[proxmark3-svn] / common / lfdemod.c

//-----------------------------------------------------------------------------
// Copyright (C) 2014 
//
// This code is licensed to you under the terms of the GNU GPL, version 2 or,
// at your option, any later version. See the LICENSE.txt file for the text of
// the license.
//-----------------------------------------------------------------------------
// Low frequency commands
//-----------------------------------------------------------------------------

#include <stdlib.h>
#include <string.h>
#include "lfdemod.h"

//by marshmellow
//takes 1s and 0s and searches for EM410x format - output EM ID
uint64_t Em410xDecode(uint8_t *BitStream,uint32_t BitLen)
{
    //no arguments needed - built this way in case we want this to be a direct call from "data " cmds in the future
    //  otherwise could be a void with no arguments
    //set defaults
    int high=0, low=128;
    uint64_t lo=0; //hi=0,

    uint32_t i = 0;
    uint32_t initLoopMax = 65;
    if (initLoopMax>BitLen) initLoopMax=BitLen;

    for (;i < initLoopMax; ++i) //65 samples should be plenty to find high and low values
    {
        if (BitStream[i] > high)
            high = BitStream[i];
        else if (BitStream[i] < low)
            low = BitStream[i];
    }
    if (((high !=1)||(low !=0))){  //allow only 1s and 0s
        // PrintAndLog("no data found");
        return 0;
    }
    uint8_t parityTest=0;
    // 111111111 bit pattern represent start of frame
    uint8_t frame_marker_mask[] = {1,1,1,1,1,1,1,1,1};
    uint32_t idx = 0;
    uint32_t ii=0;
    uint8_t resetCnt = 0;
    while( (idx + 64) < BitLen) {
restart:
        // search for a start of frame marker
        if ( memcmp(BitStream+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
        { // frame marker found
            idx+=9;//sizeof(frame_marker_mask);
            for (i=0; i<10;i++){
                for(ii=0; ii<5; ++ii){
                    parityTest += BitStream[(i*5)+ii+idx];
                }
                if (parityTest== ((parityTest>>1)<<1)){
                    parityTest=0;
                    for (ii=0; ii<4;++ii){
                        //hi = (hi<<1)|(lo>>31);
                        lo=(lo<<1LL)|(BitStream[(i*5)+ii+idx]);
                    }
                    //PrintAndLog("DEBUG: EM parity passed parity val: %d, i:%d, ii:%d,idx:%d, Buffer: %d%d%d%d%d,lo: %d",parityTest,i,ii,idx,BitStream[idx+ii+(i*5)-5],BitStream[idx+ii+(i*5)-4],BitStream[idx+ii+(i*5)-3],BitStream[idx+ii+(i*5)-2],BitStream[idx+ii+(i*5)-1],lo);
                }else {//parity failed
                    //PrintAndLog("DEBUG: EM parity failed parity val: %d, i:%d, ii:%d,idx:%d, Buffer: %d%d%d%d%d",parityTest,i,ii,idx,BitStream[idx+ii+(i*5)-5],BitStream[idx+ii+(i*5)-4],BitStream[idx+ii+(i*5)-3],BitStream[idx+ii+(i*5)-2],BitStream[idx+ii+(i*5)-1]);
                    parityTest=0;
                    idx-=8;
                    if (resetCnt>5)return 0;
                    resetCnt++;
                    goto restart;//continue;
                }
            }
            //skip last 5 bit parity test for simplicity.
            return lo;
        }else{
            idx++;
        }
    }
    return 0;
}

//by marshmellow
//takes 2 arguments - clock and invert both as integers
//attempts to demodulate ask while decoding manchester 
//prints binary found and saves in graphbuffer for further commands
int askmandemod(uint8_t * BinStream,uint32_t *BitLen,int *clk, int *invert)
{
    int i;
    int high = 0, low = 128;
    *clk=DetectASKClock(BinStream,(size_t)*BitLen,*clk); //clock default

    if (*clk<8) *clk =64;
    if (*clk<32) *clk=32;
    if (*invert != 0 && *invert != 1) *invert=0;
    uint32_t initLoopMax = 200;
    if (initLoopMax>*BitLen) initLoopMax=*BitLen;
    // Detect high and lows
    for (i = 0; i < initLoopMax; ++i) //200 samples should be enough to find high and low values
    {
        if (BinStream[i] > high)
            high = BinStream[i];
        else if (BinStream[i] < low)
            low = BinStream[i];
    }
    if ((high < 158) ){  //throw away static
        //PrintAndLog("no data found");
        return -2;
    }
    //25% fuzz in case highs and lows aren't clipped [marshmellow]
    high=(int)((high-128)*.75)+128;
    low= (int)((low-128)*.75)+128;

    //PrintAndLog("DEBUG - valid high: %d - valid low: %d",high,low);
    int lastBit = 0;  //set first clock check
    uint32_t bitnum = 0;     //output counter
    int tol = 0;  //clock tolerance adjust - waves will be accepted as within the clock if they fall + or - this value + clock from last valid wave
    if (*clk==32)tol=1;    //clock tolerance may not be needed anymore currently set to + or - 1 but could be increased for poor waves or removed entirely
    int iii = 0;
    uint32_t gLen = *BitLen;
    if (gLen > 3000) gLen=3000;
    uint8_t errCnt =0;
    uint32_t bestStart = *BitLen;
    uint32_t bestErrCnt = (*BitLen/1000);
    uint32_t maxErr = (*BitLen/1000);
    //PrintAndLog("DEBUG - lastbit - %d",lastBit);
    //loop to find first wave that works
    for (iii=0; iii < gLen; ++iii){
        if ((BinStream[iii]>=high)||(BinStream[iii]<=low)){
            lastBit=iii-*clk;
            errCnt=0;
            //loop through to see if this start location works
            for (i = iii; i < *BitLen; ++i) {
                if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
                    lastBit+=*clk;
                } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
                    //low found and we are expecting a bar
                    lastBit+=*clk;
                } else {
                    //mid value found or no bar supposed to be here
                    if ((i-lastBit)>(*clk+tol)){
                        //should have hit a high or low based on clock!!

                        //debug
                        //PrintAndLog("DEBUG - no wave in expected area - location: %d, expected: %d-%d, lastBit: %d - resetting search",i,(lastBit+(clk-((int)(tol)))),(lastBit+(clk+((int)(tol)))),lastBit);

                        errCnt++;
                        lastBit+=*clk;//skip over until hit too many errors
                        if (errCnt>(maxErr)) break;  //allow 1 error for every 1000 samples else start over
                    }
                }
                if ((i-iii) >(400 * *clk)) break; //got plenty of bits
            }
            //we got more than 64 good bits and not all errors
            if ((((i-iii)/ *clk) > (64+errCnt)) && (errCnt<maxErr)) {
                //possible good read
                if (errCnt==0){
                    bestStart=iii;
                    bestErrCnt=errCnt;
                    break;  //great read - finish
                }
                if (errCnt<bestErrCnt){  //set this as new best run
                    bestErrCnt=errCnt;
                    bestStart = iii;
                }
            }
        }
    }
    if (bestErrCnt<maxErr){
        //best run is good enough set to best run and set overwrite BinStream
        iii=bestStart;
        lastBit=bestStart-*clk;
        bitnum=0;
        for (i = iii; i < *BitLen; ++i) {
            if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
                lastBit+=*clk;
                BinStream[bitnum] =  *invert;
                bitnum++;
            } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
                //low found and we are expecting a bar
                lastBit+=*clk;
                BinStream[bitnum] = 1-*invert;
                bitnum++;
            } else {
                //mid value found or no bar supposed to be here
                if ((i-lastBit)>(*clk+tol)){
                    //should have hit a high or low based on clock!!

                    //debug
                    //PrintAndLog("DEBUG - no wave in expected area - location: %d, expected: %d-%d, lastBit: %d - resetting search",i,(lastBit+(clk-((int)(tol)))),(lastBit+(clk+((int)(tol)))),lastBit);
                    if (bitnum > 0){
                        BinStream[bitnum]=77;
                        bitnum++;
                    }

                    lastBit+=*clk;//skip over error
                }
            }
            if (bitnum >=400) break;
        }
        *BitLen=bitnum;
    } else{
        *invert=bestStart;
        *clk=iii;
        return -1;
    }
    return bestErrCnt;
}

//by marshmellow
//take 10 and 01 and manchester decode
//run through 2 times and take least errCnt
int manrawdecode(uint8_t * BitStream, int *bitLen)
{
    int bitnum=0;
    int errCnt =0;
    int i=1;
    int bestErr = 1000;
    int bestRun = 0;
    int ii=1;
    for (ii=1;ii<3;++ii){
        i=1;
        for (i=i+ii;i<*bitLen-2;i+=2){
            if(BitStream[i]==1 && (BitStream[i+1]==0)){
            } else if((BitStream[i]==0)&& BitStream[i+1]==1){
            } else {
                errCnt++;
            }
            if(bitnum>300) break;
        }
        if (bestErr>errCnt){
            bestErr=errCnt;
            bestRun=ii;
        }
        errCnt=0;
    }
    errCnt=bestErr;
    if (errCnt<20){
        ii=bestRun;
        i=1;
        for (i=i+ii;i<*bitLen-2;i+=2){
            if(BitStream[i]==1 && (BitStream[i+1]==0)){
                BitStream[bitnum++]=0;
            } else if((BitStream[i]==0)&& BitStream[i+1]==1){
                BitStream[bitnum++]=1;
            } else {
                BitStream[bitnum++]=77;
                //errCnt++;
            }
            if(bitnum>300) break;
        }
        *bitLen=bitnum;
    }
    return errCnt;
}


//by marshmellow
//take 01 or 10 = 0 and 11 or 00 = 1
int BiphaseRawDecode(uint8_t * BitStream, int *bitLen, int offset)
{
    uint8_t bitnum=0;
    uint32_t errCnt =0;
    uint32_t i=1;
    i=offset;
    for (;i<*bitLen-2;i+=2){
        if((BitStream[i]==1 && BitStream[i+1]==0)||(BitStream[i]==0 && BitStream[i+1]==1)){
            BitStream[bitnum++]=1;
        } else if((BitStream[i]==0 && BitStream[i+1]==0)||(BitStream[i]==1 && BitStream[i+1]==1)){
            BitStream[bitnum++]=0;
        } else {
            BitStream[bitnum++]=77;
            errCnt++;
        }
        if(bitnum>250) break;
    }
    *bitLen=bitnum;
    return errCnt;
}

//by marshmellow
//takes 2 arguments - clock and invert both as integers
//attempts to demodulate ask only
//prints binary found and saves in graphbuffer for further commands
int askrawdemod(uint8_t *BinStream, int *bitLen,int *clk, int *invert)
{
    uint32_t i;
    // int invert=0;  //invert default
    int high = 0, low = 128;
    *clk=DetectASKClock(BinStream,*bitLen,*clk); //clock default
    uint8_t BitStream[502] = {0};

    if (*clk<8) *clk =64;
    if (*clk<32) *clk=32;
    if (*invert != 0 && *invert != 1) *invert =0;
    uint32_t initLoopMax = 200;
    if (initLoopMax>*bitLen) initLoopMax=*bitLen;
    // Detect high and lows
    for (i = 0; i < initLoopMax; ++i) //200 samples should be plenty to find high and low values
    {
        if (BinStream[i] > high)
            high = BinStream[i];
        else if (BinStream[i] < low)
            low = BinStream[i];
    }
    if ((high < 158)){  //throw away static
        //   PrintAndLog("no data found");
        return -2;
    }
    //25% fuzz in case highs and lows aren't clipped [marshmellow]
    high=(int)((high-128)*.75)+128;
    low= (int)((low-128)*.75)+128;

    //PrintAndLog("DEBUG - valid high: %d - valid low: %d",high,low);
    int lastBit = 0;  //set first clock check
    uint32_t bitnum = 0;     //output counter
    uint8_t tol = 0;  //clock tolerance adjust - waves will be accepted as within the clock if they fall + or - this value + clock from last valid wave
    if (*clk==32)tol=1;    //clock tolerance may not be needed anymore currently set to + or - 1 but could be increased for poor waves or removed entirely
    uint32_t iii = 0;
    uint32_t gLen = *bitLen;
    if (gLen > 500) gLen=500;
    uint8_t errCnt =0;
    uint32_t bestStart = *bitLen;
    uint32_t bestErrCnt = (*bitLen/1000);
    uint8_t midBit=0;
    //PrintAndLog("DEBUG - lastbit - %d",lastBit);
    //loop to find first wave that works
    for (iii=0; iii < gLen; ++iii){
        if ((BinStream[iii]>=high)||(BinStream[iii]<=low)){
            lastBit=iii-*clk;
            //loop through to see if this start location works
            for (i = iii; i < *bitLen; ++i) {
                if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
                    lastBit+=*clk;
                    BitStream[bitnum] =  *invert;
                    bitnum++;
                    midBit=0;
                } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
                    //low found and we are expecting a bar
                    lastBit+=*clk;
                    BitStream[bitnum] = 1-*invert;
                    bitnum++;
                    midBit=0;
                } else if ((BinStream[i]<=low) && (midBit==0) && ((i-lastBit)>((*clk/2)-tol))){
                    //mid bar?
                    midBit=1;
                    BitStream[bitnum]= 1-*invert;
                    bitnum++;
                } else if ((BinStream[i]>=high)&&(midBit==0) && ((i-lastBit)>((*clk/2)-tol))){
                    //mid bar?
                    midBit=1;
                    BitStream[bitnum]= *invert;
                    bitnum++;
                } else if ((i-lastBit)>((*clk/2)+tol)&&(midBit==0)){
                    //no mid bar found
                    midBit=1;
                    BitStream[bitnum]= BitStream[bitnum-1];
                    bitnum++;
                } else {
                    //mid value found or no bar supposed to be here

                    if ((i-lastBit)>(*clk+tol)){
                        //should have hit a high or low based on clock!!
                        //debug
                        //PrintAndLog("DEBUG - no wave in expected area - location: %d, expected: %d-%d, lastBit: %d - resetting search",i,(lastBit+(clk-((int)(tol)))),(lastBit+(clk+((int)(tol)))),lastBit);
                        if (bitnum > 0){
                            BitStream[bitnum]=77;
                            bitnum++;
                        }


                        errCnt++;
                        lastBit+=*clk;//skip over until hit too many errors
                        if (errCnt>((*bitLen/1000))){  //allow 1 error for every 1000 samples else start over
                            errCnt=0;
                            bitnum=0;//start over
                            break;
                        }
                    }
                }
                if (bitnum>500) break;
            }
            //we got more than 64 good bits and not all errors
            if ((bitnum > (64+errCnt)) && (errCnt<(*bitLen/1000))) {
                //possible good read
                if (errCnt==0) break;  //great read - finish
                if (bestStart == iii) break;  //if current run == bestErrCnt run (after exhausted testing) then finish
                if (errCnt<bestErrCnt){  //set this as new best run
                    bestErrCnt=errCnt;
                    bestStart = iii;
                }
            }
        }
        if (iii>=gLen){ //exhausted test
            //if there was a ok test go back to that one and re-run the best run (then dump after that run)
            if (bestErrCnt < (*bitLen/1000)) iii=bestStart;
        }
    }
    if (bitnum>16){

        // PrintAndLog("Data start pos:%d, lastBit:%d, stop pos:%d, numBits:%d",iii,lastBit,i,bitnum);
        //move BitStream back to BinStream
        // ClearGraph(0);
        for (i=0; i < bitnum; ++i){
            BinStream[i]=BitStream[i];
        }
        *bitLen=bitnum;
        // RepaintGraphWindow();
        //output
        // if (errCnt>0){
        //   PrintAndLog("# Errors during Demoding (shown as 77 in bit stream): %d",errCnt);
        // }
        // PrintAndLog("ASK decoded bitstream:");
        // Now output the bitstream to the scrollback by line of 16 bits
        // printBitStream2(BitStream,bitnum);
        //int errCnt=0;
        //errCnt=manrawdemod(BitStream,bitnum);

        //   Em410xDecode(Cmd);
    } else return -1;
    return errCnt;
}
//translate wave to 11111100000 (1 for each short wave 0 for each long wave) 
size_t fsk_wave_demod(uint8_t * dest, size_t size, uint8_t fchigh, uint8_t fclow)
{
    uint32_t last_transition = 0;
    uint32_t idx = 1;
    uint32_t maxVal=0;
    if (fchigh==0) fchigh=10;
    if (fclow==0) fclow=8;
    // we do care about the actual theshold value as sometimes near the center of the
    // wave we may get static that changes direction of wave for one value
    // if our value is too low it might affect the read.  and if our tag or
    // antenna is weak a setting too high might not see anything. [marshmellow]
    if (size<100) return 0;
    for(idx=1; idx<100; idx++){
        if(maxVal<dest[idx]) maxVal = dest[idx];
    }
    // set close to the top of the wave threshold with 25% margin for error
    // less likely to get a false transition up there.
    // (but have to be careful not to go too high and miss some short waves)
    uint8_t threshold_value = (uint8_t)(((maxVal-128)*.75)+128);
    //  idx=1;
    //uint8_t threshold_value = 127;

    // sync to first lo-hi transition, and threshold

    // Need to threshold first sample

    if(dest[0] < threshold_value) dest[0] = 0;
    else dest[0] = 1;

    size_t numBits = 0;
    // count cycles between consecutive lo-hi transitions, there should be either 8 (fc/8)
    // or 10 (fc/10) cycles but in practice due to noise etc we may end up with with anywhere
    // between 7 to 11 cycles so fuzz it by treat anything <9 as 8 and anything else as 10
    for(idx = 1; idx < size; idx++) {
        // threshold current value

        if (dest[idx] < threshold_value) dest[idx] = 0;
        else dest[idx] = 1;

        // Check for 0->1 transition
        if (dest[idx-1] < dest[idx]) { // 0 -> 1 transition
            if ((idx-last_transition)<(fclow-2)){            //0-5 = garbage noise
                //do nothing with extra garbage
            } else if ((idx-last_transition) < (fchigh-1)) { //6-8 = 8 waves
                dest[numBits]=1;
            } else {                                                    //9+ = 10 waves
                dest[numBits]=0;
            }
            last_transition = idx;
            numBits++;
        }
    }
    return numBits; //Actually, it returns the number of bytes, but each byte represents a bit: 1 or 0
}

uint32_t myround2(float f)
{
    if (f >= 2000) return 2000;//something bad happened
    return (uint32_t) (f + (float)0.5);
}

//translate 11111100000 to 10 
size_t aggregate_bits(uint8_t *dest,size_t size,  uint8_t rfLen, uint8_t maxConsequtiveBits, uint8_t invert,uint8_t fchigh,uint8_t fclow )// uint8_t h2l_crossing_value,uint8_t l2h_crossing_value, 
{
    uint8_t lastval=dest[0];
    uint32_t idx=0;
    size_t numBits=0;
    uint32_t n=1;

    for( idx=1; idx < size; idx++) {

        if (dest[idx]==lastval) {
            n++;
            continue;
        }
        //if lastval was 1, we have a 1->0 crossing
        if ( dest[idx-1]==1 ) {
            n=myround2((float)(n+1)/((float)(rfLen)/(float)fclow));
            //n=(n+1) / h2l_crossing_value;
        } else {// 0->1 crossing
            n=myround2((float)(n+1)/((float)(rfLen-2)/(float)fchigh));  //-2 for fudge factor
            //n=(n+1) / l2h_crossing_value;
        }
        if (n == 0) n = 1;

        if(n < maxConsequtiveBits) //Consecutive
        {
            if(invert==0){ //invert bits
                memset(dest+numBits, dest[idx-1] , n);
            }else{
                memset(dest+numBits, dest[idx-1]^1 , n);
            }
            numBits += n;
        }
        n=0;
        lastval=dest[idx];
    }//end for
    return numBits;
}
//by marshmellow  (from holiman's base)
// full fsk demod from GraphBuffer wave to decoded 1s and 0s (no mandemod)
int fskdemod(uint8_t *dest, size_t size, uint8_t rfLen, uint8_t invert, uint8_t fchigh, uint8_t fclow)
{
    // FSK demodulator
    size = fsk_wave_demod(dest, size, fchigh, fclow);
    size = aggregate_bits(dest, size,rfLen,192,invert,fchigh,fclow);
    return size;
}
// loop to get raw HID waveform then FSK demodulate the TAG ID from it
int HIDdemodFSK(uint8_t *dest, size_t size, uint32_t *hi2, uint32_t *hi, uint32_t *lo)
{

    size_t idx=0; //, found=0; //size=0,
    // FSK demodulator
    size = fskdemod(dest, size,50,0,10,8);

    // final loop, go over previously decoded manchester data and decode into usable tag ID
    // 111000 bit pattern represent start of frame, 01 pattern represents a 1 and 10 represents a 0
    uint8_t frame_marker_mask[] = {1,1,1,0,0,0};
    int numshifts = 0;
    idx = 0;
    //one scan
    while( idx + sizeof(frame_marker_mask) < size) {
        // search for a start of frame marker
        if ( memcmp(dest+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
        { // frame marker found
            idx+=sizeof(frame_marker_mask);
            while(dest[idx] != dest[idx+1] && idx < size-2)
            {
                // Keep going until next frame marker (or error)
                // Shift in a bit. Start by shifting high registers
                *hi2 = (*hi2<<1)|(*hi>>31);
                *hi = (*hi<<1)|(*lo>>31);
                //Then, shift in a 0 or one into low
                if (dest[idx] && !dest[idx+1])  // 1 0
                    *lo=(*lo<<1)|0;
                else // 0 1
                    *lo=(*lo<<1)|1;
                numshifts++;
                idx += 2;
            }
            // Hopefully, we read a tag and      hit upon the next frame marker
            if(idx + sizeof(frame_marker_mask) < size)
            {
                if ( memcmp(dest+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
                {
                    //good return
                    return idx;
                }
            }
            // reset
            *hi2 = *hi = *lo = 0;
            numshifts = 0;
        }else   {
            idx++;
        }
    }
    return -1;
}

uint32_t bytebits_to_byte(uint8_t* src, int numbits)
{
    uint32_t num = 0;
    for(int i = 0 ; i < numbits ; i++)
    {
        num = (num << 1) | (*src);
        src++;
    }
    return num;
}

int IOdemodFSK(uint8_t *dest, size_t size)
{
        static const uint8_t THRESHOLD = 140;
    uint32_t idx=0;
    //make sure buffer has data
    if (size < 66) return -1;
    //test samples are not just noise
        uint8_t justNoise = 1;
        for(idx=0;idx< size && justNoise ;idx++){
                justNoise = dest[idx] < THRESHOLD;
        }
        if(justNoise) return 0;

        // FSK demodulator
        size = fskdemod(dest, size,64,1,10,8);  //  RF/64 and invert
        if (size < 65) return -1;  //did we get a good demod?
        //Index map
        //0           10          20          30          40          50          60
        //|           |           |           |           |           |           |
        //01234567 8 90123456 7 89012345 6 78901234 5 67890123 4 56789012 3 45678901 23
        //-----------------------------------------------------------------------------
        //00000000 0 11110000 1 facility 1 version* 1 code*one 1 code*two 1 ???????? 11
        //
        //XSF(version)facility:codeone+codetwo
        //Handle the data
        uint8_t mask[] = {0,0,0,0,0,0,0,0,0,1};
        for( idx=0; idx < (size - 65); idx++) {
                if ( memcmp(dest + idx, mask, sizeof(mask))==0) {
                        //frame marker found
                        if (!dest[idx+8] && dest[idx+17]==1 && dest[idx+26]==1 && dest[idx+35]==1 && dest[idx+44]==1 && dest[idx+53]==1){
                                //confirmed proper separator bits found
                                //return start position
                                return (int) idx;
                        }
                }
        }
    return 0;
}

// by marshmellow
// not perfect especially with lower clocks or VERY good antennas (heavy wave clipping)
// maybe somehow adjust peak trimming value based on samples to fix?
int DetectASKClock(uint8_t dest[], size_t size, int clock)
{
    int i=0;
    int peak=0;
    int low=128;
    int clk[]={16,32,40,50,64,100,128,256};
    int loopCnt = 256;  //don't need to loop through entire array...
    if (size<loopCnt) loopCnt = size;

    //if we already have a valid clock quit
    for (;i<8;++i)
        if (clk[i]==clock) return clock;

    //get high and low peak
    for (i=0;i<loopCnt;++i){
        if(dest[i]>peak){
            peak = dest[i];
        }
        if(dest[i]<low){
            low = dest[i];
        }
    }
    peak=(int)((peak-128)*.75)+128;
    low= (int)((low-128)*.75)+128;
    int ii;
    int clkCnt;
    int tol = 0;
    int bestErr=1000;
    int errCnt[]={0,0,0,0,0,0,0,0};
    //test each valid clock from smallest to greatest to see which lines up
    for(clkCnt=0; clkCnt<6;++clkCnt){
        if (clk[clkCnt]==32){
            tol=1;
        }else{
            tol=0;
        }
        bestErr=1000;
        //try lining up the peaks by moving starting point (try first 256)
        for (ii=0; ii<loopCnt; ++ii){
            if ((dest[ii]>=peak) || (dest[ii]<=low)){
                errCnt[clkCnt]=0;
                // now that we have the first one lined up test rest of wave array
                for (i=0; i<((int)(size/clk[clkCnt])-1); ++i){
                    if (dest[ii+(i*clk[clkCnt])]>=peak || dest[ii+(i*clk[clkCnt])]<=low){
                    }else if(dest[ii+(i*clk[clkCnt])-tol]>=peak || dest[ii+(i*clk[clkCnt])-tol]<=low){
                    }else if(dest[ii+(i*clk[clkCnt])+tol]>=peak || dest[ii+(i*clk[clkCnt])+tol]<=low){
                    }else{  //error no peak detected
                        errCnt[clkCnt]++;
                    }
                }
                //if we found no errors this is correct one - return this clock
                if(errCnt[clkCnt]==0) return clk[clkCnt];
                //if we found errors see if it is lowest so far and save it as best run
                if(errCnt[clkCnt]<bestErr) bestErr=errCnt[clkCnt];
            }
        }
    }
    int iii=0;
    int best=0;
    for (iii=0; iii<6;++iii){
        if (errCnt[iii]<errCnt[best]){
            best = iii;
        }
    }
    return clk[best];
}