Handbook for Sound Engineers

The Fundamentals of Display Technologies 1581

Designed for analog NTSC television, interlaced
scanning is where each picture, referred to as a frame, is
made up of two separate subpictures, referred to as
fields, so two fields make up a frame. An interlaced
picture is drawn on the screen in two passes, by first
scanning the horizontal lines of the first field and then
retracing to the top of the screen and scanning the hori-
zontal lines for the second field in between the first set.
Field 1 consists of lines 1 through 262½, and field 2
consists of lines 262½ through 525. A television scans
60 fields every second (thirty odd and thirty even).
These two sets of thirty fields are combined to create a
full frame every 1 » 30th of a second, resulting in a
display of thirty frames per second. Drawbacks to inter-
laced scanning compared to progressive scanning
include lower resolution, flicker, aliasing, and image
artifact quality issues.

Progressive scan differs from interlaced scan in that
each line (or row of pixels) in the signal is drawn in a
sequential order rather than an alternate order, as is done
with interlaced scan. In short, with progressive scan, the
image lines (or pixel rows) are scanned in numerical
order (1, 2, 3) down the screen from top to bottom,
instead of in an alternate order as done in interlaced
scanning. By progressively scanning the image onto a
screen every 60th of a second rather than “interlacing”
alternate lines every 30th of a second, a smoother, more
detailed, image can be produced on the screen. The
benefit is the viewing fine details, such as text, and is
also less susceptible to interlace flicker and basically
eliminates aliasing on the edges of objects in a picture.
The drawback to progressive scan is that it requires
more bandwidth to display the images onscreen.

44.5 Aspect Ratios and Screen Formats

Aspect ratio refers to the shape of the images we see on

screen, but just what comprises an aspect ratio? Aspect

ratio is typically described as the ratio of screen width

to screen height. There are two common aspect ratios.

The first is that of a standard televisio, which has a 4:3

(referred to as 4 by 3) aspect ratio. Also note that the

television aspect ratio is listed as 1.33:1. This is another

way of listing aspect ratios—dividing the width by the

height (e.g., 4 » 3 = 1.33). This is referred to as 1.33:1 or

1.33 to 1. A widescreen display, such as a plasma panel,

will usually have a 16 by 9 aspect ratio (16:9). Since

16/9 = 1.78, the aspect ratio is also known as 1.78:1 or

1.78 to 1.

44.5.1 Common Aspect Ratios

• 4 × 3 (1.33:1). This is the standard television format
  used throughout the second half of the 20th century.
  This is both typical computer and NTSC broadcast
  video.
  Note: 1280 × 1024 is actually 5:4 aspect, not 4:3.


• 16 × 9 (1.78:1). This is the common format for wide-
  screen DVD movies, HDTV (720p and 1080i) and
  widescreen computer resolutions (1280×720,
  1920 ×1080, etc.).


• 13 × 7 (1.85:1). This is the standard aspect ratio for
  theatrical release film prints.


• 29 × 9 (Cinemascope–2.35:1). A very wide screen
  format used for theatrical release movies, and some
  new DVDs.

44.6 Scaling

In the realm of digital display technologies, there is

quite often a mismatch between the resolutions of the

display itself and the signals or sources coming into the

display. This mismatch necessitates the incorporation of

a process known as scaling or scan conversion.

By definition, a digital display can also be referred to

as a fixed matrix display, with a finite number of hori-

zontal and vertical pixels—e.g., 1024×768.

In many instances the actual resolution of the input

signals and the physical resolution of the display do not

match. The mismatch requires what is known as scaling.

While a scaler can be an outboard device, in most

instances today, it is built into the display device.

Scaling, which is sometimes called scan conversion,

refers to a process of taking a higher-resolution signal,

and modifying it to be displayed on a lower-resolution

device, or a lower-resolution signal, and modifying it to

be displayed on a higher-resolution device.

44.6.1 Analog Image Display

In an analog display, such as a CRT, scan conversion is

not required, because the output of the display is infi-

nitely adjustable to match the input signal entering the

display.

The pixels, more properly referred to as rare earth

phosphor spots, are adjusted in width and position on

the CRT to match the source image.