Video timing

Although it looks like video is continuous motion, it is actually a series of still images, changing fast enough that it looks like continuous motion, as shown in Figure 1.1. This typically occurs 50 or 60 times per second for consumer video, and 70–90 times per second for computer displays. Special timing information, called vertical sync, is used to indicate when a new image is starting.

FIGURE 1.1. Video is composed of a series of still images. Each image is composed of individual lines of data.

Each still image is also composed of scan lines, lines of data that occur sequentially one after another down the display, as shown in Figure 1.1. Additional timing information, called horizontal sync, is used to indicate when a new scan line is starting.

The vertical and horizontal sync information is usually transferred in one of three ways:

1. Separate horizontal and vertical sync signals
2. Separate composite sync signal
3. Composite sync signal embedded within the video signal

The composite sync signal is a combination of both vertical and horizontal sync.

Insider Info

Computer and consumer equipment that uses analog RGB video usually uses technique 1 or 2. Consumer equipment that supports composite video or analog YPbPr video usually uses technique 3. For digital video, either technique 1 is commonly used or timing code words are embedded within the digital video stream.

Interlaced vs. Progressive

Since video is a series of still images, it makes sense to simply display each full image consecutively, one after another.

This is the basic technique of progressive, or noninterlaced, displays. For progressive displays that “paint” an image on the screen, such as a CRT, each image is displayed starting at the top left corner of the display, moving to the right edge of the display. The scanning then moves down one line, and repeats scanning left-to-right. This process is repeated until the entire screen is refreshed, as seen in Figure 1.2.

FIGURE 1.2. Progressive displays “paint” the lines of an image consecutively one after another.

In the early days of television, a technique called interlacing was developed to reduce the amount of information sent for each image. By transferring the odd-numbered lines, followed by the even-numbered lines (as shown in Figure 1.3), the amount of information sent for each image was halved. Today, most broadcasts (including HDTV) are still transmitted as interlaced. Most CRT-based displays are still interlaced while LCD, plasma, and computer displays are progressive.

FAQs

Given the advantage of interlacing, why bother to use progressive?

With interlace, each scan line is refreshed half as often as it would be if it were a progressive display. Therefore, to avoid line flicker on sharp edges due to a too-low frame rate, the line-to-line changes are limited, essentially by vertically lowpass filtering the image. A progressive display has no limit on the line-to-line changes, so it is capable of providing a higher-resolution image (vertically) without flicker.

FIGURE 1.3. Interlaced displays “paint” first one-half of the image (odd lines), then the other half (even lines).