Basic Video Concepts & Common Aspect Ratios

Basic Video Concepts

The term resolution is used to quantify a display device’s ability to reproduce fine detail in a video image. In a solid state imaging device (LCD, DILA, DLP), the resolution is simply the number of pixels on the imaging elements. In a raster scanned CRT-based device,it is a very different mechanism and there is a significant difference between horizontal and vertical resolution.

Vertical Resolution of a CRT:

Below we have a diagram that shows how an electron beam is “scanned” across a picture tube faceplate to form a NTSC video image. The technique of interlacing the images was developed to minimize that bandwidth of the signal and reduce flicker in the display.

The maximum vertical resolution is simply the number of scan lines visible in the display. This number is the number of horizontal scan lines (525) minus the retrace lines (43). Thus the maximum vertical resolution of a NTSC display is 525 -43 = 482 lines. With an HDTV image, the image is swept faster with the result of more lines of resolution.

Horizontal Resolution of a CRT:

Horizontal resolution is a completely different mechanism in a CRT-based device. The horizontal resolution is a function of how fast you can turn the electron beam on and off.

Note: By convention, video resolution is measured in picture heights, whether it is vertical resolution or horizontal resolution. So the horizontal resolution is the number of resolvable vertical lines across a width of the display equal to the picture height. For a 4:3 display this is equivalent to 75% of the resolvable lines across the full width of the display.

Understanding Aspect Ratios

The Four Most Common Aspect Ratios

transferring film to video requires that a telecine (video transfer) operator crop a smaller 4:3 section out of a widescreen movie while panning around following the movie's action. This technique, when properly done, actually works pretty well, but not everyone likes the artistic compromise of "throwing away part of the director's vision".

Not the least of which is the film directors themselves, and one of the first to really object to this process was Woody Allen. In 1979, when his film Manhattan was being transferred for television release, he steadfastly refused to have it panned and scanned. He insisted that the feature be shown with the widescreen aspect ratio intact, and this lead to the technique of "letterboxing".

Letterboxing, a method where the middle of a 4:3 image is filled with a smaller, but wider, aspect ratio image, may have had the blessing of Hollywood directors but was originally shunned by the viewing public. The objection was the black bars on the top and the bottom of the picture, people just didn't like them.

Today, letterboxing has gained much broader acceptance and you can find it available from sources such as prerecorded tapes (occasionally), broadcast television (occasionally), on cable and DSS (AMC and other movie channels broadcast in letterbox), on laserdisc (fairly common), and DVD releases (very common).

So, what about displaying letterbox material with a projection display? On a standard 4:3 display, the situation is pretty simple, letterboxed software can be seen basically one way: as a stripe across the center of the display with black bars top and bottom. On a widescreen display, you can do something different. The letterbox section of the frame can be "zoomed into" so that the image fills the wider screen essentially eliminating the black bars.

What is interesting about this technique is that it is conceptually similar to what is done in professional cinemas with standard widescreen releases with "matting". Our diagrams following at the end of this chapter illustrate this. By zooming the letterbox section in to fill the screen, the audience simply sees a widescreen image.

The main difference between video display and film display, however, is the way the zooming is done. In a movie theater, an optical zoom lens is used. In a CRT-based video display, it
is done by increasing the size of the picture electronically in the picture tube, but with an LCD/DLP-based device it is again done with an optical lens.

(Note: some solid state projectors does "zoom" electronically, the SONY VPLVW10HT is one.)

Are there any drawbacks to letterboxing on a 4:3 display as a general technique ?

As we mentioned, with the right equipment, letterboxed software can be zoomed to fill a wide screen, but you should know that this comes at a certain price. The issue is loss of vertical resolution. Let's take a matted widescreen film frame, as an example.

There is finite amount of resolution in a 35MM frame and, unfortunately, a great deal is taken up with matting. In video, the same principle applies. In a standard video frame there is some 480 lines of vertical resolution.