Technical information

by Forest Key and Chris Hock

When working with compressed movies in a format like Flash Video, the

more frames that have to be displayed in a second the higher the file size.

To manage the final file size, you have to lower either the frame rate or data

rate. If you lower the data rate and leave the frame rate unchanged, the

image quality is reduced to yield a smaller file size. If you lower the frame

rate and leave the data rate unchanged, the file size is reduced but the

movie might appear to stutter and motion may look less fluid than desired.

Whenever the frame rate is reduced, it is always a good idea to use an

evenly divisible ratio of the original frame rate. If your source has a frame

rate of 24 fps, then you should drop the frame rate to 12 fps, 8 fps, 6 fps, 4

fps, 3 fps, or 2 fps. If the source frame rate is 30 fps, in most cases you can

adjust the frame rate to 30 fps, 15 fps, 10 fps, 6 fps, and so on. If your video

is more than 10 minutes long, then audio will drift noticeably out of synch if

you do not adhere to the 29.97 fps rate or an accurate even division for

lower frame rates (such as 29.97/2 = 14.98).

Pixel Aspect Ratio

The D1/DV NTSC and PAL specification specify non -square pixels (often

called D1 aspect ratio), while computer monitor pixels are square. D1 pixels

are vertically shorter. For this reason when looking at a D1 video image on a

computer monitor, the images will appear to be squashed vertically—

making actors appear to be shorter. When this image is output and

displayed on a broadcast monitor, the pixels are wider than they are tall

and will show perfectly normal (see Figure 1).

Figure 1: The same video image displayed on a television monitor (left) and computer

monitor (right). Note the image looks vertically compressed on the computer monitor,

but normal on the television monitor.

For this reason video images that are intended for display on computer

monitors must be pixel aspect corrected by scaling the image to a valid 4:3

aspect ratio. For NTSC, the full square pixel resolution is 720 x 540 (vertical

compensation), and for PAL it is 768 x 572 (horizontal compensation).

Commonly used final video display resolutions on the Internet include 640 x

480, 512 x 384, 320 x 240, and 160 x 120.

Most video editing applications compensate for the pixel aspect ratio

discrepancy by scaling the video image in real time while rendering it on the

computer monitor. This is done because eventually the images are intended

to return to television monitors for final display, and scaling the actual

pixels in the video file would needlessly introduce a subtle distortion from

the scaling operation. However, for web display, this real -time compensation

is not a valid approach, given that the video sequence is destined to be