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Understanding TV Terms

What are analog TV, digital TV, and high-definition (HDTV)?

Devices such as monitors, tuners, and integrated monitor/tuner combinations are designed to handle any of the 18 formats officially sanctioned by the industry-led standard setting, Advanced Television Systems Committee (ATSC), for the digital transmission of video signals. There are three subgroups of digital televisions depending on their capabilities: High-definition (HDTV), enhanced-definition (EDTV), and standard-definition (SDTV).

In contrast, U.S. analog television has been available for over 50 years in the single National Television Standards Committee (NTSC) format. This is the signal that has been broadcast over the air and through cable systems. NTSC television offers slightly more than 480 interlaced lines of resolution.

■Interlaced (i) means that the picture appears in two segments: every other horizontal line of the picture is displayed in one frame, and the other half of the horizontal lines are drawn in the next frame of the picture. This works because it happens so fast that the eye is deceived into believing that only one picture is being shown.

The number refers to the number of lines of resolution, and generally the higher the number, the better the picture. For example, with digital TV (DTV) there are two progressive formats for broadcasters to choose from: 480p and 720p. A 480p image is much sharper than a regular TV broadcast, although it is only considered a DTV signal and not a true high-definition signal. 720p has 720 horizontal scan lines and is considered to be true HDTV. 720p can rival a 1080i picture, which is 1,080 horizontal scan lines displayed as an interlaced image. This is because of the sharper and more accurate (progressive) 720p picture, even though a 1080i picture has many more scan lines and a much higher resolution.

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What is the difference between progressive and interlaced signal formats for digital TVs?

There are 18 standard digital TV signal formats. Each one provides a different picture quality. Digital TV formats are named using a number followed by either progressive (p) or interlaced (i). The letter refers to the way the TV uses the signals to create the picture:

■Progressive (p) scanning presents all the horizontal lines of the picture in one frame. This provides a sharper picture to the eye and looks much better than an interlaced image of the same resolution.

What is variable aspect ratio?

The aspect ratio describes the relationship of screen width to screen height. Standard TV signals are based on the aspect ratio of 4:3 (four units wide by three units high). HDTV signals are based on the aspect ratio of 16:9, making the image 33 percent wider than a normal TV and allowing viewing TV and movies as if you were watching them in a theater.

In addition, many DVDs look better on the widescreen display because most movies made in the last 50 years were filmed in an aspect ratio of either 1.85:1

(very close to 16:9, which is equal to 1.78:1) or 2.35:1 (even wider than 16:9).

Variable aspect ratio support enables scaling of 4:3 and anamorphic video to fill the screen.

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