase           PhaseA;
	struct Flex_Phase           PhaseB;
	struct Flex_Phase           PhaseC;
	struct Flex_Phase           PhaseD;
};


struct Flex_Decode {
	enum Flex_PageTypeEnum      type;
	int                         long_address;
	long int                    capcode;
	struct BCHCode *            BCHCode;
};


struct Flex {
	struct Flex_Demodulator     Demodulator;
	struct Flex_Modulation      Modulation;
	struct Flex_State           State;
	struct Flex_Sync            Sync;
	struct Flex_FIW             FIW;
	struct Flex_Data            Data;
	struct Flex_Decode          Decode;
};


inline int is_alphanumeric_page(struct Flex * flex) {
	if (flex==NULL) return 0;
	return (flex->Decode.type == FLEX_PAGETYPE_ALPHANUMERIC ||
			flex->Decode.type == FLEX_PAGETYPE_SECURE);
}


inline int is_numeric_page(struct Flex * flex) {
	if (flex==NULL) return 0;
	return (flex->Decode.type == FLEX_PAGETYPE_STANDARD_NUMERIC ||
			flex->Decode.type == FLEX_PAGETYPE_SPECIAL_NUMERIC  ||
			flex->Decode.type == FLEX_PAGETYPE_NUMBERED_NUMERIC);
}


inline int is_tone_page(struct Flex * flex) {
	if (flex==NULL) return 0;
	return (flex->Decode.type == FLEX_PAGETYPE_TONE);
}


unsigned int count_bits(struct Flex * flex, unsigned int data) {
	if (flex==NULL) return 0;
	int i;

	unsigned int retval=0;
	for (i=0; i<32; i++) {
		retval+=data&1;
		data>>=1;
	}

	return retval;
}

static int bch3121_fix_errors(struct Flex * flex, unsigned int * data_to_fix, char PhaseNo) {
	if (flex==NULL) return -1;
	int i=0;
	int recd[31];

	/*Convert the data pattern into an array of coefficients*/
	unsigned int data=*data_to_fix;
	for (i=0; i<31; i++) {
		recd[i] = (data>>30)&1;
		data<<=1;
	}

	/*Decode and correct the coefficients*/
	int decode_error=BCHCode_Decode(flex->Decode.BCHCode, recd);

	/*Decode successful?*/
	if (!decode_error) {
		/*Convert the coefficient array back to a bit pattern*/
		data=0;
		for (i=0; i<31; i++) {
			data<<=1;
			data|=recd[i];
		}
		/*Count the number of fixed errors*/
		int fixed=count_bits(flex, (*data_to_fix & 0x7FFFFFFF) ^ data);
		if (fixed>0) {
			verbprintf(3, "FLEX: Phase %c Fixed %i errors @ 0x%08x  (0x%08x -> 0x%08x)\n", PhaseNo, fixed, (*data_to_fix&0x7FFFFFFF) ^ data, (*data_to_fix&0x7FFFFFFF), data );
		}

		/*Write the fixed data back to the caller*/
		*data_to_fix=data;

	} else {
		verbprintf(3, "FLEX: Phase %c Data corruption - Unable to fix errors.\n", PhaseNo);
	}

	return decode_error;
}

static unsigned int flex_sync_check(struct Flex * flex, unsigned long long buf) {
	if (flex==NULL) return 0;
	// 64-bit FLEX sync code:
	// AAAA:BBBBBBBB:CCCC
	//
	// Where BBBBBBBB is always 0xA6C6AAAA
	// and AAAA^CCCC is 0xFFFF
	//
	// Specific values of AAAA determine what bps and encoding the
	// packet is beyond the frame information word
	//
	// First we match on the marker field with a hamming distance < 4
	// Then we match on the outer code with a hamming distance < 4

	unsigned int marker =      (buf & 0x0000FFFFFFFF0000ULL) >> 16;
	unsigned short codehigh =  (buf & 0xFFFF000000000000ULL) >> 48;
	unsigned short codelow  = ~(buf & 0x000000000000FFFFULL);

	int retval=0;
	if (count_bits(flex, marker ^ FLEX_SYNC_MARKER) < 4  && count_bits(flex, codelow ^ codehigh) < 4 ) {
		retval=codehigh;
	} else {
		retval=0;
	}

	return retval;
}


static unsigned int flex_sync(struct Flex * flex, unsigned char sym) {
	if (flex==NULL) return 0;
	int retval=0;
	flex->Sync.syncbuf = (flex->Sync.syncbuf << 1) | ((sym < 2)?1:0);

	retval=flex_sync_check(flex, flex->Sync.syncbuf);
	if (retval!=0) {
		flex->Sync.polarity=0;
	} else {
		/*If a positive sync pattern was not found, look for a negative (inverted) one*/
		retval=flex_sync_check(flex, ~flex->Sync.syncbuf);
		if (retval!=0) {
			flex->Sync.polarity=1;
		}
	}

	return retval;
}


static void decode_mode(struct Flex * flex, unsigned int sync_code) {
	if (flex==NULL) return;

	struct {
		int sync;
		unsigned int baud;
		unsigned int levels;
	} flex_modes[] = {
		{ 0x870C, 1600, 2 },
		{ 0xB068, 1600, 4 },
		//  { 0xUNKNOWN,  3200, 2 },
		{ 0xDEA0, 3200, 4 },
		{ 0x4C7C, 3200, 4 },
		{0,0,0}
	};

	int i=0;
	for (i=0; flex_modes[i].sync!=0; i++) {
		if (count_bits(flex, flex_modes[i].sync ^ sync_code) < 4) {
			flex->Sync.sync   = sync_code;
			flex->Sync.baud   = flex_modes[i].baud;
			flex->Sync.levels = flex_modes[i].levels;
			break;
		}
	}
}


static void read_2fsk(struct Flex * flex, unsigned int sym, unsigned int * dat) {
	if (flex==NULL) return;
	*dat = (*dat >> 1) | ((sym > 1)?0x80000000:0);
}


static int decode_fiw(struct Flex * flex) {
	if (flex==NULL) return -1;
	unsigned int fiw = flex->FIW.rawdata;
	int decode_error = bch3121_fix_errors(flex, &fiw, 'F');

	if (decode_error) {
		verbprintf(3, "FLEX: Unable to decode FIW, too much data corruption\n");
		return 1;
	}

	// The only relevant bits in the FIW word for the purpose of this function
	// are those masked by 0x001FFFFF.
	flex->FIW.checksum = fiw & 0xF;
	flex->FIW.cycleno = (fiw >> 4) & 0xF;
	flex->FIW.frameno = (fiw >> 8) & 0x7F;
	flex->FIW.fix3 = (fiw >> 15) & 0x3F;

	unsigned int checksum = (fiw & 0xF);
	checksum += ((fiw >> 4) & 0xF);
	checksum += ((fiw >> 8) & 0xF);
	checksum += ((fiw >> 12) & 0xF);
	checksum += ((fiw >> 16) & 0xF);
	checksum += ((fiw >> 20) & 0x01);

	checksum &= 0xF;

	if (checksum == 0xF) {
		int timeseconds = flex->FIW.cycleno*4*60 + flex->FIW.frameno*4*60/128;
		verbprintf(1, "FLEX: FrameInfoWord: cycleno=%02i frameno=%03i fix3=0x%02x time=%02i:%02i\n",
				flex->FIW.cycleno,
				flex->FIW.frameno,
				flex->FIW.fix3,
				timeseconds/60,
				timeseconds%60);

		return 0;
	} else {
		verbprintf(3, "FLEX: Bad Checksum 0x%x\n", checksum);

		return 1;
	}
}


static void parse_alphanumeric(struct Flex * flex, unsigned int * phaseptr, char PhaseNo, int mw1, int mw2, int j) {
	if (flex==NULL) return;
	verbprintf(3, "FLEX: Parse Alpha Numeric\n");
	int frag;
	//   int cont;

	if (!flex->Decode.long_address) {
		frag = ( phaseptr[mw1] >> 11) & 0x03;
		//	cont = ( phaseptr[mw1] >> 10) & 0x01;
		mw1++;
	} else {
		frag = ( phaseptr[j+1] >> 11) & 0x03;
		//	cont = ( phaseptr[j+1] >> 10) & 0x01;
		mw2--;
	}

	//d_payload << frag << FIELD_DELIM;
	//d_payload << cont << FIELD_DELIM;

	int i;
	time_t now=time(NULL);
	struct tm * gmt=gmtime(&now);

	verbprintf(0,  "FLEX: %04i-%02i-%02i %02i:%02i:%02i %i/%i/%c %02i.%03i [%09li] ALN ", gmt->tm_year+1900, gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec,
			flex->Sync.baud, flex->Sync.levels, PhaseNo, flex->FIW.cycleno, flex->FIW.frameno, flex->Decode.capcode);

	for (i = mw1; i <= mw2; i++) {
		unsigned int dw =  phaseptr[i];
		unsigned char ch;

		if (i > mw1 || frag != 0x03) {
			ch = dw & 0x7F;
			if (ch != 0x03)
				verbprintf(0, "%c", ch);
		}

		ch = (dw >> 7) & 0x7F;
		if (ch != 0x03)	// Fill
			verbprintf(0, "%c", ch);

		ch = (dw >> 14) & 0x7F;
		if (ch != 0x03)	// Fill
			verbprintf(0, "%c", ch);
	}


	verbprintf(0, "\n");

}


static void parse_numeric(struct Flex * flex, unsigned int * phaseptr, char PhaseNo, int mw1, int mw2, int j) {
	if (flex==NULL) return;
	unsigned const char flex_bcd[17] = "0123456789 U -][";

	time_t now=time(NULL);
	struct tm * gmt=gmtime(&now);
	verbprintf(0,  "FLEX: %04i-%02i-%02i %02i:%02i:%02i %i/%i/%c %02i.%03i [%09li] NUM ", gmt->tm_year+1900, gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec,
			flex->Sync.baud, flex->Sync.levels, PhaseNo, flex->FIW.cycleno, flex->FIW.frameno, flex->Decode.capcode);

	// Get first dataword from message field or from second
	// vector word if long address
	int dw;
	if(!flex->Decode.long_address) {
		dw = phaseptr[mw1];
		mw1++;
		mw2++;
	} else {
		dw = phaseptr[j+1];
	}

	unsigned char digit = 0;
	int count = 4;
	if(flex->Decode.type == FLEX_PAGETYPE_NUMBERED_NUMERIC) {
		count += 10;        // Skip 10 header bits for numbered numeric pages
	} else {
		count += 2;        // Otherwise skip 2
	}
	int i;
	for(i = mw1; i <= mw2; i++) {
		int k;
		for(k = 0; k < 21; k++) {
			// Shift LSB from data word into digit
			digit = (digit >> 1) & 0x0F;
			if(dw & 0x01) {
				digit ^= 0x08;
			}
			dw >>= 1;
			if(--count == 0) {
				if(digit != 0x0C) {// Fill
					verbprintf(0, "%c", flex_bcd[digit]);
				}
				count = 4;
			}
		}
		dw = phaseptr[i];
	}
	verbprintf(0, "\n");
}


static void parse_tone_only(struct Flex * flex, char PhaseNo) {
	if (flex==NULL) return;
	time_t now=time(NULL);
	struct tm * gmt=gmtime(&now);
	verbprintf(0,  "FLEX: %04i-%02i-%02i %02i:%02i:%02i %i/%i/%c %02i.%03i [%09li] TON\n", gmt->tm_year+1900, gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec,
			flex->Sync.baud, flex->Sync.levels, PhaseNo, flex->FIW.cycleno, flex->FIW.frameno, flex->Decode.capcode);
}


static void parse_unknown(struct Flex * flex, unsigned int * phaseptr, char PhaseNo, int mw1, int mw2) {
	if (flex==NULL) return;
	time_t now=time(NULL);
	struct tm * gmt=gmtime(&now);
	verbprintf(0,  "FLEX: %04i-%02i-%02i %02i:%02i:%02i %i/%i/%c %02i.%03i [%09li] UNK", gmt->tm_year+1900, gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec,
			flex->Sync.baud, flex->Sync.levels, PhaseNo, flex->FIW.cycleno, flex->FIW.frameno, flex->Decode.capcode);

	int i;
	for (i = mw1; i <= mw2; i++) {
		verbprintf(0, " %08x", phaseptr[i]);
	}
	verbprintf(0, "\n");
}


static void parse_capcode(struct Flex * flex, unsigned int aw1, unsigned int aw2) {
	if (flex==NULL) return;
	flex->Decode.long_address = (aw1 < 0x008001L) ||
		(aw1 > 0x1E0000L) ||
		(aw1 > 0x1E7FFEL);

	if (flex->Decode.long_address)
		flex->Decode.capcode = aw1+((aw2^0x001FFFFF)<<15)+0x1F9000;  // Don't ask
	else
		flex->Decode.capcode = aw1-0x8000;
}


static void decode_phase(struct Flex * flex, char PhaseNo) {
	if (flex==NULL) return;
	unsigned int *phaseptr=NULL;
	switch (PhaseNo) {
		case 'A': phaseptr=flex->Data.PhaseA.buf; break;
		case 'B': phaseptr=flex->Data.PhaseB.buf; break;
		case 'C': phaseptr=flex->Data.PhaseC.buf; break;
		case 'D': phaseptr=flex->Data.PhaseD.buf; break;
	}
	int i, j;
	for (i=0; i<88; i++) {
		int decode_error=bch3121_fix_errors(flex, &phaseptr[i], PhaseNo);

		if (decode_error) {
			verbprintf(3, "FLEX: Garbled message at block %i\n", i);
			return;
		}

		/*Extract just the message bits*/
		phaseptr[i]&=0x001FFFFF;
	}

	// Block information word is the first data word in frame
	unsigned int biw = phaseptr[0];

	// Nothing to see here, please move along
	if (biw == 0 || biw == 0x001FFFFF) {
		verbprintf(3, "FLEX: Nothing to see here, please move along\n");
		return;
	}

	// Vector start index is bits 15-10
	// Address start address is bits 9-8, plus one for offset
	int voffset = (biw >> 10) & 0x3f;
	int aoffset = ((biw >> 8) & 0x03) + 1;
	verbprintf(3, "FLEX: BlockInfoWord: (Phase %c) BIW:%08X AW:%02i-%02i (%i pages)\n", PhaseNo, biw, aoffset, voffset, voffset-aoffset);

	// Iterate through pages and dispatch to appropriate handler
	for (i = aoffset; i < voffset; i++) {
		j = voffset+i-aoffset;		// Start of vector field for address @ i

		if (phaseptr[i] == 0x00000000 ||
				phaseptr[i] == 0x001FFFFF) {
			verbprintf(3, "FLEX: Idle codewords, invalid address\n");
			continue;				// Idle codewords, invalid address
		}

		parse_capcode(flex, phaseptr[i], phaseptr[i+1]);
		if (flex->Decode.long_address)
			i++;

		if (flex->Decode.capcode < 0) {		// Invalid address, skip
			verbprintf(3, "FLEX: Invalid address\n");
			continue;
		}

		verbprintf(3, "FLEX: CAPCODE:%016lx\n", flex->Decode.capcode);

		// Parse vector information word for address @ offset 'i'
		unsigned int viw = phaseptr[j];
		flex->Decode.type = ((viw >> 4) & 0x00000007);
		int mw1 = (viw >> 7) & 0x00000007F;
		int len = (viw >> 14) & 0x0000007F;

		if (is_numeric_page(flex))
			len &= 0x07;
		int mw2 = mw1+len;

		if (mw1 == 0 && mw2 == 0){
			verbprintf(3, "FLEX: Invalid VIW\n");
			continue;				// Invalid VIW
		}

		if (is_tone_page(flex))
			mw1 = mw2 = 0;

		if (mw1 > 87 || mw2 > 87){
			verbprintf(3, "FLEX: Invalid Offsets\n");
			continue;				// Invalid offsets
		}

		if (is_alphanumeric_page(flex))
			parse_alphanumeric(flex, phaseptr, PhaseNo, mw1, mw2-1, j);
		else if (is_numeric_page(flex))
			parse_numeric(flex, phaseptr, PhaseNo, mw1, mw2, j);
		else if (is_tone_page(flex))
			parse_tone_only(flex, PhaseNo);
		else
			parse_unknown(flex, phaseptr, PhaseNo, mw1, mw2);
	}
}


static void clear_phase_data(struct Flex * flex) {
	if (flex==NULL) return;
	int i;
	for (i=0; i<88; i++) {
		flex->Data.PhaseA.buf[i]=0;
		flex->Data.PhaseB.buf[i]=0;
		flex->Data.PhaseC.buf[i]=0;
		flex->Data.PhaseD.buf[i]=0;
	}

	flex->Data.PhaseA.idle_count=0;
	flex->Data.PhaseB.idle_count=0;
	flex->Data.PhaseC.idle_count=0;
	flex->Data.PhaseD.idle_count=0;

	flex->Data.phase_toggle=0;
	flex->Data.data_bit_counter=0;

}


static void decode_data(struct Flex * flex) {
	if (flex==NULL) return;

	if (flex->Sync.baud == 1600) {
		if (flex->Sync.levels==2) {
			decode_phase(flex, 'A');
		} else {
			decode_phase(flex, 'A');
			decode_phase(flex, 'B');
		}
	} else {
		if (flex->Sync.levels==2) {
			decode_phase(flex, 'A');
			decode_phase(flex, 'C');
		} else {
			decode_phase(flex, 'A');
			decode_phase(flex, 'B');
			decode_phase(flex, 'C');
			decode_phase(flex, 'D');
		}
	}
}


static int read_data(struct Flex * flex, unsigned char sym) {
	if (flex==NULL) return -1;
	// Here is where we output a 1 or 0 on each phase according
	// to current FLEX mode and symbol value.  Unassigned phases
	// are zero from the enter_idle() initialization.
	//
	// FLEX can transmit the data portion of the frame at either
	// 1600 bps or 3200 bps, and can use either two- or four-level
	// FSK encoding.
	//
	// At 1600 bps, 2-level, a single "phase" is transmitted with bit
	// value '0' using level '3' and bit value '1' using level '0'.
	//
	// At 1600 bps, 4-level, a second "phase" is transmitted, and the
	// di-bits are encoded with a gray code:
	//
	// Symbol	Phase 1  Phase 2
	// ------   -------  -------
	//   0         1        1
	//   1         1        0
	//   2         0        0
	//   3         0        1
	//
	// At 1600 bps, 4-level, these are called PHASE A and PHASE B.
	//
	// At 3200 bps, the same 1 or 2 bit encoding occurs, except that
	// additionally two streams are interleaved on alternating symbols.
	// Thus, PHASE A (and PHASE B if 4-level) are decoded on one symbol,
	// then PHASE C (and PHASE D if 4-level) are decoded on the next.

	int bit_a=0; //Received data bit for Phase A
	int bit_b=0; //Received data bit for Phase B

	bit_a = (sym > 1);
	if (flex->Sync.levels == 4) {
		bit_b = (sym == 1) || (sym == 2);
	}

	if (flex->Sync.baud == 1600) {
		flex->Data.phase_toggle=0;
	}

	//By making the index scan the data words in this way, the data is deinterlaced
	//Bits 0, 1, and 2 map straight through to give a 0-7 sequence that repeats 32 times before moving to 8-15 repeating 32 times
	unsigned int idx= ((flex->Data.data_bit_counter>>5)&0xFFF8) |  (flex->Data.data_bit_counter&0x0007);

	if (flex->Data.phase_toggle==0) {
		flex->Data.PhaseA.buf[idx] = (flex->Data.PhaseA.buf[idx]>>1) | (bit_a?(0x80000000):0);
		flex->Data.PhaseB.buf[idx] = (flex->Data.PhaseB.buf[idx]>>1) | (bit_b?(0x80000000):0);
		flex->Data.phase_toggle=1;

		if ((flex->Data.data_bit_counter & 0xFF) == 0xFF) {
			if (flex->Data.PhaseA.buf[idx] == 0x00000000 || flex->Data.PhaseA.buf[idx] == 0xffffffff) flex->Data.PhaseA.idle_count++;
			if (flex->Data.PhaseB.buf[idx] == 0x00000000 || flex->Data.PhaseB.buf[idx] == 0xffffffff) flex->Data.PhaseB.idle_count++;
		}
	} else {
		flex->Data.PhaseC.buf[idx] = (flex->Data.PhaseC.buf[idx]>>1) | (bit_a?(0x80000000):0);
		flex->Data.PhaseD.buf[idx] = (flex->Data.PhaseD.buf[idx]>>1) | (bit_b?(0x80000000):0);
		flex->Data.phase_toggle=0;

		if ((flex->Data.data_bit_counter & 0xFF) == 0xFF) {
			if (flex->Data.PhaseC.buf[idx] == 0x00000000 || flex->Data.PhaseC.buf[idx] == 0xffffffff) flex->Data.PhaseC.idle_count++;
			if (flex->Data.PhaseD.buf[idx] == 0x00000000 || flex->Data.PhaseD.buf[idx] == 0xffffffff) flex->Data.PhaseD.idle_count++;
		}
	}

	if (flex->Sync.baud == 1600 || flex->Data.phase_toggle==0) {
		flex->Data.data_bit_counter++;
	}

	/*Report if all active phases have gone idle*/
	int idle=0;
	if (flex->Sync.baud == 1600) {
		if (flex->Sync.levels==2) {
			idle=(flex->Data.PhaseA.idle_count>IDLE_THRESHOLD);
		} else {
			idle=((flex->Data.PhaseA.idle_count>IDLE_THRESHOLD) && (flex->Data.PhaseB.idle_count>IDLE_THRESHOLD));
		}
	} else {
		if (flex->Sync.levels==2) {
			idle=((flex->Data.PhaseA.idle_count>IDLE_THRESHOLD) && (flex->Data.PhaseC.idle_count>IDLE_THRESHOLD));
		} else {
			idle=((flex->Data.PhaseA.idle_count>IDLE_THRESHOLD) && (flex->Data.PhaseB.idle_count>IDLE_THRESHOLD) && (flex->Data.PhaseC.idle_count>IDLE_THRESHOLD) && (flex->Data.PhaseD.idle_count>IDLE_THRESHOLD));
		}
	}

	return idle;
}


static void report_state(struct Flex * flex) {
	if (flex->State.Current != flex->State.Previous) {
		flex->State.Previous = flex->State.Current;

		char * state="Unknown";
		switch (flex->State.Current) {
			case FLEX_STATE_SYNC1:
				state="SYNC1";
				break;
			case FLEX_STATE_FIW:
				state="FIW";
				break;
			case FLEX_STATE_SYNC2:
				state="SYNC2";
				break;
			case FLEX_STATE_DATA:
				state="DATA";
				break;
			default:
				break;

		}
		verbprintf(1, "FLEX: State: %s\n", state);
	}
}

//Called for each received symbol
static void flex_sym(struct Flex * flex, unsigned char sym) {
	if (flex==NULL) return;
	/*If the signal has a negative polarity, the symbols must be inverted*/
	/*Polarity is determined during the IDLE/sync word checking phase*/
	unsigned char sym_rectified;
	if (flex->Sync.polarity) {
		sym_rectified=3-sym;
	} else {
		sym_rectified=sym;
	}

	switch (flex->State.Current) {
		case FLEX_STATE_SYNC1:
			{
				// Continually compare the received symbol stream
				// against the known FLEX sync words.
				unsigned int sync_code=flex_sync(flex, sym); //Unrectified version of the symbol must be used here
				if (sync_code!=0) {
					decode_mode(flex,sync_code);

					if (flex->Sync.baud!=0 && flex->Sync.levels!=0) {
						flex->State.Current=FLEX_STATE_FIW;

						verbprintf(1, "FLEX: SyncInfoWord: sync_code=0x%04x baud=%i levels=%i polarity=%s zero=%f envelope=%f symrate=%f\n",
								sync_code, flex->Sync.baud, flex->Sync.levels, flex->Sync.polarity?"NEG":"POS", flex->Modulation.zero, flex->Modulation.envelope, flex->Modulation.symbol_rate);
					} else {
						verbprintf(3, "FLEX: Unknown Sync code = 0x%04x\n", sync_code);
						flex->State.Current=FLEX_STATE_SYNC1;
					}
				} else {
					flex->State.Current=FLEX_STATE_SYNC1;
				}

				flex->State.fiwcount=0;
				flex->FIW.rawdata=0;
				break;
			}
		case FLEX_STATE_FIW:
			{
				// Skip 16 bits of dotting, then accumulate 32 bits
				// of Frame Information Word.
				// FIW is accumulated, call BCH to error correct it
				flex->State.fiwcount++;
				if (flex->State.fiwcount>=16) {
					read_2fsk(flex, sym_rectified, &flex->FIW.rawdata);
				}

				if (flex->State.fiwcount==48) {
					if (decode_fiw(flex)==0) {
						flex->State.sync2_count=0;
						flex->Demodulator.baud = flex->Sync.baud;
						flex->State.Current=FLEX_STATE_SYNC2;
					} else {
						flex->State.Current=FLEX_STATE_SYNC1;
					}
				}
				break;
			}
		case FLEX_STATE_SYNC2:
			{
				// This part and the remainder of the frame are transmitted
				// at either 1600 bps or 3200 bps based on the received
				// FLEX sync word. The second SYNC header is 25ms of idle bits
				// at either speed.
				// Skip 25 ms = 40 bits @ 1600 bps, 80 @ 3200 bps
				if (++flex->State.sync2_count == flex->Sync.baud*25/1000) {
					flex->State.data_count=0;
					clear_phase_data(flex);
					flex->State.Current=FLEX_STATE_DATA;
				}

				break;
			}
		case FLEX_STATE_DATA:
			{
				// The data portion of the frame is 1760 ms long at either
				// baudrate.  This is 2816 bits @ 1600 bps and 5632 bits @ 3200 bps.
				// The output_symbol() routine decodes and doles out the bits
				// to each of the four transmitted phases of FLEX interleaved codes.
				int idle=read_data(flex, sym_rectified);
				if (++flex->State.data_count == flex->Sync.baud*1760/1000 || idle) {
					decode_data(flex);
					flex->Demodulator.baud = 1600;
					flex->State.Current=FLEX_STATE_SYNC1;
					flex->State.data_count=0;
				}
				break;
			}
	}
}

void Flex_Demodulate(struct Flex * flex, double sample) {
	if (flex==NULL) return;
	const long int phase_max=100 * flex->Demodulator.sample_freq;                           // Maximum value for phase (calculated to divide by sample frequency without remainder)
	const long int phase_rate=phase_max*flex->Demodulator.baud/flex->Demodulator.sample_freq;      // Increment per baseband sample
	const double phasepercent = 100.0 *  flex->Demodulator.phase/phase_max;

	/*Update the sample counter*/
	flex->Demodulator.sample_count++;

	/*Remove DC offset (FIR filter)*/
	if (flex->State.Current==FLEX_STATE_SYNC1) {
		flex->Modulation.zero=(flex->Modulation.zero*(FREQ_SAMP*DC_OFFSET_FILTER) + sample)/((FREQ_SAMP*DC_OFFSET_FILTER)+1);
	}
	sample-=flex->Modulation.zero;

	if (flex->Demodulator.locked) {
		/*During the synchronisation period, establish the envelope of the signal*/
		if (flex->State.Current==FLEX_STATE_SYNC1) {
			flex->Demodulator.envelope_sum+=fabs(sample);
			flex->Demodulator.envelope_count++;
			flex->Modulation.envelope=flex->Demodulator.envelope_sum/flex->Demodulator.envelope_count;
		}
	} else {
		/*Reset and hold in initial state*/
		flex->Modulation.envelope=0;
		flex->Demodulator.envelope_sum=0;
		flex->Demodulator.envelope_count=0;
		flex->Demodulator.baud=1600;
		flex->Demodulator.timeout=0;
		flex->Demodulator.nonconsec=0;
		flex->State.Current=FLEX_STATE_SYNC1;
	}

	/* MID 80% SYMBOL PERIOD */
	if (phasepercent > 10 && phasepercent <90) {
		/*Count the number of occurrences of each symbol value for analysis at end of symbol period*/
		if (sample > 0) {
			if (sample > flex->Modulation.envelope*SLICE_THRESHOLD)
				flex->Demodulator.symcount[3]++;
			else
				flex->Demodulator.symcount[2]++;
		} else {
			if (sample < -flex->Modulation.envelope*SLICE_THRESHOLD)
				flex->Demodulator.symcount[0]++;
			else
				flex->Demodulator.symcount[1]++;
		}
	}

	/* ZERO CROSSING */
	if ((flex->Demodulator.sample_last<0 && sample>=0) || (flex->Demodulator.sample_last>=0 && sample<0)){
		/*The phase error has a direction towards the closest symbol boundary*/
		double phase_error;
		if (phasepercent<50) {
			phase_error=flex->Demodulator.phase;
		} else {
			phase_error=flex->Demodulator.phase - phase_max;
		}

		/*Phase lock with the signal*/
		if (flex->Demodulator.locked) {
			flex->Demodulator.phase -= phase_error * PHASE_LOCKED_RATE;
		} else {
			flex->Demodulator.phase -= phase_error * PHASE_UNLOCKED_RATE;
		}

		/*If too many zero crossing occur within the mid 80% then indicate lock has been lost*/
		if (phasepercent > 10 && phasepercent < 90) {
			flex->Demodulator.nonconsec++;
			if (flex->Demodulator.nonconsec>20 && flex->Demodulator.locked) {
				verbprintf(1, "FLEX: Synchronisation Lost\n");
				flex->Demodulator.locked=0;
			}
		} else {
			flex->Demodulator.nonconsec=0;
		}

		flex->Demodulator.timeout=0;
	}
	flex->Demodulator.sample_last=sample;

	/* END OF SYMBOL PERIOD */
	flex->Demodulator.phase+=phase_rate;
	if (flex->Demodulator.phase > phase_max) {
		flex->Demodulator.phase -= phase_max;
		flex->Demodulator.symbol_count++;
		flex->Modulation.symbol_rate=1.0 * flex->Demodulator.symbol_count*flex->Demodulator.sample_freq / flex->Demodulator.sample_count;

		/*Determine the modal symbol*/
		int j;
		int decmax=0;
		int modal_symbol=0;
		for (j=0; j<4; j++) {
			if (flex->Demodulator.symcount[j] > decmax) {
				modal_symbol=j;
				decmax=flex->Demodulator.symcount[j];
			}
		}
		flex->Demodulator.symcount[0]=0;
		flex->Demodulator.symcount[1]=0;
		flex->Demodulator.symcount[2]=0;
		flex->Demodulator.symcount[3]=0;


		if (flex->Demodulator.locked) {
			/*Process the symbol*/
			flex_sym(flex, modal_symbol);
		} else {
			/*Check for lock pattern*/
			/*Shift symbols into buffer, symbols are converted so that the max and min symbols map to 1 and 2 i.e each contain a single 1 */
			flex->Demodulator.lock_buf=(flex->Demodulator.lock_buf<<2) | (modal_symbol ^ 0x1);
			unsigned long long lock_pattern = flex->Demodulator.lock_buf ^ 0x6666666666666666;
			unsigned long long lock_mask = (1LL<<(2*LOCK_LEN))-1;
			if ((lock_pattern&lock_mask) == 0 || ((~lock_pattern)&lock_mask)==0) {
				verbprintf(1, "FLEX: Locked\n");
				flex->Demodulator.locked=1;
				/*Clear the syncronisation buffer*/
				flex->Demodulator.lock_buf=0;
				flex->Demodulator.symbol_count=0;
				flex->Demodulator.sample_count=0;
			}
		}

		/*Time out after 50 periods with no zero crossing*/
		flex->Demodulator.timeout++;
		if (flex->Demodulator.timeout>50) {
			verbprintf(1, "FLEX: Timeout\n");
			flex->Demodulator.locked=0;
		}
	}

	report_state(flex);
}


void Flex_Delete(struct Flex * flex) {
	if (flex==NULL) return;

	if (flex->Decode.BCHCode!=NULL) {
		BCHCode_Delete(flex->Decode.BCHCode);
		flex->Decode.BCHCode=NULL;
	}

	free(flex);
}


struct Flex * Flex_New(unsigned int SampleFrequency) {
	struct Flex *flex=(struct Flex *)malloc(sizeof(struct Flex));
	if (flex!=NULL) {
		memset(flex, 0, sizeof(struct Flex));

		flex->Demodulator.sample_freq=SampleFrequency;
		// The baud rate of first syncword and FIW is always 1600, so set that
		// rate to start.
		flex->Demodulator.baud = 1600;

		/*Generator polynomial for BCH3121 Code*/
		int p[6];
		p[0] = p[2] = p[5] = 1; p[1] = p[3] = p[4] =0;
		flex->Decode.BCHCode=BCHCode_New( p, 5, 31, 21, 2);
		if (flex->Decode.BCHCode == NULL) {
			Flex_Delete(flex);
			flex=NULL;
		}
	}

	return flex;
}


static void flex_demod(struct demod_state *s, buffer_t buffer, int length) {
	if (s==NULL) return;
	if (s->l1.flex==NULL) return;
	int i;
	for (i=0; i<length; i++) {
		Flex_Demodulate(s->l1.flex, buffer.fbuffer[i]);
	}
}


static void flex_init(struct demod_state *s) {
	if (s==NULL) return;
	s->l1.flex=Flex_New(FREQ_SAMP);
}


static void flex_deinit(struct demod_state *s) {
	if (s==NULL) return;
	if (s->l1.flex==NULL) return;

	Flex_Delete(s->l1.flex);
	s->l1.flex=NULL;
}


const struct demod_param demod_flex = {
	"FLEX", true, FREQ_SAMP, FILTLEN, flex_init, flex_demod, flex_deinit
};