Apple Pro Training Series: Optimizing Your Final Cut Pro System: A Technical Guide to RealWorld PostProduction [Electronic resources] نسخه متنی

Video Fundamentals

IRE setup, and dot-crawl? Yet they all resulted from careful design decisions made back in analog days, and most (if not all) make perfect sense once you know their background.

When you know how an image is structured, and how it's broken down for storage and transmission, you'll be able to choose the right kinds of connections for moving pictures around, and be able to diagnose arcane but common problems associated with interlacing and field order.

Frames, Scanning, and Sync

Video is a sequence of pictures; but the pictures themselves can be structured in a variety of ways. Fortunately, the need for transmitter and receiver to work together resulted in some fundamental principles that all modern video systems follow.

When you read this book, you start at the upper-left corner of a page, scan across a line of text, then return to the left edge to read the next line of text (assuming, of course, that this is not the Japanese or Hebrew version of this book). Reading a line at a time, you traverse the entire two-dimensional page, breaking each page down into lines of text.

Video works the same way, reading a single

scanline across the image, then moving down to read another, and another, until an entire

frame of video has been scanned. The basic video signal is an analog waveform (we'll get to digital later on), where zero voltage corresponds to black, and voltage increases in proportion to scene brightness.

In NTSC television, the brightness scale is calibrated in

IRE units , with 100 divisions between zero voltage and the nominal

white level (the level at which the display shows a full-brightness white image). In PAL, brightness is measured in millivolts, mV.

Waveform monitor is calibrated in percent, from 0 percent to 100 percent nominal peak white. This is

not the same as IRE units, as you'll see.

The structure of scanlines all stacked one atop another to form a complete frame is called a

raster , hence the term

raster graphics for images broken down into a regular pattern of scanlines, or

scanning raster for the image structure displayed on the face of a CRT (cathode-ray tube).

Scanlines are separated by

horizontal sync pulses , short negative voltage spikes inserted by the camera to tell the video display where one scanline ends and another is about to begin. The camera also generates a

vertical sync pulse at the bottom of the image so that the display knows when to

retrace to the top of the screen and start a new scan.

Note

Vertical sync pulses have the same amplitude (height) as horizontal sync pulses, but are many times wider and have additional complications called

serrations and

equalizing pulses , which are interesting in their own right but beyond the scope of this lesson.

This waveform display shows scanlines complete with horizontal sync and colorburst.

FCP's waveform display shows only picture, not sync or blanking.

[View full size image]

Setup, Black, and Blanking

black level of the screen by fiddling with a "black level" or "picture" control; that control set the point at which the voltage of the signal just barely started to make the CRT's phosphors glow. If you misadjusted the picture control even slightly towards the "bright" side, you risked seeing a handful of diagonal lines running across your picture: the retrace of the electron beam as it completes one field and returns to the top of the screen to start the next one.

The NTSC standard fixed the problem by adding a

setup or

pedestal to the picture: a small voltage offset equal to 7.5 percent of the total brightness level. Setup raises the nominal

black level a bit above the zero-voltage

blanking levelthe level at which the electron beam should be completely off, or blanked. Setup gave you room for error; by the time you turned the brightness up enough to see the retrace lines, the blacks in your picture were milky gray.

Setup was a sensible solution in the early days of television, but modern TV sets blank the retrace completely, regardless of black level. Outside of North and Central America, setup is unknown, even in NTSC-using countries like Japan and South Korea. For most of the world, the black level is the same as the blanking level. But setup remains part of North American NTSC, and black is 7.5 IRE units higher than blanking, a detail that will return to haunt us in later lessons.

Fields and Frames

The more scanlines a frame has, the more vertical detail it shows. Of course, the more scanlines you have, the more time it takes to trace out the picture. Unfortunately, the phosphors in CRTs have a very short persistence: they don't remain bright for very long after the scanning electron beam passes by, and early television pioneers found that by the time they had enough scanlines to form a decent picture, they couldn't refresh the images quickly enough to prevent the displays from flickering abominably. Even though the frame rate was high enough to adequately represent motion, flicker made the pictures unwatchable.

The solution to the flicker problem was

interlace : instead of progressively scanning the entire frame a line at a time, the way you'd read a page, the frame was divided into two

fields , one comprised of all the even scanlines, the other of all the odd scanlines. Each field contains half the picture, so fields can be scanned (and displayed) twice as quickly as frames. Even though the total amount of information is the same as in progressive frame scanning, interlaced field scanning allows the CRT to be refreshed twice as quickly, so the flicker problem was overcome. The eye handily integrates the two interlaced fields into a full frame, and the net result is that you get enough scanlines to form a detailed picture while repainting the screen fast enough to minimize flicker.

Pixels setting off. (Both settings appear in the

Zoom pop-up menu in the Viewer and Canvas, or in the View > Level menu.)

Note

FCP's Viewer and Canvas show you a true pixel-for-pixel representation only when Show As Sq. Pixels is turned off and the scale is set to 100 percent. You'll find that larger scales will seem to work with raw clips, but once you apply filters, the only way to see what you really have is to use these settings.

Of course, if you can output the sequence to an NTSC monitor, you'll also see the difference in still frames there.

Because the two fields are really separate images, they have to be processed as such. That's why FCP has the Field Dominance setting in Sequence Settings: the setting tells FCP which field comes first in time. (If you get the setting wrong, rendered images show exaggerated combing artifacts when they're moving.)

Many filters and transitions work on a field basis for interleaved material, not a frame basis, to avoid combing artifacts. FCP also has to resize interlaced images using the two half-resolution fields separately instead of the entire full-resolution frame. This is one reason folks like progressive scanning when upconverting or downconverting or printing out to film: progressive images allow much crisper and smoother resizing with fewer artifacts.

Note

Interlaced images also have slightly less vertical resolution than progressive images, even on static pictures. Interlaced cameras apply slight vertical blurring during image capture to avoid

twitter (frame-rate flicker) of fine details that only appear in one field or the other, and that, combined with various perceptual factors, means that the actual vertical resolution of interlaced images is about 0.7 x the expected resolution based on the line count.

Aspect Ratio

A picture's aspect ratio is the ratio of its width to its height. In TV terms, aspect ratio is most often expressed in integers:

4x3 , or

16x9 . Most standard-definition pictures worldwide are 4x34 units wide by 3 units tallbut the 16x9 widescreen format is increasingly popular, especially in Europe, and has Widescreen pictures are often displayed on "narrowscreen" televisions by

letterboxing , shrinking the image to fit and filling the space above and below with black bands. 4x3 images can be inserted into a 16x9 program by

pillarboxing , adding black side panels on the edges of the 4x3 picture.

Note

The odd terminology refers to British mailboxes, which are tall vertical pillars with thin horizontal slots for the mail.

Bandwidth, Resolution, and Line Counts

Bandwidth determines the resolution of an analog television signal, and

line count is what measures the resolution. You'll see both terms used in equipment specifications, and in the never-ending arguments about which camera, VTR, or tape format is supposedly better than another.

Vertical Resolution

An NTSC television picture has about 483

active lines , scanlines carrying picture information. (There are 525 lines in an entire frame; the rest carry the vertical blanking and vertical sync. PAL has 576 active lines out of 625 total.) You might think, then, that the vertical resolution of an NTSC picture would be 483 lines, but interlace causes the effective resolution to be only 70 percent of the line count (the

Kell factor ), to about 338 lines.

Note

In television, resolution is measured by individual TV linesa black line beside a white line counts as two lines. In film, print, and most other media, resolution is measured in line pairs or cyclesa black line and a white line counts as only one line pair.

Horizontal Resolution

Horizontal resolution is measured over a width equal to the height of the picture, so you can compare horizontal and vertical resolution values on an equal basis. The formal measurement is thus TVl/phTV lines per picture heightalthough you'll often see it listed as "TV lines."

luma resolutions.

NTSC chroma's bandwidth of 0.5 to 1.5 MHz (depending on the exact color) yields an effective chroma resolution of only 40 to 120 TVl/ph. This low resolution doesn't bother us much because our eyes are so much less sensitive to fine detail in chroma than in luma.