Cracking the Video Time Code

Cracking the Video Time Code

Unraveling the perplexity and complexity of using video time code.

Wouldn't it be great to have a convenient way to locate the shots you want to use on your videotapes? No matter how fast your Fast Forward button is, hunting through hours of video footage is never a productive or fun pastime. Video time code is the solution to these problems. Understanding how time code works makes video production and editing a much more manageable process for videographers.

Kinds of Time

Many devices use time code of one sort or another to synchronize with other devices and with the real world of atomic clock time. For our purposes, time code is the standardized system that video cameras use to assign a digital time signature to each frame of video that's shot. Essentially, there are two kinds of time code: drop frame and non-drop frame. One of the important things to know about these time codes is that they are simply a way of counting video frames. It is a misconception that drop-frame time code actually drops any frames of video.

Instead, it merely drops frame numbers. Think of drop-frame time code as pushing back the second-hand of a clock to make synchronization adjustments. Pushing back the hand has no effect in the real world. When the system drops frame numbers, no changes are ever made to the video frames themselves. The only difference between drop and non-drop frame time code is in how they count frames.

The Society of Motion Picture and Television Engineers (SMPTE) created drop and non-drop frame time code. While it might seem logical to count in whole-number frames per second, technical issues with the NTSC video signal (the North American video signal standard) mean that 30 frames per second won't work. The NTSC signal, developed in 1953, uses 29.97 frames-per-second video signals in order to be backward compatible with black and white televisions (among other reasons). The signal starts out like a black and white signal, but then uses three frames to deliver the color information.

The result of the NTSC video signal is that time code frame-counts gradually drift away from time as it is measured in the real world. For instance, when we use non-drop frame time code (and we usually do), you'll notice that the time code duration reads slightly longer than real time: 3.5 seconds or 108 frames longer for every hour of video. By contrast, drop-frame time code remains accurate with real time by dropping (that is, not counting) all of the video frames, specifically skipping 108 numbers every hour.

LTC and VITC

Another kind of time code is longitudinal time code (LTC). This time code system converts information into a bit-stream that the camera records on audio track two on compatible machines. LTC is either recorded when shooting or dubbed onto your original tape prior to editing. Some of LTC's disadvantages include the elimination of one audio track and the fact that the time code is not visible when the tape is paused or slowly jogged during playback.

Vertical Interval time code (VITC) eliminates these problems with LTC. VITC (pronounced vit-see) is recorded as part of the off-screen video signal, but, like LTC, VITC is bit-stream information. The VITC bit-stream exists in the video signal's raster-line, however, instead of one of the audio tracks. VITC reads accurately when the tape is paused, but must be recorded during shooting or onto a copy of the original tape.

To Drop or Not

Since drop-frame and non-drop frame time codes are just two methods of counting video frames, using one or the other is sometimes a matter of the editor's preference. In some instances, however, you'll make this decision based on your client's needs or your editing system limitations. If you work with Mini DV time code, you'll use drop-frame time code, but other systems, such as DVCAM and DVCPRO, offer the choice of using drop-frame or non-drop frame time code. Remember, it is just a system of counting frames, so the image is unaffected by all of this accounting.

For personal and non-commercial purposes, using either time code will be sufficient. The decision really begins to matter primarily when you are working on commercial and professional broadcast projects. Professional broadcast video must use drop-frame time code, because of the need for timing precision.

Time to Organize

SMPTE, LTC and VITC: who cares, right? How does all this time code make life easier? Using time code is a great way to organize productions during shooting and editing.

Your camcorder writes time code (i.e. a time-stamp) in an hours:minutes:seconds:frames format (HH:MM:SS:FF). Video time code signals use a 24-hour counting standard. Therefore, every video frame on a tape is time-stamped with a value ranging between 00:00:00:00 and 23:59:59:29.

Some video systems used to confuse a zero starting point with the absence of any time code at all. Another problem with 00:00:00:00 is that some projects and gear require a pre-roll. Rewinding before zero gives you negative time code, which is not handled well by some systems.

The time-stamp is an ideal logging device when shooting video. For example, a videographer shooting a sporting event notes the time of an important play. By logging the time of the play, the tape's time code can be used to determine the exact location of the shot, rather than having to search through hours of video to find it.

Some gear allows you to choose between shooting in real time or control time. Real time means that you'll see the time as it appears on a standard clock. Control time displays a view of the actual position of the footage on the tape.

Time code also offers the ability to synchronize multiple machines, including editing decks. The unique time-stamp allows editing machines such as switchers and dubbers to run in perfect synchronization. However, some of the control protocols for consumer editing decks do not use time codes that time-stamp every frame. These decks limit the overall accuracy of editing using the time-stamp.

One of the ways that time code is regulated between synchronized equipment is through a Serial Digital Interface (SDI) connection. SDI is the standard digital video transport method used by professional and broadcast equipment and is another technical standard maintained by SMPTE.

No Time for Breaks

One of the most critical problems involving time code is discontinuity, sometimes referred to as broken time code. Broken time code happens when the camera cannot find existing time code on the tape and resets the counter to zero. This can happen when you start a new tape or when you start recording after some pre-existing footage to a blank section of the tape.

One way to prevent breaks in time code is to pre-black your tapes. To do this, insert your tape and record with the lens cap on (and, optionally, the audio muted). Record throughout the entire length of the tape to ensure that every video frame has time code on it.

Blacking tape is a preventative measure that's good when re-using old tapes and is critical when working with older analog cameras that support time code. Nowadays, however, many digital cameras are equipped with features that help eliminate broken time code, for example an end-search feature, which allows you to find the last recorded frame on a tape.

With time code, logging, synchronizing and managing footage becomes a simpler task. By understanding how it relates to shooting and editing, videographers gain an advantage during their project's production.

Pat Bailey is a digital video Technical Support Analyst and freelance writer.

Issue: 

Pat
Bailey
Fri, 07/01/2005 - 12:00am