Pyro Industries API-1394PCI Manual de usuario descargar pdf (Pagina 219)

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CHAPTER 6: IMPORTING, EXPORTING, AND DIGITAL CUT ■

the color gamut, image brightness and darkness, and so on is extraordinary. Fortunately,

we won’t be doing any hands-on gamut mapping—the system does it for you, but you

need to understand what is happening and why it affects you.

Here’s a neat little experiment. Try creating an image in Photoshop using a vivid

yellow or Kelly green. Next, try to output the same color to an NTSC monitor. The

Kelly green turns into a less saturated green, and the vivid yellow turns more into a

mellow tone. Why? Because NTSC doesn’t quite have an accurate Kelly green or vivid

yellow in its color gamut.

That’s a lot of information, but you need to know that what you see in your

monitors on the computer can vary when it is sent to an NTSC video signal.

Field Ordering

The NTSC and PAL video frame is composed of two fields, and each field scans every

other line in the picture. These two fields “interlace” with each other. For example, if

we number each line of video, one field displays the odd-numbered lines: line, 1, line 3,

line 5, etc. The other field scans the even-numbered lines: line 2, line 4, line 6, etc.

When the two fields are combined or interlaced, they create what we perceive to be a

full-resolution frame. Because of a human trait known as “persistence of vision,” we

think that we see a full frame, even though the image is displayed on a television one

dot at a time. If you’ve ever shot a picture of a television display at high shutter speed,

you’ll only see a small dot as the image.

But I’m getting ahead of myself here. The question here is this: When a source is

recording to videotape or to a file, do the even-numbered lines of the video frame play

back first, or do the odd-numbered lines? The order of fields is determined by the play-

back of the source, which is important because these fields are recorded so that they

are “temporally displaced” from each other. In other words, the second field that is

recorded, whether it is odd or even, happens later in time than the first field, so if you

switch the order, the resulting images can look like a horror show (see Figure 6.6).

Figure 6.6 Poor field ordering. Note the positions of the hockey players in each of the fields.

Allow me to elucidate by example. We shoot a video of Drew Bledsoe throwing

a football. As each field is recorded, Mr. Bledsoe’s arm moves forward. We record this

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Pyro Industries API-1394PCI Manual de usuario Pagina 219