About Timecode

This page includes a brief description of Timecode. What it is and how it is used.

Many people will know that a real of film is made up of a series of individual still images called frames. You can see this clearly if you examine some film.  The individual pictures are sequentially alligned along the roll of film.

Each picture is called a frame. For standard film, as seen at a cinema, 24 frames are displayed per second. This frame rate is measured in frames per second and is subseqently know as 24 frames per second (fps). A method of measuring and labeling each from with respect to time was required and hence timecode came into being.

In the same way that time is measure in hours minutes and seconds (Often displayed as in the hh:mm:ss format such as 12:23:15) timecode is displayed by simply adding an additional field to indicate the field. For standard film this will run from 00 to 23 in the same way that minutes and seconds run from 00 to 59 hours runs from 00 to 23.

A television picture is similarly built up of a number of frames although, unlike a film frame, which is a clearly defined single picture projected all at the same time, a television picture frame is actually build up of a sequence of lines which scan the complete frame. In fact, to reduce percieved flicker, each frame is built up of two interlaced fields.

In the USA, and countried that have used thier television system (mainly North and South America), a standard know as NTSC (National Television System Commitee) has been used for many years. In Europe and other parts of the world a slightly different system known as PAL (Phase Alternating Line)  is used.

When both system were designed in the 1950s and 1960s it was decided due to limitation of the electronics available at that time, to use a picture field rate that was the same as the electrical mains supply frequency. Hence, 60Hz in the USA and 50Hz for PAL. This limited the visibility of any mains hum interferrence that would be seend on the picture. As each television frame is actually made of two fields, the frame rate is half of this, namely 30 frames per second and 25 frames per second respectivly.

So NTSC timecode uses frames that run from 00 to 29 and PAL timecode uses frames that run from 00 to 24.

If this were the end of the story then things would be relativly simple and staight forward, but it's not!

Towards the end of the NTSC colour standardisation procedure, a testing problem lead to the decision to change the actual NTSC colour frame rate from the 30fps used for the black and white used at that time (which ran at 30fps). In hindsite the problem could have been overcome using different techniques but to avoid some interferrene with the audio television signal it was decided to make the frame rate actually 29.97 frames per second instead of 30. This would keep the system (almost) compatible with the old system).

The problem with this is that if you are counting the frames as if they were 30fps (The count must be an integer value) but they are actually running only at 29.97 frames per second the timecode value will slowly drift away from real time (by 0.03 frames per second, which is 1.8 frames per minute or 18 frames every 10 minutes). This only leads to an error of 3.6 seconds per hour but that is just too much in some circumstances.

To avoid this a technique know as "drop frame" is used. Every minute, except from the 10 minute boundries two frames are skipped. Note that is the labels assigned to the frames are skipped not the frames themselves. This means that 18 frames are skipped every 10 minutes which brings real time and timecode back into synchronisation at that point.

So, a sequence of frames runs like:

09:23:36:27

09:23:36:28

09:23:36:29

09:24:37:02

09:24:37:03

09:24:37:04

The frame number 09:24:37:00 and 09:24:37:01 are dropped or skipped.

As you can imagine, this can cause problems when trying to add two timecode values or doing any mathmatical manipulation on them

Luckily PAL timecode does not suffer from this sort of problems, as the frame rate is defined as exactly 25 frames per second.

For more information you can visit the following links:

NTSC information.

PAL information.

SMPTE Timecode.

Timecode is used whenever it is important to keep track of timing when using film or television. In the same way that time is used to measure and document the progression of events, timecode is used to measure and document video or film.

With the advent of high definition television, new frame rates have been introduced in the last few years.

Frame rates double that of the standard definition NTSC and PAL signals are used. In additional an odd frame rate just slightly slower that the standard film rate of 24fps is used to make it easier to convert from film to HD NTSC and back again.