HD Frame Rates And NTSC Compatibility

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HD Frame Rates And NTSC Compatibility

Although it is becoming increasingly popular to shoot and/or master to some flavor of HD video today, because of the need for backward compatibility for NTSC compatible equipment like down-conversion from HD to SD for broadcast, or for the creation of SD DVD's, and "forward" compatibility for converting NTSC material to HD, there has been a need to create standards that make these conversions somewhat simple.

NTSC Compatible Frame Rates

The major issue at hand: frame rate. As long as you shoot with an "NTSC compatible" frame rate in HD, the video can be easily down-converted through a number of options, both hardware and software based that won't compromise quality because of mis-matching frame rates. And NTSC can be up-converted to HD also using a number of options if we use appropriate frame rate options.

As we saw earlier, there are flavors of HD video in the 720 and 1080 line formats for both 23.98 and 24 frame video. There is both 29.97 and 30. There is both 59.94 and 60. Let's start with 24p.

24p: Used to describe a video frame rate of 23.98 or 24 frames per second, scanned progressively. It does not imply standard def or hi def, as there are 24p flavors in both worlds. In the U.S., the most common flavor of 24p is 23.98. By the way, when we refer to 23.98 fps, we are actually referring to a specific rate of 23.976 fps, but the term most widely used is simply 23.98.

Working at 23.98 fps is very important as it allows for compatibility with NTSC. There are only three frame rates that are directly (easily) compatible with NTSC:

• 23.98
• 29.97
• 59.94

It doesn't matter whether the HD video originates as interlaced or progressive, but the frame rate is crucial. Since NTSC is 29.97 fps, HD video shot at 29.97 fps is easily down-converted as there is no frame rate conversion. 23.98 is also easily down-converted to 29.97, via a method referred to as a "pulldown", or "2:3 pulldown", which has also been the long-standing method for converting 24 fps film to 29.97 NTSC video. Refer to the section on Pulldown for more information.

HD Broadcast Compatibility

The frame rates mentioned above are not only important for compatibility with SD video, but HD broadcasters in the U.S. have all adopted frame rates that are NTSC compatible. So even if your program is destined straight for HD delivery, it will still need to be mastered in one of the three NTSC compatible frame rates. Check with your intended broadcaster to find out their specifications.

Even when final specs for delivery are unknown, there is one HD format that is easily converted to other HD formats: 1080p23.98. This format can be converted to 1080i, 720p or down-converted to standard definition NTSC. In addition, this format can be easily converted to any other NTSC compatible frame rate in HD: 29.97 or 59.94 through a simple pulldown.

PAL Countries

In the PAL world, life is a bit more simple (to put it mildly). Since film in PAL countries is shot at 25 fps, there is an easy transfer to PAL video at 25 fps. When working in HD, there are 25p, 50i (25 fps, interlace), and 50p frame variations. One thing that makes workflows straightforward is the lack of confusion- all frame rates are whole numbers or "integer" numbers- 25 is exactly 25. Additionally, since the film standard and digital video standards share the exact same frame rate (with the exception of 50p), there is no need for a pulldown to convert film to video.