Interlaced or Progressive are terms you may have heard in connection with video. Interlacing is a technology that has been used for the last 70-80 years for scanning the pictures on cathode ray tube TV screens. It was invented to overcome the limitations inherent in the electronics available during the development of TV – i.e., it is a fudge. In the UK the picture is scanned 50 times per second for our TV system, but each scan covers only half the lines of the picture. This means all the odd-numbered lines are scanned first and then the interleaving even-numbered lines are scanned. So although the scan rate is 50 times per second, as only half the field is scanned you get 25 frames per second with interlaced video.
Computer monitors and flat screen TVs are capable of displaying progressive images at 25 full frames per second (in the UK and PAL areas around the World). That is, as 25 complete still high resolution pictures each second, each one slightly different as the subject moves. We shoot using the progressive format with our PMW XDCAM EX3 camera. The benefit is that we can freeze fast motion into a series of crystal clear high-resolution still images if desired, which has applications where the viewer wishes to pause a video and examine, for example, a sports movement for training purposes or read a detailed text paragraph in the video. Editing progressive video also has many advantages for the clarity of the end result, especially when viewed on modern display devices – even if only in standard definition. In fact, our EX3 (and even more recent Canon 7D DSLR) cameras allow us to shoot at up to 60 frames per second in progressive mode in HD which gives incredibly clear slow motion.
Copyright © 2009 Andy K Wilkinson PhD
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