Digital Video: Frames, Fields and Dropping Them

There are two main defects unique to digital video productions that can make them unnacceptable and must be avoided. The first is severe compression, which, when applied too liberally, causes image quality to be inferior. This is discussed in the page on QuickTime Quality and Compression. The other is dropped frames, which will be covered here.

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Framerates, Fields and Interlacing

NTSC video displays video as a series of still images (frames) that run at a rate of 29.97 per second (often erroneously rounded to 30), causing the illusion of moving images. In comparison, PAL video, a format common in many parts of the world, runs at 25 fps, and film generally runs at 24fps.

Interlacing and Fields

Each frame of video is separated into two fields, each containing every other line on the screen, and together making up a complete frame. They are displayed sequentially -- first one field and then the other. Originally, this was to compensate for the inadequacies of early components in the beginning of television -- the scan would not have been fast enough to draw a complete frame, line by line, in 1/30th of a second, without causing visible tearing.

Drawing a frame with two fields, called interlacing, has a hidden benefit: To the human eye, each field appears to be a complete frame.

If, for example, a car is in position A in one frame, and position B in the next frame, the second field drawn in the first frame will display the car halfway between the two points. The visibly seamless 30fps actually appears to be an ultra-smooth 60 frames per second.

This is why the advent of 60-field video in QuickTime was so important to Quicktime's acceptance in the video community. 30 f(rame)ps video is simply not as smooth or professional as 60 f(ield)ps video. Visually, it can be the difference between video that looks "good" versus video that looks, well, like what you'd see on a TV.

This is also why finely-detailed computer-generated graphics flicker when presented on an interlaced device. Click here for more about flicker and convolution deflickering.

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Dropped Frames

When a digital video system fails to capture every frame and field in a video sequence, it is known as dropping frames. The visual characteristic is a "skipping" or "stuttering" look, wherein the missing frame is substituted with the previous frame. This may not be noticeable on an RGB computer monitor, which is not interlaced. But on an interlaced device like a TV, it can look terrible, particularly during sequences with a lot of fast motion. If the digitizer dropped frame B in the car example mentioned above, the car would be drawn in the starting position in the first field, then on its way to position B in the next field, and then back in the starting position in the next field, on its way to position B again in the next field, and then abruptly past position B in the fifth field. This jarring back-and-forth stutter is what makes even a single dropped frame or field unnacceptable.

Using RGB Monitors for Previewing Edited Video

The interlaced scheme in NTSC, PAL, et. al. is what makes an RGB computer monitor inferior for previewing edited video. The lack of interlacing eliminates the 60-field smoothness, and dropped frames are substatially less apparent. Additionally, it is best to develop content while previewing on an interlaced monitor so that, for example, the colors of your titles can be accurately previewed, and the bleeding associated with using colors outside of NTSC's "legal" colorspace will be immediately apparent. Finally, interlaced video does not look good on a progressive scan monitor, due to the field differentiation, also known as "field distortion".

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More info about Framerates and Timebases

To recap:

When comparing accelerated QuickTime systems, the question is no longer "What framerate can I get with a [fill in the blank]". VideoVision Studio can record at 60 fields per second, without missing a frame, on any configuration, unless it has an element that is incompatible, and causes dropped frames at any settings. The question then really becomes: "At what Quality can I record and play back video without dropped frames on my [fill in the blank]?"

Framerate reports

Following digitization, a great deal of attention is often paid to the framerate report of Premiere's Movie Analysis window. When the framerate fails to read 30.00fps, users often wonder what happened.

This figure is largely meaningless, for several reasons. QuickTime reports inaccurate values for the first and last frames of a movie, which screws up the averaging. Also, NTSC video does not actually run at 30fps, but runs at 29.97fps, for technical reasons. So your captures should never reach 30.00.

In any case, all that matters is that the Movie Analysis window has not reported any DROPPED FRAMES. How many frames are dropped is only useful as an indicator of how close you are to finding the correct data rate for your system. If you have any dropped frames at all, you should rerecord at a lower data rate.

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Conforming Movies

(Coming Soon!)

Alternate Frame Rates

(Coming Soon!)

Film->Video and Back

(Coming Soon!)

Timebases

(Coming Soon!)

Pull-up

(Coming Soon!) contents