Pyro Industries API-1394PCI Manual do Utilizador descarregar pdf (Página 89)

CHAPTER 2: CAPTURING ■

Video does not really record at a rate of 30 frames per second. It’s slightly off

that number; it’s actually 29.97 frames per second. This difference might seem insignifi-

cant, but in these days of automated broadcast equipment, it is significant. For exam-

ple, if we were to measure the duration of a television program that is supposed to last

28 minutes and 38 seconds using non–drop frame (30fps) timecode, the duration

would read 28:36:08 because although the clock duration needs to be 28:38:00, the

30fps count would read 28:36:08 when it reached that point in real time. To solve this

inconsistency, engineers developed drop frame timecode, a system of counting frames in

which recorded time is the equivalent of clock time, so that video timecode remains the

same as actual clock time, and the two numbers match.

This is achieved by skipping two timecode numbers every minute except on the

tens. So at minutes 1 through 9, you will see the timecode jump like this: 00;01;59;29

to 00;02;00;02. The same holds true for minutes 11 through 19. On minutes 0, 10, 20,

and so on there are no skipped numbers. Here, the timecode will count 00;09;59;29 to

00;10;00;00. Don’t worry about your picture and sound, though; even though this

frame code mode is called drop frame, there are no missing frames! Only the counting

method is skipping numbers, and your media is all present and accounted for.

Which frame code mode should you use? Common sense might tell you that

drop frame timecode should always be the choice, because it always keeps up with real

time. But there are occasions where non–drop frame timecode would be better. Here

are some examples:

• Anyone using animated frames knows that precise frame counts are necessary. For

every frame of animation used, there should be a corresponding timecode num-

ber. Drop frame timecode “skips” some numbers in counting frames so that the

code keeps up with clock time rather than corresponding to actual frame counts.

Thus, non-drop frame code is necessary to avoid an incorrect frame count.

• Thirty-second commercials rarely use drop frame code because the frame count

does not change until a minute duration has passed. Thus advertisers usually

prefer using non-drop frame code so that the beginning and end of each 30-sec-

ond spot land on an even-numbered minute. For example, if we used drop frame

timecode, the first commercial on a reel could begin at one hour even (1:00:00:00),

but the second commercial would begin at the top of the next minute at 1;00;00;02

because drop frame code skips two frames in the count at the beginning of each

minute (except every tenth minute in the count).

It’s not as important to remember the actual counting scheme of drop frame

video as it is to understand what it does:

• Frame code modes are only methods of counting frames. In other words, all the

pictures will appear regardless of whether the frame count uses drop frame or

non-drop frame coding.

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Pyro Industries API-1394PCI Manual do Utilizador Página 89