Module 57

Updated: 01/04/2008

  Video Editing,  Part VIII

Making Use

of Time-Code

Although we've mentioned time-code previously, we now need to more fully explore how it works and its role in the editing process.

SMPTE/EBU time-code (or just " time-code") is an eight-digit code that allows you to specify precise video and audio editing points.

A designated time-code point (set of numbers) cannot vary from one editing session to another or from one machine to another.

Editing instructions like, "cut the scene when Whitney smiles at the camera," leave room for interpretation -- especially if Whitney tends to smile a lot.

But even though a tape may be four hours long, "00:01:16:12" refers to one very precise point within that total time.

Breaking the Code

Although a string of eight numbers like 02:54:48:17 might seem imposing, their meaning is very simple: 2 hours, 54 minutes, 48 seconds and 17 frames.

Since time-code numbers move from right to left when they are entered into an edit controller, you must enter hours, minutes, seconds and frames, in that order.

If there is anything tricky about time-code, it's the fact that you don't add and subtract from a base of ten the way you do with most math problems.

The first two numbers are based on 24 hours. This is so-called military time and the time designation used in some countries.

Instead of the time starting again at 1:00 p.m. in the afternoon, the time at that point becomes 13-hundred (13:00) hours and goes all the way to 23-hundred hours, 59-minues-59 seconds, at which point things start over again. (By the way, if it's afternoon or night as you read this, what time is it in military time?)

In time code the minutes and seconds numbers  range from 00 to 59, just the way they do on any clock, and the frames go from 00 to 29. (Recall there are 30 frames per second in NTSC video.  The PAL and SECAM systems use 25 as a base.)

Thirty frames, like 5/5 of a mile, would be impossible in time-code display, because 30 frames in NTSC equal one second. (The next frame after 29 would add a complete second and the frame counter would start counting over again on the next second.) Likewise, "60 minutes" would be impossible in time-code (but not necessarily impossible on CBS).

Question: What comes after 04 hours, 59 minutes, 59 seconds and 29 frames (04:59:59:29)? If you said 05:00:00:00 you would be right.

Now let's look at some more complex time-code problems.

If one video segment is 8 seconds, 20 frames long, and a second segment is 6 seconds, 19 frames long, what is the total time?

Note in this example that as we add the total number of frames we end up with 39. But, since there can be only 30 frames in a second, we add one second to the seconds' column and we end up with 9 frames. (39 minus 30 = 09 frames). Adding 9 seconds (8 plus the 1 we carried over) and 6 gives us 15 seconds, for a total of 15:09.

Let's look at this question. If the time-code point for entering a video segment is 01:22:38:25, and the out-point is 01:24:45:10, what is the total time of the segment?

Getting the answer is a matter of subtracting the smaller time code (second line above) from the larger time code (top line).

Note that since we can't subtract 25 frames from 10 frames we have to change the 10 to 40 by borrowing a second from the 45.

For people who regularly do time-code calculations, computer programs and small hand held calculators are available.  A computer pop-up time-code calculator called WTCC is available for free downloading from the Internet.

Drop-Frame Time-Code

Basic SMPTE/EBU time-code assumes a frame rate of 30 per-second in NTSC video, or 25 per-second in the other countries we've mentioned.

Although 30 in NTSC video is a nice even number, it actually only applies to black and white television.

For technical reasons, when the NTSC color television was introduced, a frame rate of 29.97 frames per second was adopted. This frame rate is also used in the U.S. version of DTV/HDTV.

Although the difference between 30 and 29.97 may seem insignificant, in some applications it can result in significant timing problems. If you assume a rate of 30 frames per second instead of 29.97, you end up with a 3.6-second error every 60 minutes.

Since broadcasting is a to-the-second business, a way had to be devised to correct this error. Just lopping off 3.6 seconds at the end of every hour was not practical way of doing this  -- especially from the viewpoint of a sponsor that gets the end of a commercial cut off as a result.

The Solution

So how do you fix this error?

A little math tells you that 3.6 seconds equal an extra 108 video frames each hour (3.6 times 30 frames per second). So, to maintain accuracy, 108 frames must be dropped each hour and done in a way that will minimize confusion. Unfortunately, we're not dealing with nice even numbers here.

First, it was decided that the 108-frame correction had to be equally distributed throughout the hour. (Better to lose a bit here and there instead of everything all at once.)

If you dropped 2 frames per-minute, you would end up dropping 120 frames per-hour instead of 108. That's nice and neat, but it's 12 frames too many. But, since you can't drop half frames, this is as close as you can get by making a consistent correction every minute.

So what to do with the 12 extra frames? The solution is every 10th minute not to drop the 2 frames.

In one hour that equals 12 frames, since there are six ten-minute intervals in an hour.

So, using this approach you end up dropping 108 frames every hour -- exactly what you need to get rid of.

Since the frame dropping occurs right at the changeover point from one minute to the next, you'll see the time-code counter on an editor suddenly jump over the dropped frames every time the correction is made.

For example, when you reach 01:07:59:29, the next frame would be 01:08:00:02. In drop-frame time-code frames 00 and 01 don't exist.

Maybe this is not the most elegant solution in the world, but it works, and now it should be obvious why it's called drop-frame time-code.

For non-critical applications, such as news segments, industrial television productions, etc., drop-frame isn't needed. However, if you are involved with producing 15-minute or longer programs for broadcast, you should use an editor with drop-frame capability.

On most edit controllers you will find a switch that lets you select either a drop-frame or non-drop frame mode. Software programs typically have a drop-down box where you can select the approach you want.

When you use the drop-frame mode, a signal is added to the SMPTE/EBU time-coded video that automatically lets the equipment know that drop-frame is being used.

There is more to editing with time code, and you will find additional information here.

Time-Code Display

Some editing systems have small time-code displays on the top of the edit controller, as shown here.

More sophisticated editing systems superimpose the time-code numbers over the video, itself, as we see below. 

In the latter case the time-code numbers may be either temporarily superimposed over the video (keyed-in code), or they may become a permanent part of the picture (burned-in time-code).

In the case of keyed-in code, an electronic device reads the digital time-code information from the tape and generates the numbers to be temporarily keyed (superimposed) over the video.

The disadvantage of this approach is that you can only see the code if you are using special equipment, such as an appropriate editing system.

Once the time-code-numbers have been burned in (permanently superimposed into the video), the video and time-code can be viewed on any video playback system.

Although this approach requires making a special copy of the original footage, it can be an advantage if you want to use standard playback equipment, such as a VCR, to review tapes at home or on location and make time code notes on the segments you want. Reviewing a tape in this way and making what's called an initial paper-and-pencil edit can save a great deal of time later on.