22 February 2011
The video that you see on a standard-definition television screen follows standards established in the 1950s when color television was first introduced. The leading formats in use today are NTSC (National Television System Committee) and PAL (Phase Alternating Line). Generally speaking, NTSC is the standard used in the Americas and Japan, whereas PAL is used in Europe, Australia, the Middle East, and Asia.
This section of the Video Learning Guide for Flash provides an overview of the key concepts related to the NTSC and PAL video standards. This information is relevant only for older standard-definition formats. Many issues related to those formats are no longer of concern when using the newer, high-definition (HD) video formats.
Conventional television screens are made up of horizontal lines, while computer monitors consist of a series of horizontal and vertical pixels. The standard line resolution for an NTSC television is 525 lines; for PAL, it is 576 lines. Most modern computer monitors have much higher vertical resolutions (measured in pixels), such as 768 or 1024, requiring vertical upscaling during playback in order to fill the monitor.
For NTSC video images, the SMPTE 259M professional standard specifies that the 525 lines be represented as 720 × 486—that is, 720 horizontal pixels by 486 vertical pixels. This default video size is commonly known as D1. Capturing footage with most modern video capture cards from a professional BetaSP or Digital Betacam source results in a D1-sized frame. Capturing footage from a DV (digital video) source, however, yields a 720 × 480 frame. The difference between the D1 specification and the DV specification is only 6 vertical pixels. Many compression algorithms, including DV compression, work best with pixel dimensions that are multiples of 16. By removing 6 pixels from the D1 resolution, the DV format achieved a native resolution with a multiple of 16.
For PAL video images, frames are always 720 × 576 pixels, regardless of video source. Because PAL's vertical resolution, 576, is a multiple of 16, no change is necessary for DV compression.
Video is essentially a sequence of images flashed on the screen in rapid succession, giving the illusion of motion. The number of frames displayed every second is known as the frame rate, and the speed of the movie's playback is measured in frames per second (fps). The higher the frame rate, the more frames per second will be used to display the sequence of images, resulting in smoother motion. The trade-off, however, is that higher frame rates require a larger amount of data to display the video, and therefore require more bandwidth.
NTSC video is usually said to run at approximately 30 fps, and PAL runs at 25 fps. To be technically accurate, NTSC video runs at 29.97 fps. The reason for the fractional frame rate dates back to legacy technical limitations.
When working with compressed video in a format like FLV video, frame rate can affect the quality of the video in hard-to-predict ways depending on how you encode the video and its specific content. Lower frame rates ostensibly provide less content to encode, which theoretically improves quality or decreases file size. At the same time, however, it makes it more likely that there are noticeable changes in the pixels from one frame to the next, which requires more data to encode. If you lower the frame rate and leave the data rate unchanged, the video may appear to stutter and motion may look less fluid than desired.
Whenever the frame rate is reduced, it is always a good idea to use an evenly divisible ratio of the original frame rate. This is so that the software doesn't have to make up (interpolate) the image information for the output frames that don't coincide with the times of the input frames. If your source has a frame rate of 24 fps, then reduce the frame rate to 12 fps, 8 fps, 6 fps, 4 fps, 3 fps, or 2 fps. If the source frame rate is 30 fps, in most cases you can adjust the frame rate to 30 fps, 15 fps, 10 fps, 6 fps, and so on. If your video is more than 10 minutes long, then audio will drift noticeably out of synch if you do not adhere to the 29.97 fps rate or use an accurate even division for lower frame rates (such as setting the frame rate to 14.98, which is half of 29.97).
The D1/DV NTSC and PAL specifications specify non-square pixels (often called D1 aspect ratio), while computer monitor pixels are square. In fact, DV/D1 NTSC pixels have a 0.91 pixel aspect ratio (PAR), which means they're tall and skinny; DV/D1 PAL pixels have a PAR of 1.09, so they're short and squat. For this reason, when you look at a D1 video image on a computer monitor, the images appear to be distorted (see Figure 1).
For some video formats, images consist of two interlaced fields that together make up a frame. This approach was introduced when TV was first invented, due to a technical limitation that prevented a full frame from being "progressively" drawn on the monitor (from top to bottom) without a noticeable visual shuttering; as an image was being displayed, it appeared as though it was being wiped on the screen. By breaking up the image into two fields (halves) and displaying one after the other (see Figure 2), this artifact was eliminated.
Although many newer video standards for high-definition television and all digital cinema and web formats use progressive video (images are drawn in one pass from top to bottom), many HD standards use interlaced video for temporal resolution issues. For a given data rate, you can choose to have either a perfect whole image every n units of time or a less perfect image every n/2 units of time. Sports viewers actually prefer having half the image refreshed twice as often so that they can see the movement of fast-moving objects better. Partially for this reason, the major TV manufacturers want to keep interlacing.
With real video footage, the two interlaced fields often look very similar to each other, and no visible artifacts appear when looking at a video frame on a computer monitor. However, if you are working with video footage that includes high-motion material that changes quickly (including movement of the camera or subjects in the frame) noticeable field artifacts can appear when the fields are displayed together on a computer monitor, giving the video image a blurry and ghost-like quality. This is due to the composition of two moments of time together in one frame. Therefore, to display crisp video on a computer monitor, video frames must be de-interlaced by combining the two fields to create a single, whole frame.
Progressive scan video cameras usually have the ability to switch back from progressive scan to interlaced video, and most of these cameras have a variety of frame rates that can be used with and without interlacing. Typical frame rates are described as 60p (60 fps progressive), 30i (30 fps interlaced), 30p (30 fps progressive), and 24p (24 fps progressive). When you are working with progressive video footage there is no need to de-interlace the video clips before deploying them to the web.
For more technical details, read the Interlaced video and separating fields section of the Using After Effects CS5 online documentation. Also check out All About Video Fields on the Lurker's Guide to Video.