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Horizontal scan frequency is the number of horizontal scan lines a display or video signal generates each second. It is measured in hertz (Hz) or, far more commonly, kilohertz (kHz). On a CRT it is also called the horizontal sync rate or line rate, and in digital display specifications it usually appears as “horizontal frequency.” It describes timing, not the number of pixels you see across the screen.
What the term means
A raster display builds an image one horizontal line at a time. Each line starts with a synchronization pulse, and the frequency at which those pulses occur is the horizontal scan frequency. A vendor glossary from Tektronix defines it as “the frequency at which horizontal sync pulses start the horizontal retrace for each line.” Christie Digital’s glossary describes horizontal frequency as “the frequency at which scan lines are generated, which varies amongst sources,” and lists horizontal scan rate and line rate as alternate names.
Because a single frame or field contains hundreds of lines, the horizontal figure is always numerically much larger than the vertical refresh rate. A display refreshing 60 times per second may still be drawing tens of thousands of lines per second.
Horizontal scan frequency versus refresh rate and resolution
These three terms are often confused, but they answer different questions.
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| Term | What it counts | Typical unit | Example of what it tells you |
|---|---|---|---|
| Horizontal scan frequency | Line timing cycles per second | kHz | How fast the beam or scan moves from line to line |
| Vertical refresh rate | Complete frames or fields per second | Hz | How often the whole picture is redrawn |
| Horizontal resolution | Pixels or detail across one line | Pixels | How much detail fits on a line |
The first two are linked by the timing of the video mode: the number of lines per frame or field, together with how often frames or fields are sent. Resolution is a separate property. A signal can have a high horizontal frequency and still show limited detail if the bandwidth or the display cannot resolve it, and a mode’s visible pixel count does not by itself reveal its line timing.
How the numbers relate
A simple approximation is that horizontal frequency equals total lines per frame multiplied by frames per second. Real modes complicate this because the total line count includes blanking intervals, which are lines that carry no visible picture but are still scanned. That is why the total timing, not the visible resolution, determines the horizontal figure.
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An older educational web page uses 800 × 600 at 75 Hz as an example that yields about 60 kHz. Treat this as an illustration of the method rather than a universal result. Another mode with the same visible size and refresh rate can have a different total line count and therefore a different horizontal frequency.
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Most published figures for horizontal frequency come from analog television standards. They are useful for understanding the concept but are not current monitor specifications.
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| Figure | Context | Source and date | Scope |
|---|---|---|---|
| 15.75 kHz | NTSC horizontal sync frequency | Tektronix glossary; publication year not stated | Approximate NTSC figure |
| 15,734.263 Hz | Mandated NTSC horizontal scan frequency; vertical frequency derived by dividing by 262.5 | Video Images catalog, 1991 | Legacy NTSC analog video; not a monitor specification |
| 15.75 kHz and 800 × 600 at 75 Hz giving about 60 kHz | Teaching examples | The Video System educational page; publication year not stated | Illustrative only |
| 15.75 kHz to 31.5 kHz | Range of monitor horizontal frequencies in the past | REMark article, 1992 | Historical; not a current market range |
The NTSC figure divided by 262.5 lines per field gives a field rate of roughly 59.94 Hz, which is the familiar NTSC field rate. Note that the 262.5 figure refers to fields, not frames, which is one reason interlaced terminology needs care.
CRT and flat-panel displays
The beam-and-retrace description applies directly to cathode-ray tubes, where an electron beam physically sweeps each line and returns (retraces) to start the next one. Horizontal frequency on a CRT therefore determines how fast that sweep must run, and a monitor’s usable horizontal range is limited by its deflection circuitry.
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Flat-panel displays such as LCDs do not sweep a beam. They receive a timing signal and scan the incoming data onto a pixel grid. The horizontal frequency still describes the input signal’s line timing, but it is best understood as a property of the signal or mode rather than a physical motion inside the panel.
Interlaced and progressive signals
Interlaced video sends alternating fields, each containing half the lines of a frame. When describing an interlaced signal, state whether a figure is a field rate or a frame rate. Converting a field rate into a frame rate without saying so produces incorrect comparisons. Older sources do not always use these terms consistently.
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Checking whether a specific monitor supports a signal
A generic horizontal frequency figure cannot tell you whether a particular monitor will accept a signal. Compatibility depends on the input timing the source sends and the horizontal range the device supports. To check:
- Find the exact model number on the device or its label.
- Open the manufacturer’s specification sheet or user manual for that model.
- Look for the listed horizontal frequency range (usually given in kHz) and the supported timing or mode list.
- Compare the mode you want against that list, including the total timing where it is published, not only the visible resolution.
If the manual lists only visible resolutions and refresh rates, the manufacturer may not publish the full timing, and you should test the mode on the device before relying on it.
Evidence limits
The sources reviewed for this article establish the definition, the relationship to refresh rate, and several historical examples. They do not establish a current general range for modern monitor horizontal frequencies, and the historical sources listed above have varying or unstated publication dates. Any contemporary figure should come from a current manufacturer document for the specific product.
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Horizontal scan frequency is a timing measure expressed in kHz, distinct from vertical refresh and from horizontal resolution. Use the figure with the mode’s full timing and the device’s stated range, and treat historical NTSC numbers as examples of a legacy standard.
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