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), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...
, is the number of times per second that a
raster-scan video system transmits or displays a complete horizontal line, as opposed to
vertical scan rate, the number of times per second that an entire screenful of image data is transmitted or displayed.
Cathode ray tubes
Within a
cathode-ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...
(CRT), the horizontal scan rate is how many times in a second that the
electron beam
Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy.
Mechani ...
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
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
s, 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, instantaneous phase and frequency, ...
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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