Flash Video Encoder User Guide

Flash Video Encoder User Guide > Understanding video standards and terminology > Frame rates

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). 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), Adobe 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 video clip with little motion, cutting the frame rate in half might save only 20% 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.

Flash CS3