Alan Freedman -- The Computer Language Company - Computer Desktop Encyclopedia
Computer Desktop Encyclopedia
Longest-Running Tech Reference on the Planet

A CDE Definition

You'll love The Computer Desktop Encyclopedia (CDE) for Tech Term of the Day (TTOD)


vertical scan frequency

Also called "refresh rate," it is the number of times an entire CRT screen was refreshed, or redrawn, per second. Measured in Hertz, display systems typically ranged from 56 Hz to over 100, but at least 70 was recommended to help prevent eyestrain.

The earliest CRTs were unable to support both the highest resolution along with a high refresh rate; however, later CRTs provided adequate rates at all resolutions. Vertical scan frequency is not an issue with LCD displays because they do not use phosphors that have to be continuously energized. However, the data from the frame buffer is continuously sent to the LCD panel in order to display animation and video. Thus, the flat panel does have a maximum refresh rate to support motion, but not to refresh static images. Contrast with horizontal scan frequency. See interlace.


To illuminate a screen by displaying all odd lines in the frame first and then all even lines. Interlacing uses half frames per second (fields per second) rather than full frames per second.

The interlace method was developed for TV broadcasting because the allotted bandwidth for TV channels in the 1940s was not sufficient to transmit 60 full frames per second. It was decided that interlacing with 60 half frames was visually better than 30 non-interlaced full frames.

Interlace Vs. Progressive Scan (I vs. P)
Interlaced screens display odd lines first: 1-3-5, etc.; then even lines: 2-4-6, etc. Non-interlaced "progressive scan" screens display lines consecutively: 1-2-3-4-5-6, etc.

All old tube TVs (CRTs) were interlaced. Very old CRT computer monitors were interlaced at their highest resolution and progressive at lower resolutions. Today, some digital TV standards are interlaced, such as the high-definition 1080i format, and 1080i material is commonly transmitted by the TV networks. HDTV sets support both interlace and progressive scan methods (see HDTV and DTV). See deinterlace, vertical scan frequency, 4K 3D TV and interlaced GIF.

The Difference
In one second, interlacing displays 30 sets of odd lines and 30 sets of even lines, resulting in 60 fields per second. For every half frame of interlaced scan, progressive scan displays a full frame. Modern DVDs and monitors convert the 60 fields in interlaced NTSC content to 60 frames by interpolating the missing lines.

Personal Use Only

Before/After Your Search Term
vertical market softwarevertical scan rate
vertical MOSvertical search
vertical MOSFETvertical search engine
vertical portalvertical software
vertical recordingvertical solution
vertical redundancy checkvertical sync
vertical refreshVerticalNet
vertical resolutionvertices
vertical retracevery high frequency
vertical scalingvery large instruction word

Terms By Topic
Click any of the following categories for a list of fundamental terms.
Computer Words You Gotta KnowSystem design
Job categoriesUnix/Linux
Interesting stuffPersonal computers
InternetIndustrial Automation/Process Control
Communications & networkingAssociations/Standards organizations
HistoryDesktop publishing
ProgrammingHealthcare IT
System design