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What Are NTSC/PAL/SECAM Broadcast Formats?

One of the biggest challenges for DV and digital TV (DTV) is that for the foreseeable future they must remain compatible with analog television (ATV) standards that were established half a century ago.

As a videomaker, you need to understand this arcane system well enough to make the right choices about equipment that may be compatible with some standards but incompatible with others. (For the most part, filmmakers don't have to worry about this stuff, aside from framing their shots for the small screen.)

ATV broadcast formats apply to network transmissions, cable systems, camcorders, VCRs, and DVD decks. And to make matters worse, there are different ATV broadcast standards for different parts of the world.

DV camcorders are designed to work with either NTSC (the North American TV standard) or PAL/SECAM (the British and French/Asian TV standards). Although there are multiformat VCRs and DVD players, camcorders come in one format or the other, not both.

The basic differences between NTSC and PAL/SECAM come down to frame rates and scan lines. (They also differ in color space, a topic we've already mentioned in this chapter and cover in greater depth in Appendix A.)

Frame Rates

The standard frame rate for motion-picture film cameras anywhere in the world is 24 frames per second (fps). If only video were so simple!

NTSC displays pictures at nearly 30 fps—the actual number is 29.97. PAL/SECAM uses exactly 25 fps. These frame rates are based on the local frequency of alternating current (AC) electrical power: 60 Hz in the States; 50 Hz in most of the rest of the world. Early television engineers used the AC frequency to control raster scanning ( Figure 3.9 ).

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Figure 3.9 TV's complex system of scan lines and interlaced fields is inherited from the early days of television. The scanning beam is turned on during the left-to-right trace that creates visible scan lines, and turned off during the right-to-left (horizontal flyback) and bottom-to-top (vertical flyback) traces.

Fields and Scan Lines

In theory, new digital TV sets could make a clean break with the past. The technology exists for making video broadcasting simpler, but it won't happen anytime soon. Anyone who produces video is stuck with the complicated legacy of decades-old analog television. If it were otherwise, millions of older sets all over the world would go blank, and no one in the industry is ready for that to happen.

Early TV picture tubes couldn't hold an image for very long before it started to fade, so television engineers added an additional complication to reduce flicker: They divided each frame into two fields, each composed of alternate sets of scan lines (first odd-numbered lines, then even-numbered lines).

Under this method, NTSC scanning becomes a succession of horizontal scans at close to 60 fields per second. The scan lines of the second field are painted in between the scan lines of the first. The result is a near-30 fps shown at nearly 60 fields per second—with little or no flicker. PAL/SECAM TV sets do 50 fields per second to achieve 25 fps.

The alternating scan lines of the two fields that compose each frame are said to be interlaced.

In the world of analog video, the number of horizontal scan lines on the screen is one way to measure picture resolution. And once again, NTSC and PAL/SECAM employ different specs: NTSC uses 525 horizontal scan lines; PAL and SECAM use 625.

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