What We Must Understand Introduction to Video What We Must Understand

Introduction It is important to understand the basic principles, terms, and standards associated with video before beginning a project.

Persistence of Vision Persistence of vision theory states that the human eye holds each still image for a fraction of a second, and that it remains on the retina long enough to blend with the next image.

Persistence of Vision (con’t.) Film displays 24 still images each second (frames per second, or fps)

Scanning A CRT has an electron gun in the back and a screen in the front. The inside of the screen is covered with thousands of tiny phosphorus dots called pixels. The back of a color CRT holds three electron guns: red, green, and blue (RGB).

Scanning (con’t.) National Television System Committee (NTSC) video is the standard for American video. Pixels placed in lines that extend across the screen are called scan lines. Half of a frame is called a field. Two fields fit together to display a complete frame

Scanning (con’t.) Interlaced video uses two fields to create a complete image. Bandwidth is the size of the pipe that information travels through. Progressive scanning scans every line in order. High definition (HD) uses 720 progressively scanned lines or 1080 interlaced lines.

Time Code Each frame has its own unique identifying number called a time code. In the time code, 13:42:27:15, 13=hours, 42=minutes, 27=seconds, and 15=frame number. Frame accurate editing is efficient and effective due to time code.

Aspect Ratio and Resolution The aspect ratio for standard definition television is 4:3, meaning for every four pixels across, you have three pixels up. HD uses a 16:9 aspect ratio.

Analog Versus Digital Signals An analog signal is a faithful copy of the original wave. One drawback to analog is little glitches in the signal, called noise.

Analog (Cont.) Another problem with analog signal is that it is linear, meaning you cannot just jump from the beginning to the end, or go back and forth.

Analog (Con’t.) The last problem with analog is bandwidth because analog is difficult to make smaller. Digital solves these three problems because it uses sampling to reduce the noise, it is nonlinear, and it can be compressed to solve bandwidth problems.

FYI…..BOXES Note: NTSC created the standards used in American video in the 1940s and 1950s. In the early days of television, different manufacturers wanted their systems to be used, so NTSC was formed to examine the options and create a standard. NTSC is based on compromises, which, like all compromises, do not address every need but meet the majority. NTSC standards include the use of 525 scan lines and the interlaced 29.97 frame rate. (You might be wondering “Why 29.97?” You learn why in the “Time Codes” section.) The Advanced Television Systems Committee (ATSC) created higher-quality television standards, but they have not been implemented completely yet

Why is video 30 frames per second, not 24 as with film Why is video 30 frames per second, not 24 as with film? The CRT’s electron gun gets the electrons it shoots from the electricity in your house. The electric current alternates 60 times every second, which is why the field rate is 60 fields per second. Original black-and-white NTSC video had a frame rate of 30 fps.

When color was introduced to the signal, however, older black-and-white TV sets could not display the images. Engineers found that color programs could be seen on those older sets if the frame rate was dropped to 29.97. Most editing so ftware allows you to work with video at 29.97 fps, but make your life easier and just go with the 30 fps option. In Europe and many other parts of the world, electrical current alternates 50 times each second instead of 60. Because the flow of electricity is different, the field rate in those parts of the world is 50, so the frame rate is 25 fps. To address these differences, they need a different video standard. Two common ones in other parts of the world are PAL and SECAM.

Introduction to Digital Video