Horizontal scan rate, or horizontal frequency, usually expressed in
kilohertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...
, is the number of times per second that a
raster-scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image ...
video system transmits or displays a complete horizontal line, as opposed to
vertical scan rate
The refresh rate (or "vertical refresh rate", "vertical scan rate", terminology originating with the cathode ray tubes) is the number of times per second that a raster-based display device displays a new image. This is independent from frame rate ...
, the number of times per second that an entire screenful of image data is transmitted or displayed.
Cathode ray tubes
Within a CRT, the horizontal scan rate is how many times in a second that the electron beam moves from the left side of the display to the right and back. The number of horizontal lines displayed per second can be roughly derived from this number multiplied by the vertical scan rate.
The horizontal scan frequencies of a CRT include some intervals that occur during the vertical blanking interval, so the horizontal scan rate does not directly correlate to visible display lines unless the quantity of unseen lines are also known.
The horizontal scan rate is one of the primary figures determining the resolution capability of a CRT, since it is determined by how quickly the electromagnetic deflection system can reverse the current flowing in the deflection coil in order to move the electron beam from one side of the display to the other. Reversing the current more quickly requires higher voltages, which require more expensive electrical components.
In
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog ...
systems the horizontal frequency is between 15.625 kHz and 15.750 kHz.
Other technologies
While other display technologies such as
liquid-crystal displays do not have the specific electrical characteristics that constrain horizontal scan rates on CRTs, there is still a horizontal scan rate characteristic in the signals that drive these displays.
References
Television technology
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