Understanding video standards and terminology
9
Frame rates
Video is a sequence of images that appear on the screen in rapid succession, giving the illusion
of motion. The number of frames that appear every second is known as the
frame rate
, and it
is measured in frames per second (fps). The higher the frame rate, the more frames per second
are used to display the sequence of images, resulting in smoother motion. The trade-off for
higher quality, however, is that higher frame rates require a larger amount of data to display
the video, which uses more bandwidth.
When working with digitally compressed video in a format such as Flash Video, the higher
the frame rate, the larger the file size. To reduce the file size, you must lower either the frame
rate or the data rate (for more information, see
“Data rates” on page 10
). If you lower the data
rate and leave the frame rate unchanged, the image quality is reduced. If you lower the frame
rate and leave the data rate unchanged, the video motion may look less smooth than desired.
Because video looks much better at native frame rates (the frame rate at which the video was
originally filmed), Macromedia recommends leaving the frame rate high if your delivery
channels and playback platforms allow it. For full-motion NTSC, (the standard defined by
the National Television System Committee in the U.S.), use 29.97 fps; for PAL (Phase
Alternating Line, the dominant television standard in Europe), use 25 fps. If you lower the
frame rate (which can significantly reduce the video data that must be encoded), Flash Video
Encoder drops frames at a linear rate to achieve the new fps rate. However, if you need to
reduce the frame rate, the best results come from dividing evenly. For example, 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 15 fps, 10
fps, 6 fps, and so on.
If your video clip is encoded with a higher data rate, a lower frame rate can improve playback
on lower-end computers. For example, if you are compressing a talking-head clip with little
motion, cutting the frame rate in half might save only 20 percent of the data rate. However, if
you are compressing high-motion video, reducing the frame rate has a much greater effect on
the data rate.
NO
TE
If a video clip is longer than 10 minutes, the audio will drift noticeably out of sync if you do
not adhere to the 29.97 fps rate or an accurate even division for lower frame rates (such
as 14.98 fps, which is half of 29.97).
000_FLVEncoder.book Page 9 Friday, August 12, 2005 3:50 PM
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