Monday, October 5, 2009

High Speed Sports Video on TV

We all already have or want that high definition plasma, LCD or even better, LED television. If you have watched sports on high definition TV, you know the difference immediately. The players are recognizable. No more asking who made that tackle or scored that goal?

Now a number of TV sports programs also show high speed replays, with the most prominent example being CBS Sports Swing Vision, which is used for CBS Golf coverage as well as Tennis and numerous other sports. We also see high speed video being used on TV and in sports to help make critical refereeing or umpiring decisions, such as the Hawk-Eye system for Tennis line calling and for the LBW decision in Cricket and high speed video replays for NFL challenge decisions.

Obviously there is a difference between high definition (or high resolution) and high speed video.
Normal TV images have up to 576 vertical lines of pixel data and the video is recorded at either 24 or 30 frames per second, depending on whether you are in Europe and Africa or in the Americas and Asia. The 576 lines describes the resolution or standard definition, while the 30 frames per second describes the speed of playback. Now your high definition television may have a resolution of 1080 progressive (1920x1080 or 2.07 MPixels) but the TV program you are watching is often recorded at the same frame rate as your standard definition TV, 24 or 30 frames per second. These days as more people have high definition TV's that can play back the higher frame rates, there are more sports broadcasts being filmed at 60 frames per second or even a little higher. This however is still not high speed video.

Therefore you can see that high definition television does not necessarily mean that we can always see slow motion replays of high speed motion. Special high speed cameras are needed to record broadcast quality high speed images such as those seen with swing vision.

High speed slow motion replays require cameras that record video data at 500, 1000 or 2000 frames per second and sometimes even higher. At the same time, they need to record with resolutions as high as 1080p, to retain their broadcast quality.

Swing Vision claims to record high speed footage with frame rates up to 40,000 frames per second. It is certainly possible for high speed cameras these days to record video at 1920x1080 pixel resolution at speeds of 2000 frames per second. To get higher speeds of capture broadcasters can trade in some of the resolution and for example record at 1280x720 pixel resolution (still HD) at 10,000 frames per second.

Below are 2 examples showing a HD video of surfing recorded at normal frame rate and played back in slow motion and a HD video of surfing recorded with a high frame rate and played back in slow motion.

If you pause the first video at any stage you will notice that the image goes a little out of focus. So although this video was filmed in HD, it was not filmed with a high frame rate. The fast motion of the surfer causes the paused video image to blur a little as her frequency of movement is faster than the frame rate of the camera.

The second video was recorded with the Typhoon HD4 high speed camera which can record video at 1280X1024 pixel resolution (1024 HD) at 1000 frames per second for full resolution. If you pause this video you will notice the clarity of the image. The surfer and wave frequency of movement is slower the camera frame rate.

We can see the difference between HD video of sport recorded at normal speed and high speed. The high speed recording allows for much more clarity in viewing high speed movement and therefore the chance to analyze performance in much more detail. In these surfing videos, you are probably thinking that the higher speed camera was not necessary to analyze the surfers movement as the clarity of the first video was more than enough. This is true but for higher speed movement such as the club during a golf swing, the higher frame rate can help identify those small improvements that can make a difference.

When capturing video of your own performance you may not be able to afford or for that matter need a high definition, high speed camera. Human movement, even fast human movement is predicted to have a maximum frequency of around 250 hertz. This suggests that even for the fastest human movement (not including equipment such as the golf club or tenis racket, which will move faster) 250 frames per second capture speed will be enough. But, the higher the frame rate you can get on your camera the more detail you will have to analyze in your video. There are numerous HD video cameras available and many of them can record at 60 frames per second and some even higher. To analyze your performance this is a great place to start.

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