HD video is commonly presented in 720 or 1080 sizes, with a ‘p’ or’ i’ after the number, which is a shorthand for either ‘progressive’ or’ interlaced’. But what exactly do these two terms mean? In this second article in a series, which attempts to break down the technical aspects of video recording and editing, I explain how a displayed picture is composed.
When watching a video on screen the picture will be transmitted in one of two ways. The first method, used by computers and some media players, is to output full-frame images, displaying whole frames as single images, one after the other. This is called progressive (p) video. The second method, used by TV broadcasters, is to use interlaced images (i), that are made by blending elements from two adjacent frames one after the other, splicing odd and even lines together to form each frame. When a TV processes interlaced video it separates the lines from the adjacent frames and blends them together in the correct sequence to display the progressive video you’re used to watching. This process is called deinterlacing.
Progressive video contains more data than interlaced video and as a result images displayed appear sharper and more detailed, with no motion artefacts visible when watching sport or other quick motion sequences. The downside however is that because progressive video requires more data to be transmitted it is not a great choice for broadcasters, that instead choose to put out interlaced video, or owners of video cameras with limited storage capabilities. Modern cameras offer the ability to record both ‘p’ and ‘i’ video, with most editing software supporting both formats too. If you have plenty of storage space on your camera and want to record the best quality HD video possible choose progressive, otherwise opt for interlaced.
The last descriptor in this technical shorthand is a number sometimes seen at the end of a video description , e.g. 1080p/24 or 1080i/50. This last number after the slash denotes the speed of the video or TV, measured in frames per second, fps, or Hz (cycles per second). The greater the frame rate, the greater the quality of video displayed. Common video standards are 24Hz for movies and 50 or 60 for TV broadcasts and gaming. Interlaced video sources at 50 or 60Hz are usually deinterlaced and displayed by an HD TV at 25 or 30 progressive video frames per second.
If you have a question about this post, or would like to know more about a particular aspect of video recording & editing, let me know and I’ll try my best to answer it over the coming weeks.
Read the first post in the series, which explains the differences between standard and high definition video.