When producing a motion picture, a video, or even in audio only applications, the need for identifying a specific point in time has always been important. Though there are many ways of working without using time code, life is much easier when you do use it.

Time Code consists in representing addresses for events in a precise way, using...

in different forms

Audio :	LTC, for Longitudinal TC	XLR Connector, Jack, even RCA !
Video :	VITC, for Vertical Interval TC	BNC connector (typical of the video world)
MIDI :	MTC, for MIDI TC	DIN connector (the one used for MIDI !)
Optical :	there's even a way to print the 8 digits of a TC as a bar-code or a dot matrix along side the film negative !	Specific connectors chosen by camera manufacturers

The Time Code is perfectly normalised since 1972, by the SMPTE (Society of Motion Picture & Television engineers) and the EBU (European Broadcasting Union)

"Why subdivide the seconds in frames, and not milliseconds ?"

Because Time Code has been developped primarly for the needs of television and cinema. After that, recording studios working solely with audio have also found this method of addressing events quite convenient (syncing tape recorders, automations, etc...), even if there isn't a single video or film frame around the place. Since then, when working with audio, we subdivide time in "frames" or "samples" (now that digital audio is a common thing in a studio) more often than in "milliseconds".

"Where does the Time Code information fit on a medium ?"

We'll have to split the different types of media in 5 categories :

Audio Tape Recorders (analog or digital)
Video Tape Recorders (analog or digital)
Sequencers (and drum machines)
Direct to Disks
Film

On an audio tape recorder, designed to record audio, an audio representation of the TC digits must therefore be used, called LTC. On analog ATRs, the last track is generally prefered to record the signal (track 24, on a 24 track recorder). On digital ATRs, there is a dedicated track for that signal (except on consumer products).

On a video tape recorder, designed to record picture but also audio, they first used LTC recorded on one of the audio tracks, despite the audio engineer weeping his heart out ! But since the beginning of the 80's, two mini-revolutions occured : an additional track was created to record the LTC, and VTRs now also record the TC in the invisible lines of the video signal (vertical interval blanking), which enables the TC to be read in pause. Hence, this form of TC is called VITC and coexists with the LTC track. Most of the time, it's the VITC version they use on these machines.

On a sequencer, designed to record MIDI bytes, they had to find a way to express the HH:MM:SS:FF digits of a TC in bytes recognizeable by the MIDI language. It's easily done and called MTC. Since a sequencer is a virtual machine (what I mean is that there is no physical medium such as a tape moving, it's just RAM emptying at a specific rate), the MTC can be generated in real time, to suit the needs of other machines, or to sync-up to other machines.

On a Direct to Disk system, designed to record audio, video, and even MIDI, well.... Everything's possible. It just depends on the input/output card of your DtD system. The most professional ones will sport BNC connectors, as well as XLR and DIN. A more limited system will only offer XLR (or 1/4" Jack) and DIN. The consumer cards will generally only offer DIN connectors (MIDI). Since a Direct to Disk is also a virtual machine, the TC (any form, even all three at the same time) can be generated in real time, to suit the needs of other machines, or to sync-up to other machines.

On a movie camera, designed to record pictures optically, they developped systems that enable the digits of the TC to be printed as a bar-code, like on Campbell's Soup cans (another system uses a tiny dot matrix), alongside the pictures, on the negative.

on the right, the Arriflex bar code.

on the left, the Aatoncode dot matrix, with man readable equivalent numbers (HH:MM:SS).

"The Hour, the Minute and the Second are universal notions of time, but what about the frame ?"

Indeed, there are three main Time Code formats, corresponding to the different needs in frame rates for television and film around the world :

Film :	24 frames/second	For motion picture
EBU :	25 f/s	For TV in countries where the mains' frequency is 50 Hz.
SMPTE :	Ouch !	For TV in countries where the mains' frequency is 60 Hz.

"Why Ouch ?"

There are several variations around the SMPTE frame rate, to accomodate the problems of the various frame rates associated with television systems running on a 60 Hz mains.

In black & white 60 Hz television, there are 30 f/s.

But in color, each frame lasts a little bit longer than a 1/30^th of a second. This frame rate is called 29,97 f/s.

(in reality, the exact frame rate is even goofier : 29,97002617 f/s !!!)

We can therefore already distinguish 30 f/s SMPTE and 29,97 SMPTE, where each frame doesn't have exactly the same duration in time. But things can get tougher...

"We were expecting that one : what's a DF or NDF time code ?"

"Why is it that, even though I live in Europe, the word SMPTE is used again and again (on TC output & input connectors, for example, or in stores when you ask a salesman some advice) rather than the correct terms, like LTC, VITC... We don't even have SMPTE around here, we run at EBU frame rates !?!?"

ecause most people still confuse the SMPTE, EBU, FILM format (numer of frames/second) and LTC, VITC, MTC : its' electrical representation (audio/video/MIDI/optical) on the medium...

You can very well have a SMPTE time code, represented in LTC, or an EBU time code represented in VITC, an EBU in LTC, a FILM time code (24 f/s) in LTC, a FILM time code (24 f/s) in MTC or VITC... etc... etc...