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LTC LinearTimecode (SMPTE)

A LinearTimecode decode library for Arduino.cc

Use this library to decode linear timecodes. You can generate such timecodes for example on the elteesee website made by pehrhovey. You can find more references in this wikipedia article.

Skip to How to use the library

What is LTC?

"LTC" stands for "Linear Timecode" and is used to encode/decode timecode data in an audio signal. Basically you have an audio signal that runs synchronously with the music or the film and in which the timecode is stored as a bit sequence. You can also store the date in a timecode, but I'll just cover the time for now.

How does LTC work?

The time code consists of 8 bytes and a fixed sync word at the end. So 10 bytes in total. The fixed sync word at the end is used to tell the decoder that the sequence has ended.

A long bit period corresponds to a 0 bit
A double short bit period corresponds to a 1 bit

Example for one frame

This example shows the timecode 00:00:00.00

signwave

Raw image

Which byte does what?

Byte Bit Visuel Meaning
A 0 - 3 --:--:--.-X Frame number units (0–9)
B 0 - 3 --:--:--.X- Frame number tens (0-2)
C 0 - 3 --:--:-X.-- Seconds number units (0–9)
D 0 - 3 --:--:X-.-- Seconds number tens (0–5)
E 0 - 3 --:-X:--.-- Minutes number units (0-9)
F 0 - 3 --:X-:--.-- Minutes number tens (0-5)
G 0 - 3 -X:--:--.-- Hours number units (0-9)
H 0 - 3 X-:--:--.-- Hours number tens (0-2)
I 0 - 7 Sync word
J 0 - 7 Sync word

Binary decimal numbers wiki

How does the library decode the timecode?

Standards

First, there are different timecode standards, which differ in the number of frames or the length of a frame as you can see on the elteesee website.

  • 23.976 F
  • 24 F
  • 25 F
  • 29.97ndf (no drop frame)
  • 30 F

Calculate one bit lenght

To calculate how long one bit lasts, we need to do a bit of math. Got it? A bit of math? Nahh lets go on 😅:

// convert framerate to ms
float framerate = 25.00;
float ms = 1000 / framerate;

// now calculate how long one bit is
// a frame consists of 80 bits
// this gives us back how long one bit is in ms
float oneBit = ms / 80;

// convert ms to µs and cast to int and we have the bit length in µs
const int bitLength = int(oneBit * 1000);

Now the library just waits for an interrupt. An interrupt occurs when the sine wave crosses 0. Then the library checks the time difference between the two interrupts. A short time between two interrupts means that a 1 bit has arrived, and a long time between two interrupts means that a 0 bit has arrived.

How to use the library?

Syntax

constructor
LinearTimecode::LinearTimecode(LinearTimecode::frameTypes frameType);

enum frameType : { FRAME_23_976, FRAME_24, FRAME_25, NO_DROP_FRAME_29_97, FRAME_30 }


LinearTimecode::LinearTimecode(const unsigned int frameDuration);

const unsigned int frameDuration : duration


methodes
const int LinearTimecode::getBitLength();

Returns the bit length of one bit.


uint8_t LinearTimecode::getByteBuffer(const unsigned int index);

const unsigned int index : index

Returns the byte buffer at the specified index.


float LinearTimecode::getFramerate();

Returns the set frame rate as a floating point number


const int LinearTimecode::getFrames();

Returns the current frame


const int LinearTimecode::getSeconds();

Returns the current second


const int LinearTimecode::getMinutes();

Returns the current minute


const int LinearTimecode::getHours();

Returns the current hour


String LinearTimecode::getTimecode(LinearTimecode::formats format);

enum format : { FORMAT_DOT, FORMAT_COLON, FORMAT_DOT_COLON, FORMAT_SPACE }

Returns the timecode in a string format

Format Visual
FORMAT_DOT HH.MM.SS.FF
FORMAT_COLON HH:MM:SS:FF
FORMAT_DOT_COLON HH:MM:SS.FF
FORMAT_SPACE HH MM SS FF 

void LinearTimecode::onEdgeChange();

Needs to be executed on an interrupt change on the audio pin


void LinearTimecode::onSync(void (*callback)());

void (*callback)() : function to execute

Gets called when the sync pattern is detected


void LinearTimecode::setShortEdgeDuration(const unsigned int min, const unsigned int max);

const unsigned int min : the minimum short edge duration
const unsigned int max : the maximum short edge duration

Set the minimum and maximum duration of the short edge


void LinearTimecode::setLongEdgeDuration(const unsigned int min, const unsigned int max);

const unsigned int min : the minimum long edge duration
const unsigned int max : the maximum long edge duration

Set the minimum and maximum duration of the long edge


void LinearTimecode::setSyncPattern(word pattern);

word pattern : sync pattern

Set the sync pattern


Example

code
// include
// include the libary
#include <LinearTimecode.h>

// define
// the audio in pin needs to be an change intrrupt pin!
#define AUDIO_IN 2

// Create a new LinearTimecode class instance (I'll call it "ltc") with a frame rate of 25 frames
LinearTimecode ltc(ltc.FRAME_30);

// the setup function
void setup() {
  // begin serial
  Serial.begin(115200);

  // attach the interrupt pin to pin 2
  attachInterrupt(2, [](){
    
    // call the onEdgeChange() method of your class instance
    // this needs to be in a lambda function!
    ltc.onEdgeChange();
  }, CHANGE);
  
  // call the onSync() method of your class instance
  ltc.onSync([](){
    
    // print the result in the console
    Serial.println(ltc.getTimecode(ltc.FORMAT_DOT_COLON));
  });
}

void loop() {

}
output
10:03:12.13

Example timecode