Module 57

      

Updated: 01/12/2014

 

  Video Editing,  Part VIIIModule 57

 

Making Use

of Time-Code

 

 

SMPTE/EBU time-code

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

Once time-code become a part of a video file, 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 video recording 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 simple: 2 hours, 54 minutes, 48 seconds and 17 frames.

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.

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.

In time code the minute and second 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?

8 seconds, 20 frames, plus
6 seconds, 19 frames
= 15:09

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?

segment out-point - 01:24:45:10    
segment in-point - 01:22:38:25
= total segment time - 00:02:06:15      


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.  An Internet search will bring up Windows and Mac time-code calculators available for downloading.

Drop-Frame Time-Code

Basic time-code assumes a frame rate of 30 per-second or 25 per-second, depending on the country. For the following discussion we'll use 30, which the U.S and a number of other countries use.

Although 30 is a nice even number, it actually only applies to black and white television. For technical reasons when color television was introduced in the NTSC video system a frame rate of 29.97 frames per second had to be used. This frame rate is also used in the U.S. version of DTV/HDTV.

So we have NTSC video running at 29.97 frames per second and time code based on a nice, even 30 frames per second.

That creates a problem.

If you assume a rate of 30 frames per second instead of 29.97, you end up with a 3.6-second time-code error every 60 minutes.

Since broadcasting is a to-the-second business, a way had to be devised to correct this error.

The Drop-Frame Solution

So what's a practical way of fixing this?

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 synchronization between the video and the time-code108 frames had to be dropped from the time-code numbers each hour and done in a way that will minimize confusion.

First, it was decided that the 108-frame correction in the time-code 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. (That part, at least, works out nicely.)

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.

Remember, this only affects the time-code reference and no actual video frames are skipped.

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.

If you happen to punch in one of these non-existent numbers the editing system will just jump ahead to the next valid address.

Maybe this NTSC drop-frame solution is not the most elegant, 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 hardware or software switch that lets you select either a drop-frame or non-drop frame mode.

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.

Drop frame is usually represented with a semi-colon (;) or period (.) between the seconds and frames whereas non-drop retains the colon (:). The period is usually used on devices that don't have the ability to display a semi-colon.

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 time code displaytop 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 display 2time-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 the latter approach requires making a special copy of the original footage, it can be an advantage if you want to make editing notes using standard playback equipment.

Reviewing segments in this way and making what's called a paper-and-pencil edit can save a great deal of time later on.  


red dot The on-line Final Cut Pro Users Manual has more information on these subjects.



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