Digispark MPX Stereo Modulator

MPX_Stereo_Modulator.ino

/*  Digispark MPX Stereo Modulator
    Based on a design found at
    "http://cappels.org/dproj/FM_MPX_STEREO/
      SIMPLE FM STEREO MULTIPLEX ENOCDER CIRCUIT.html"
    Be sure to use adequate low paths pants filtering and stay FCC
      part 15 compliant.
    ATTinyCore board library v1.5.2
    7/9/2022 ©The Lightning Stalker
*/

#define MUXL_PIN        1  // The left speaker
#define MUXR_PIN        2  // The right speaker
#define PILOT_TONE_PIN  0  // 19KHz pilot tone

// initialize state machine at binary 011 (decimal 3)
volatile int DDRSHADOW = 3, PORTSHADOW = 0, othertimes = 0;

ISR(TIMER0_COMPA_vect)  // Interrupt driven 2 * 38KHz
{
  cli();
  DDRSHADOW ^= 6;       // Use of shadow registers for indirect ->
  if (othertimes == 0)  // half time for 19KHz
  {
    PORTSHADOW ^= 1;    // XOR bit toggling
    PORTB = PORTSHADOW;
    DDRB = DDRSHADOW;   // -> port manipulation
  }
  else
  {
    PORTSHADOW ^= 0;    // Let's try making these two things the same.
    PORTB = PORTSHADOW;
    DDRB = DDRSHADOW;   // to even out the timing a little
  }
  othertimes ^= 1;      // since the pilot tone is half or 19KHz
  sei();                // for some reason booleans don't work here.
}

void setup() {
  // put your setup code here, to run once:
  pinMode(MUXL_PIN, OUTPUT);  // Why reinvent the wheel when you can
  pinMode(MUXR_PIN, OUTPUT);  // grab the wheel, stick it on your
  pinMode(PILOT_TONE_PIN, OUTPUT);  // cart and make it your bitch

  // set up counter interrupt
  TCCR0A = 0x00;           // Normal port operation
  TCCR0B = 0x00;
  TCCR0A |= (1 << WGM01);  // set compare match CTC mode
  TCCR0B |= (1 << CS00);   // no prescaling
  OCR0A = 217;             // 38KHz * 2 at the 16.5MHz Digispeak default
  TIMSK |= (1 << OCIE0A);  // enabling timer0 compare match interrupt
}

/* The Digispeak uses the internal oscillator of the Attiny85 which
    is very unstable. It's best to use an external crystal and
    change the above OCR1C compare match value accordingly.  Most
    easily done by just ditching the Digispock entirely and using a
    bare Attiny85 to get all of the USB components out of the way
    You will lose the USB functionality anyway.
*/

void loop() {
  // put your main code here, to run repeatedly:
  // You can put something else here if you need to.
}

It's advisable to use a crystal because there is some instability in the timing that is most likely an aspect of the internal RC oscillator.