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This article was previously published under Q269068
Frames per Second (FPS) is a measure of how motion video is displayed. The term applies equally to film video and digital video. Each frame is a still image.
BackgroundTechnological means can be used to suggest the appearance of movement. To create the perception of motion, the brain automatically adds or fills in missing information. It does this first through a concept known as persistence of vision, where a visual stimulus continues to be registered by the brain for a very short time after the stimulus ends. Secondly, it takes advantage of what is known as the phi function. For example, if two adjacent lights alternately flash on and off, we see a single light shifting back and forth. This is because we tend to fill in gaps between closely spaced objects of vision. These are exploited by motion pictures, which consist of rapid successions of still frames in which the "moving" objects are displaced a very short distance from one another.
Because of these phenomena, the higher the FPS, the smoother the motion appears. In general, the minimum FPS needed to avoid jerky motion is about 30 FPS. For high-motion content, an encoding session around 60 FPS may be more beneficial.
When dealing with FPS, it is important to also understand other terms that are used throughout the Windows Media Encoder:
Understanding Key FramesWhen content is streamed, it is very costly (in terms of CPU and network bandwidth) to send all of the video data from each single frame. To solve this problem, the Encoder has a key frame frequency setting. The key frame (also known as the I-frame) is a data frame that contains all of the video data. The intermittent frames only send changes, or deltas, from the key frame.
Take for example a speaker at a podium. It makes more sense to only send the area of the screen that is changing (the speakers mouth, and maybe hands), then it does to send the background information as well. In this example, a higher key frame interval can be selected without harming video quality.
If the content is of higher motion, then it is more beneficial to set the key frame rate lower. Examples of high-motion video are action movies or sporting events. In these instances, the entire frame is changing very quickly so a lot of data is going to be sent, and a higher-end computer is required to achieve desired results.
Even though the key frame rate is set, it is possible for a key frame to be sent before the time interval elapses. If the delta is high enough, the Encoder automatically creates a new key frame.
Factors That Affect FPSThere are primarily three factors that affect the frame rate, and they are all interrelated:
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