Videographica: Video Glossary

bandwidth - Technically speaking, "bandwidth" is the width of the electromagnetic "band" of energy used to transmit some stream of information. For example, each television channel in over-the-air NTSC takes up roughly 4 MHz of the radio frequency spectrum. The term bandwidth has been generalized from its origin in physics to be roughly equivalent to "data rate" — the amount of bits or bytes a system transmits per second. This extension is natural, because higher data rates generally require a wider EM band to transmit. This isn't really accurate since you can get a higher data rate without increasing the bandwidth by using a given amount of the EM spectrum more cleverly. This usage persists, however, so you'll need to take context into account to know whether a person is talking about the actual width of a band of the EM spectrum or they're just using the term to mean "data rate".

D1 - D1 is a resolution standard. In the NTSC system, "Full D1" means 720x480 pixels, and in the PAL and SECAM systems full D1 is 720x576. You also see "cropped D1", which is 704xNN, which is useful because the 8 pixels on either edge of the video frame aren't supposed to contain useful information. Therefore, some programs will prefer the cropped D1 resolution to save bandwidth. Other popular resolutions are often described in terms of D1: the SVCD resolution is 2/3 D1 (480xNN) and 352xNN is 1/2 D1. Occasionally you see SIF somewhat inaccurately described as 1/4 D1.

field - The NTSC, PAL and SECAM television systems are all interlaced. Each half-frame is called a field.

fps - Frames or (less commonly) fields per second. If context doesn't provide a clue, you can usually assume that "fps" means frames per second.

frame - Video systems work like film projectors: they show still pictures in rapid succession; if the frame rate is high enough, the small differences between each frame trick the mind into thinking it is seeing motion. See also field

interlaced - In an interlaced video system, only half of the lines making up the frame are shot at a time. For example, the camera might capture only the odd frame lines first, then capture the even lines second. If the frame rate of an interlaced video system is 25 fps, it has 50 fields per second. Since each field is shot separately, interlacing makes a 25 fps system look like a smoother-running 50 fps system without actually having to use the doubled bandwidth required by a true 50 fps sytem.

NTSC - The National Television Standards Committee. This committee defined the analog over-the-air television standard originally used in the United States. Since its inception there, its use has spread to the rest of North America, to parts of South America and to Japan. It has 480 horizontal lines of video data and 29.97 frames per second. See also resolution. Compare PAL and SECAM.

PAL - Phase Alternating Line. This term describes the over-the-air video encoding used in Australia and most of Eurasia. It has 576 horizontal lines of video data and 25 frames per second. See also resolution. Compare NTSC and SECAM.

resolution - Measuring and describing resolution in analog video systems is tricky. In digital video systems, you can simply use pixel dimensions: for example, you might say a particular digital video system has 352x240 resolution. In the analog world (e.g. over-the-air television, cable TV, composite and S-video...), the only firm resolution is the "horizontal lines of resolution". For example, NTSC has 480 horizontal lines of resolution. (By the way, be careful to note that when you're talking about the vertical resolution of a picture, you speak of horizontal lines — 480 horizontal lines of resolution means that you can display up to 480 alternating black and white horizontal lines and see them all distinctly. Similarly, the horizontal resolution is defined in terms of vertical lines.) Horizontal resolution is trickier: in an analog system, there is no way to tell how many vertical lines of resolution the system can show without measuring it, either with physics or by running a test signal through a system and viewing the result. For example, the low-resolution VHS format will only distinguish about 240 vertical lines of resolution. The tape format simply can't store any more detail than this. Your television probably has 400-500 vertical lines of resolution, and professional equipment has at least 720 vertical lines of resolution.

resolution standard - There are a number of named resolution standards: D1 and SIF being the most popular. All of these standards are designed to put a name to a particular data rate that is independent of the frame rate difference among video systems — NTSC is 29.97 fps and PAL/SECAM is 25 fps. Consider D1: it's defined as 720x480 for NTSC, and 720x576 for PAL and SECAM. You will find that 720x480x30 is equal to 720x576x25; therefore, "full D1" implies the same amount of data per second in all television systems for which it is defined. (29.97 fps is only one frame in a thousand lower than 30 fps.)

SECAM - A French acronym whose expansion I forget. :) This system is similar to PAL, but of course not so similar as to be directly compatible. It was invented in France and today is used there almost exclusively. It used to be used elsewhere in the world, but most everyone else has converted to either PAL or NTSC.

SIF - The Source Interchange Format is a resolution standard defined as 352x240 for NTSC and 352x288 for PAL and SECAM.

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