Specifications
5
A Digital Video Primer
Resolution
e quality of the images you see in lm and video is not only dependent upon frame rate. e
amount of information in each frame, or image resolution, is also a factor. All other things being
equal, a higher resolution results in a better quality image.
e resolution of analog video is represented by the number of scan lines per image, or, the num-
ber of lines the electron beam draws across the screen or vertical resolution.
• NTSC is based on 525 vertical lines of resolution, displayed as two interlaced elds. However,
some of these lines are used for synchronization and blanking, so only 486 lines are actually
visible in the active picture area. e blanking part of a television signal can be seen as the
black bars above and below the active area.
• PAL is based on 625 vertical lines of resolution, displayed as two interlaced elds. As with
NTSC, some of these lines are used for synchronization and blanking, so only 576 lines are
actually visible in the active picture area.
Horizontal resolution in analog video refers to the number of black-to-white transitions or the
detail that can be displayed on each scan line. e horizontal resolution of video equipment is
usually described in lines of resolution to depict an equivalent resolution to the actual lines of
resolution in the vertical domain. Be careful when comparing the horizontal resolution of an
analog signal (NTSC or PAL) to a digital signal. With an analog signal, you are really looking at
bandwidth (or frequency response), which translates to the sharpness of an image or how much
detail can be seen. With digital video, the horizontal resolution is more easily measurable, because
there are a xed number of pixels.
A section of a resolution chart expanded to show how resolution degrades with DV25 and MPEG-2 compression,
and after being transferred to VHS tape.
Analog signals have no limitation on horizontal resolution, and many high-end studio cameras
can produce images with very high horizontal resolution (greater than 720 lines). However, when
the signal is recorded on an analog tape, processed, or run through a broadcast transmitter, you
eectively reduce the horizontal resolution. In general, the horizontal resolution of VHS is con-
sidered to be ~250 lines, and SVHS is considered to be ~410 lines.
Resolution for digital images, on computer displays and digital television sets, for example, is
represented by a xed number of individual picture elements (pixels) on the screen, and is oen
expressed as a dimension: the number of horizontal pixels by the number of vertical pixels. For
example, 640 x 480 and 720 x 486 are full-frame SD resolutions, and 1920 x 1080 is a full-frame
HD resolution.
In the U.S., there is one FCC-approved standard for analog video and 18 standards for digital TV.
Currently, the three most commonly encountered digital resolutions are:
• 480p* or 480 lines of vertical resolution, scanned progressively
• 720p or 720 lines of vertical resolution, scanned progressively
• 1080i or 1080 lines of vertical resolution, interlaced
* Note that when the letter P is used to denote progressive in the 24P video format, it is capitalized; but when it
is used to refer to television display resolution, it is typically lower case, such as 720p.