Technical information

by Forest Key and Chris Hock

Table 1 provides an overview of the differences between these standards.

Table 1: Video Standards at a Glance

Image Size Frame Rate Aspect Ratio Display

NTSC 720 x 480 29.97 D1 Interlaced

PAL 720 x 576 25 D1 Interlaced

Computer Varies

(much larger)

-- Square Progressive

Frame Size

Conventional television screens are made up of horizontal lines while

computer monitors consist of a series of horizontal and vertical pixels. The

standard line resolution for an NTSC television is 525 lines; for PAL it is 576

lines. Most modern computer monitors have much higher vertical

resolutions (measured in pixels), such as 768 or 1024, requiring vertical

upscaling during playback in order to fill the monitor.

For NTSC video images, the SMPTE 259M professional standard specifies

that the 525 lines be represented as 720 x 486—that is, 720 horizontal

pixels by 486 vertical pixels. This default video size is commonly known as

D1. Capturing footage with most modern video capture cards from a

professional BetaSP or Digital Betacam source result in a D1-sized frame.

Capturing footage from a DV source, however, yields a 720 x 480 frame. The

difference between the D1 spec and the DV spec is only 6 vertical pixels.

Many compression algorithms, including DV compression, prefer image

sizes to be a multiple of 16. By shaving off the 6 pixels from a D1 resolution,

the DV format was able to have a native resolution with a multiple of 16.

For PAL video images, frames are always 720 x 576, regardless of video

source. Because PAL’s vertical resolution, 576, is a mul tiple of 16, no

change is necessary for DV compression.

Frame Rate

Video is essentially a sequence of images flashed on the screen in rapid

succession, giving the illusion of motion. The number of frames displayed

every second is known as the frame rate, and it is measured in frames per

second (fps). The higher the frame rate, the more frames per second will be

used to display the sequence of images, resulting in smoother motion. The

trade-off, however, is that higher frame rates require a higher amount of

data—or system bandwidth —to display the video.

In a broad sense, NTSC video runs at 30 fps, and PAL runs at 25 fps. In

actuality NTSC runs at 29.97 fps. The reason for the odd frame rate dates

back to the transition from black and white television to color TV signals,

where the 29.97 fps rate was chosen to ensure backwards compatibility

with existing television sets. The fractional rate is more of a mathematical

issue than anything else—there are still 30 frames, but they run 0.1 percent

slower than actual time, giving you a frame rate of 29.97 fps.