CTCSS decoder with Arduino

From a friend of mine i got an ideea for a complex repeater controller, able to work with 4 radios. Two of them forming a UHF repeater and, at least, two other, forming a second repeater or simplex radios for linking the first repeater with other remote repeaters.
But this is not the subject here...



The first step was to check the possibility to decode CTCSS and to generate it back. But decoding was the biggest challenge.
I was looking for some CML circuits but the representative here asked me to buy a large quantity at a big price.
So I was turning to my Arduino trying to figure it out how to make it able to decode CTCSS in no more than 100 msec.
Finally, I did it by measuring the pulses between interrupts and find this method pretty nice and precise, at least for a crystal resonator driven Arduino.

*** Refinements of the initial code was further made by Jean-Jacques ON7EQ   and  Paul ON4ADI. Please, read to the end.

Here is the code, just copy/paste it in the Arduino IDE:
// Frequency counter sketch, for measuring frequencies low enough to execute an interrupt for each cycle
// Connect the frequency source to the INT0 pin (digital pin 2 on an Arduino Uno)
// By Adrian, YO3HJV
// This work is released to Public Domain
// First published on January, 18th 2015 on
// http://yo3hjv.blogspot.ro/2015/01/ctcss-decoder-with-arduino.html
// This code can be used for free but I will appreciate if you mention the author.
// 73 de Adrian, YO3HJV


#include
#include
/ *May be uncomment for standard Arduino LCD library.
I am using a weird library here... */
LiquidCrystal lcd(8, 9, 4, 5, 6, 7); // set for my configuration. Let PIN2 to be the input for the signal.
volatile unsigned long timpPrimImpuls;
volatile unsigned long timpUltimImpuls;
volatile unsigned long numImpuls;
void setup()
{
//Serial.begin(19200); // print for debugging. Uncomment if necessary
lcd.begin(16, 2);
}
// Measure the frequency over the specified sample time in milliseconds, returning the frequency in Hz
float readFrequency(unsigned int sampleTime)
{
numImpuls = 0; // start a new reading
attachInterrupt(0, counter, RISING); // enable the interrupt
delay(sampleTime);
detachInterrupt(0);
return (numImpuls < 3) ? 0 : (996500.0 * (float)(numImpuls - 2))/(float)(timpUltimImpuls - timpPrimImpuls);
}
// NOTE: 996500.0 is the value find by me. The theoretic value is "1000000000.0"
// Start with this value and check the precision against a good frequency meter.
void loop()
{
float freq = readFrequency(100);
lcd.setCursor(0, 0);
lcd.print("Freq: ");
lcd.print (freq);
lcd.print(" ");
lcd.print("Hz ");
lcd.setCursor (0, 1);
// Too low but over 10 Hz
if ((freq > 10) && (freq < 65.8))
{
lcd.print(" TOO LOW ");
//DO SOMETHING
}
else if ((freq > 66.00) && (freq < 68.00))
{
lcd.print("CT: 67.0 Hz, XZ ");
//DO SOMETHING
}
else if ((freq > 68.30) && (freq < 70.30))
{
lcd.print("CT: 69.3 Hz, WZ ");
//DO SOMETHING
}
else if ((freq > 70.90) && (freq < 72.90))
{
lcd.print("CT: 71.9 Hz, XA ");
//DO SOMETHING
}
else if ((freq > 73.40) && (freq < 75.40))
{
lcd.print("CT: 74.4 Hz, WA ");
//DO SOMETHING
}
else if ((freq > 76.00) && (freq < 78.00))
{
lcd.print("CT: 77.0 Hz, XB ");
//DO SOMETHING
}
else if ((freq > 78.70) && (freq < 79.70))
{
lcd.print("CT: 79.70 Hz, WB ");
//DO SOMETHING
}
else if ((freq > 81.50) && (freq < 83.50))
{
lcd.print("CT: 82.5 Hz, YZ ");
//DO SOMETHING
}
else if ((freq > 84.30) && (freq < 86.50))
{
lcd.print("CT: 85.4 Hz, YA ");
//DO SOMETHING
}
else if ((freq > 87.40) && (freq < 89.60))
{
lcd.print("CT: 88.5 Hz, YB ");
//DO SOMETHING
}
else if ((freq > 90.40) && (freq < 92.60))
{
lcd.print("CT: 91.5 Hz, ZZ ");
//DO SOMETHING
}
else if ((freq > 93.7) && (freq < 95.90))
{
lcd.print("CT: 94.8 Hz, ZA ");
//DO SOMETHING
}
else if ((freq > 96.30) && (freq < 98.5))
{
lcd.print("CT: 97.4 Hz, ZB ");
//DO SOMETHING
}
else if ((freq > 99.00) && (freq < 101.00))
{
lcd.print("CT: 100.0 Hz, 1Z ");
//DO SOMETHING
}
else if ((freq > 102.40) && (freq < 104.60))
{
lcd.print("CT: 103.5 Hz, 1A ");
//DO SOMETHING
}
else if ((freq > 106.10) && (freq < 108.30))
{
lcd.print("CT: 107.2 Hz, 1B ");
//DO SOMETHING
}
else if ((freq > 109.80) && (freq < 112.00))
{
lcd.print("CT: 110.9 Hz, 2Z ");
//DO SOMETHING
}
else if ((freq > 113.60) && (freq < 116.00))
{
lcd.print("CT: 114.8 Hz, 2A ");
//DO SOMETHING
}
else if ((freq > 117.60) && (freq < 119.90))
{
lcd.print("CT: 118.8 Hz, 2B ");
//DO SOMETHING
}
else if ((freq > 122.00) && (freq < 124.00))
{
lcd.print("CT: 123.0 Hz, 3Z ");
//DO SOMETHING
}
else if ((freq > 126.20) && (freq < 128.40))
{
lcd.print("CT: 127.3 Hz, 3A ");
}
else if ((freq > 130.40) && (freq < 133.00))
{
lcd.print("CT: 131.8 Hz, 3B ");
}
else if ((freq > 135.00) && (freq < 138.00))
{
lcd.print("CT: 136.5 Hz, 4Z ");
//DO SOMETHING
}
else if ((freq > 140.00) && (freq < 142.80))
{
lcd.print("CT: 141.3 Hz, 4A ");
//DO SOMETHING
}
else if ((freq > 145.00) && (freq < 147.80))
{
lcd.print("CT: 146.2 Hz, 4B ");
//DO SOMETHING
}
else if ((freq > 150.00) && (freq < 152.80))
{
lcd.print("CT: 151.4 Hz, 5Z ");
//DO SOMETHING
}
else if ((freq > 156.00) && (freq < 158.80))
{
lcd.print("CT: 157.7 Hz, 5A ");
//DO SOMETHING
}
// NON-STANDARD
else if ((freq > 159.00) && (freq < 161.00))
{
lcd.print("CT: 159.8 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 161.00) && (freq < 163.50))
{
lcd.print("CT: 162.2 Hz, 5B ");
//DO SOMETHING
}
// NON-STANDARD
else if ((freq > 164.00) && (freq < 166.30))
{
lcd.print("CT: 165.5 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 166.60) && (freq < 169.00))
{
lcd.print("CT: 167.9 Hz, 6Z ");
//DO SOMETHING
}
// NON-STANDARD
else if ((freq > 170.00) && (freq < 172.40))
{
lcd.print("CT: 171.3 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 172.60) && (freq < 175.00))
{
lcd.print("CT: 173.8 Hz, 6A ");
//DO SOMETHING
}
//NON-STANDARD
else if ((freq > 176.00) && (freq < 178.50))
{
lcd.print("CT: 177.3 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 178.6) && (freq < 181.00))
{
lcd.print("CT: 179.9 Hz, 6Z ");
//DO SOMETHING
}
//NON-STANDARD
else if ((freq > 182.00) && (freq < 184.80))
{
lcd.print("CT: 183.5 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 185.00) && (freq < 187.50))
{
lcd.print("CT: 186.2 Hz, 7Z ");
//DO SOMETHING
}
//NON-STANRDARD
else if ((freq > 188.40) && (freq < 191.30))
{
lcd.print("CT: 189.9 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 191.00) && (freq < 194.00))
{
lcd.print("CT: 192.8 Hz, 7A ");
//DO SOMETHING
}
//NON-STANDARD
else if ((freq > 195.40) && (freq < 198.00))
{
lcd.print("CT: 196.6 Hz, -- ");
//DO SOMETHING
}
//NON-STANDARD
else if ((freq > 198.30) && (freq < 201.00))
{
lcd.print("CT: 199.5 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 202.00) && (freq < 204.00))
{
lcd.print("CT: 203.5 Hz, M1 ");
//DO SOMETHING
}
else if ((freq > 205.00) && (freq < 208.00))
{
lcd.print("CT: 206.5 Hz, 8Z ");
//DO SOMETHING
}
else if ((freq > 209) && (freq < 212.00))
{
lcd.print("CT: 210.7 Hz, M2 ");
//DO SOMETHING
}
else if ((freq > 217.00) && (freq < 219.30))
{
lcd.print("CT: 218.1 Hz, M3 ");
}
else if ((freq > 224.00) && (freq < 227.00))
{
lcd.print("CT: 225.7 Hz, M4 ");
}
else if ((freq > 227.60) && (freq < 231.30))
{
lcd.print("CT: 229.1 Hz, 9Z ");
//DO SOMETHING
}
else if ((freq > 231.70) && (freq < 235.00))
{
lcd.print("CT: 233.6 Hz, -- ");
//DO SOMETHING
}
else if ((freq > 239.60) && (freq < 243.00))
{
lcd.print("CT: 241.8 Hz, M6 ");
//DO SOMETHING
}
else if ((freq > 248.00) && (freq < 252.00))
{
lcd.print("CT: 250.3 Hz, M7 ");
//DO SOMETHING
}
else if ((freq > 252.70) && (freq < 256.80))
{
lcd.print("CT: 254.1 Hz, 0Z ");
//DO SOMETHING
}
else if (freq > 256.80)
{
lcd.print(" TOO HIGH ");
//DO SOMETHING
}
else
{
lcd.setCursor (0, 1);
lcd.print(" NOISE ");
// or, comment the line above and
// uncomment the line below for an empty LCD line
// lcd.print(" ");
}
delay(50);
// lcd.clear(); //Not necessary. uncomment but will flicker!
} //END OF LOOP
void counter()
{
unsigned long now = micros();
if (numImpuls == 1)
{
timpPrimImpuls = now;
}
else
{
timpUltimImpuls = now;
}
++numImpuls;
}


After fine tuning the code, the second issue was to make the Arduino read from real life signals, a.k.a from the discriminator (FM detector).
But there, unfortunately, there is also "noise" above 300 Hz from the voice and the Arduino will tend to detect it too, asuming that we have a 5Vpp signal on pin 2 of the UNO board.
Therefore, a preamplifier and a Low Pass Filter to cut all the frequencies below* above 260 Hz with at least 12db/octave.
I will not bother you with more than necessary details, I post the schematics here and if someone has questions i will answer.

73 de Adrian YO3HJV


A "Later Edit" is a must!
After some emails with Carlo IU1DOR found some mistakes in the schematics. One is about the parallel diodes at U3a which are drawn as conductive in one sense; must be corrected as anti-parallel diodes.
The second one is R10 which is not in series with that diodes.

Here is the original part from Motorola GM300 schematics:

Anyway, Carlo kindly give me the permission to post here the PCB layout for the filter (TNX!):

The code is available on GITHUB. 
*-TNX  IU1DOR for correction!

LATER EDIT  (15 feb. 2016)
I recently received a e-mail from Jean-Jacques ON7EQ   who pointed me to an upgrade to this project.
The upgrade was made possible by the work of Paul ON4ADI who refine the code and added a necessary software filter to make the CTCSS Arduino decoder able to withstand to real world conditions.
The project of the new CTCSS decoder can be seen here.