FSK Modem

The purpose of this project was to see if I could improve the performance of my Bell 103A2 modem. Initially to see if I could extract the baud clock to mitigate output jitter. Well I have not looked at that (have to investigate other start bit synchronisation methods as well), the FSK Modem is a good platform to test the PLL as most of the code has been written (and debugged) and it has a built in loop-back test.

All I need to do is replace the demodulation code subject to a decision on (1) a binary XOR or (2) a mixer style phase detector.

An unreasonable good results should be expected from the loop-back as the baud generator is not really asynchronous. Well this has to be tested.

The First Port

For the first port I dropped the sample frequency to 8065 Hz to match the FSK modem. I tested bothe the Anwer and Originate mark/space frequencies. This uncovered a problem for the PID version of the PLL. Although adjustment of the parameters found an acceptable performance solution, the simple low pass PLL worked better. It appears that damping factor and natural frequency are not that important performance controls!

The Second Port

The second port is based on the low pass PLL and an internal loop back test. I am on holidays so no test work, but here is the code:

//  Bell 103A2 Modem
//  ================

#define ModemMode false // Answer Modem -> Receives Low Channel
#define Baud        300 // Baud rate
#define TimerConst   30 // 8065 Hz sample rate
// Phase steps based on PWM frequency of 31372.5 Hz (=16000000/510)
#define BaudPhase   627 // Baud phase increment (for 31372.5 Hz clock)
#define OriFspace  2235 // Originate modem Space frequency 1070 Hz
#define OriFmark   2653 // Originate modem Mark  frequency 1270 Hz
#define AnsFspace  4230 // Answer modem    Space frequency 2025 Hz
#define AnsFmark   4648 // Answer modem    Mark  frequency 2225 Hz
#define AnsPM      7882 // Answer modem tuning word for 970 Hz
#define OriPM     15643 // Originate modem tuning word Fore 1925 Hz

// Constants
const unsigned int SR=12000;        // Sample Rate
const unsigned int PL=870;          // Originate PLL Low Frequency
const unsigned int PH=1825;         // Answer PLL Low Frequency
const unsigned int PML=PL*65536/SR; // Originater DDS Tunning Number
const unsigned int PMH=PH*65536/SR; // Answer DDS Tunning Number


// Define various ADC prescaler
const byte PS_16=(1<<ADPS2);
const byte PS_32=(1<<ADPS2)|(1<<ADPS0);
const byte PS_64=(1<<ADPS2)|(1<<ADPS1);
const byte PS_128=(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);

// A 64 step byte sized sine table
const byte iSin[64] = {
  128,140,153,165,177,188,199,209,
  218,226,234,240,245,250,253,254,
  255,254,253,250,245,240,234,226,
  218,209,199,188,177,165,153,140,
  128,116,103, 91, 79, 68, 57, 47,
   38, 30, 22, 16, 11,  6,  3,  2,
    1,  2,  3,  6, 11, 16, 22, 30,
   38, 47, 57, 68, 79, 91,103,116
};

// Debug variables
// volatile int I=0;
// volatile int D0[324];
// volatile int D1[324];

// Get a signal sample
volatile bool dataAvailable=false;
volatile int SX0=0;
volatile int SX1=0;
volatile int SX2=0;
ISR(TIMER0_COMPA_vect) {
  // Roll the samples
  SX2=SX1;SX1=SX0;
  // SX0=analogRead(A0)-512;
  SX0=OCR2A-128;
  // Flag new sample available
  dataAvailable=true;
}

// TRANSMIT
volatile byte DataOut=1;
volatile bool baudUpdate=true;  
volatile unsigned int baudTick=0;
ISR(TIMER2_OVF_vect) {
  static unsigned int phase=0;
  OCR2A = iSin[(phase>>10)]; // Output on pin 11
  if (!ModemMode) {          // Set true for Answer modem - Invert for audio loopback test
    // Answer modem
    if (DataOut==0) {
      phase+=AnsFspace;
    } else {
      phase+=AnsFmark;
    }
  } else {
    // Originate modem
    if (DataOut==0) {
      phase+=OriFspace;
    } else {
      phase+=OriFmark;
    }      
  }
  // Update baud tick
  baudTick+=BaudPhase;
  if (baudTick<BaudPhase) baudUpdate=true;
}

void setup() {
  pinMode(A0,INPUT);                               // Audio input
  pinMode(12,OUTPUT);                              // Digital data output
  pinMode(11,OUTPUT);                              // Audio output (PWM)
  pinMode(10,OUTPUT);                              // Digital data input
  pinMode(LED_BUILTIN,OUTPUT);                     // Signal detected
  digitalWrite(LED_BUILTIN,LOW);
  
  // Set ADC prescaler (assume a 16 MHz clock)
  ADCSRA&=~PS_128;                                 // Remove bits set by Arduino library
  ADCSRA|=PS_16;                                   // 16 prescaler (1 MHz)

  // Disable interrupts
  cli();
  
  // Use Timer 0 for sample rate (8065 Hz)
  // ATmega48A/PA/88A/PA/168A/PA/328/P 
  TIMSK0 = 0;                                      // Timer interrupts off
  TCCR0A = (2 << WGM00);                           // CTC mode
  TCCR0B = (3 << CS00);                            // prescaler 64, 16 MHz clock
  TIMSK0 = (1 << OCIE0A);                          // COMPA interrupt
  OCR0A = TimerConst;                              // Sample rate: 8065 Hz = 250 kHz / (30+1)

  // Use Timer 2 for Audio PWM
  // ATmega48A/PA/88A/PA/168A/PA/328/P
  TIMSK2 = 0;                                      // Timer interrupts off
  TCCR2A = (2 << COM2A0)|(1 << WGM20);             // Phase correct PWM (31.25 kHz), toggle output on OC2A (PB3/D11)
  TCCR2B = (0 << WGM22)|(1 << CS20);               // 16 MHz clock (no pre-scaler)
  OCR2A = 128;                                     // Set 50% duty
  TIMSK2 = (1<<TOIE2);                             // Set interrupt on overflow (=BOTTOM)

  // Enable interrupts 
  sei();                                           
}


int Demodulate(bool Mode) {
  static unsigned int K=3000; // PLL Gain
  static unsigned int SX=0;   // Signal
  static unsigned int PX=0;   // PLL Frequency
  static unsigned int PA=0;   // PLL Phase Accumulator
  static unsigned int PM=0;   // PLL Phase Increment
  static unsigned int PD=0;   // PLL Phase Detector
  static unsigned int LP=0;   // VCO Control
  static unsigned int OP=0;   // Output filter
  // Bandpass filter
  static unsigned int BA0=0;
  static unsigned int BA1=0;
  static unsigned int BA2=0;
  static unsigned int BB0=0;
  static unsigned int BB1=0;
  static unsigned int BB2=0;
  static unsigned int BP0=0;
  static unsigned int BP1=0;
  static unsigned int BP2=0;

  // SIGNAL DECODER
  BA2=BA1;BA1=BA0;    
  BB2=BB1;BB1=BB0;    
  BP2=BP1;BP1=BP0;    
  if (Mode) {                         // Set true for Answer modem
    // PLL DDS TUNING NUMBER
    PM=AnsPM;
    // ANWSER MODEM BANDPASS FILTER (Fc = 1170 Hz)
    BA0=(8*(SX0-SX2)+7*BA1-4*BA2)>>3;
    BB0=(3*(BA0-BA2)+7*BB1-4*BB2)>>3;
    BP0=(2*(BB0-BB2)+7*BP1-4*BP2)>>3;
  } else {
    // PLL DDS TUNING NUMBER
    PM=OriPM;
    // ORIGINATE MODEM
    BA0=(8*(SX0-SX2)-BA1-4*BA2)>>3;   // Bandpass (Fc = 2125 Hz)
    BB0=(3*(BA0-BA2)-BB1-4*BB2)>>3;
    BP0=(2*(BB0-BB2)-BP1-4*BP2)>>3;
  }

  // BINARY SIGNAL 
  SX=(BP0>=0)?1:0;
  
  // PLL VC0
  PA=PA+PM+LP;
  PX=PA>>15;
  
  // XOR PHASE DETECTOR
  PD=(SX==PX)?PD=0:PD=K;
  
  // LOOP FILTER
  LP=PD+7*(LP-PD)>>3;

  // OUTPUT FILTER
  OP=OP+7*(OP-LP)>>3;
  
  // Debug: Record data for post analysis
  // if (I<324) {
  //    D0[I]=SX0;
  //    D1[I]=BP0;
  //    I++;
  // }

  // Return correlation flag
  if (OP>=0) {
    return(1);
  } else {
    return(0);    
  }  
}

void loop() {
  static bool Mode=ModemMode;
  static byte DataIn=1;
  static byte i=0;

  // SEND DATA
  if (baudUpdate) { // Counter has overflowed
    i++;
    if (i==1) DataOut=1;
    if (i==2) DataOut=0;
    if (i==3) DataOut=1;
    if (i==4) DataOut=1;
    if (i==5) DataOut=0;
    if (i==6) DataOut=0;
    if (i==7) DataOut=1;
    if (i==8) DataOut=1;
    if (i==9) DataOut=1;
    if (i==10) DataOut=0;
    if (i==11) DataOut=0;
    if (i==12) DataOut=0;
    if (i>=12) i=0;
    baudUpdate=false;
    // digitalWrite(10,DataOut);
  }
  
  // RECEIVE DATA
  if (dataAvailable) {
    // Demodulate signal and set D12
    DataIn=Demodulate(Mode);
    digitalWrite(12,DataIn);
    // Done, wait for next sample tick
    dataAvailable=false;
  }
  
  // Debug output
  // if (I>=324) {
  //   cli(); 
  //   Serial.begin(9600);
  //   for (I=0;I<324;I++) {
  //     Serial.print(I);
  //     Serial.print(",");
  //     Serial.print(D0[I]);
  //     Serial.print(",");
  //     Serial.println(D1[I]);
  //   }
  //   Serial.flush();
  //   Serial.end();
  //   while (true);  
  // }
  
}

TBC ...

AlanX