Horizontal scan rate
Encyclopedia
Horizontal scan rate, or horizontal frequency, usually expressed in kilohertz
, is the frequency
at which a CRT
moves the electron beam from the left side of the display to the right and back, and therefore describes the number of horizontal lines displayed per second. CRT timings actually include some horizontal scans before the visible display, after it, and during the travel from bottom to top (known as vertical back porch, vertical front porch, and vertical sync width, respectively, and collectively known as vertical blank time
), so the horizontal scan rate does not directly correlate to visible display lines, unless the unseen lines are also known, but it can still be used to approximate the display lines, as the total blank time is usually a small but significant portion of the total lines.
It is usually the most limiting factor of a CRT display. This limit is due to how quickly the CRT's yoke can move the electron beam from one side of the display to the other.
Compare vertical scan rate (refresh rate
), which indicates how often the electron beam is moved from the bottom of the display to the top. Given the horizontal scan rate, the refresh rate can be approximated by dividing the horizontal scan rate by the number of horizontal lines multiplied by 1.05 (since about 5% of the time it takes to scan the screen is spent moving the electron beam back to the top). For instance, a monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280 x 1024 has a refresh rate of 96,000 / (1024 x 1.05) ≈ 89 Hz (rounded down).
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, is the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
at which a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
moves the electron beam from the left side of the display to the right and back, and therefore describes the number of horizontal lines displayed per second. CRT timings actually include some horizontal scans before the visible display, after it, and during the travel from bottom to top (known as vertical back porch, vertical front porch, and vertical sync width, respectively, and collectively known as vertical blank time
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
), so the horizontal scan rate does not directly correlate to visible display lines, unless the unseen lines are also known, but it can still be used to approximate the display lines, as the total blank time is usually a small but significant portion of the total lines.
It is usually the most limiting factor of a CRT display. This limit is due to how quickly the CRT's yoke can move the electron beam from one side of the display to the other.
Compare vertical scan rate (refresh rate
Refresh rate
The refresh rate is the number of times in a second that a display hardware draws the data...
), which indicates how often the electron beam is moved from the bottom of the display to the top. Given the horizontal scan rate, the refresh rate can be approximated by dividing the horizontal scan rate by the number of horizontal lines multiplied by 1.05 (since about 5% of the time it takes to scan the screen is spent moving the electron beam back to the top). For instance, a monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280 x 1024 has a refresh rate of 96,000 / (1024 x 1.05) ≈ 89 Hz (rounded down).
Example
- Display ModeComputer display standardComputer display standards are often a combination of aspect ratio, display resolution, color depth, and refresh rate.This article describes the different display standards for computer displays.-History:...
at 1280x1024 @ 60Hz means it draws 1024 lines x 60 per second - Taking Vertical blanking intervalVertical blanking intervalThe vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
into account, it would actually draw 1024x1.05 lines for a full frame cycle - Therefore, it draws 1024x1.05x60 lines per second = 1 line per 1/(1024x1.05x60) second
- Horizontal scan rate: 1/(1024x1.05x60) sec ~= 1.55009921E-5 sec or 64.512 kHz