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It all used to be very simple. Analog outputs to lousy display quality on crampled displays at prices you could afford or ferociously expensive high-end monitors. Now we have digital outputs and endless varieties of bandwidth hungry resolutions. This one-pager is meant to make it all easier - dream on!
Almost all current monitors/displays use an uncompressed full screen refresh when updating the display. The exception is HDTV which relies on a compressed video stream at a receiver which them uncompresses it to drive the display using an uncompressed interface which may be either internal (embedded within the TV) or external - typically connected using HDMI - such as a DVD/Blue-Ray Player or a set-top box or even a specially equipped PC. For monitors or displays which use a digital interface (as opposed to an analog VGA interface) this means that the bandwidth of the connection between the host (PC, DVD Player etc.) and the monitor must be capable of drawing the entire screen (a frame) at its resolution (defined by the vertical pixels, horizontal pixels) and color depth (defined by the number of bits in each color for each pixel). To calculate the bandwidth required for a single frame the following equation is used:
Horizontal pixels x Vertical pixels x pixel (color) depth
In order to be able to use the display interface for a variety of purposes including motion it is necessary to update the display (one frame at a time) multiple times per second - this is known typically as the Refresh Rate (for historical reasons) or occasionally as the Frame Rate and is expressed in Hertz (number of times per second). Thus, the total bandwidth required by the display/monitor interface now becomes:
Horizontal pixels x Vertical pixels x pixel (color) depth x frame rate (in Hertz)
To understand the relationship between the Refresh Rate (the number of times the display is redrawn expressed in Hertz) and Frame Rate (the number of times the display changes expressed in Frames per second (FPS)) we need to understand - roughly - how the graphic/video susbsystem typically works. The graphic card/chip in the PC or DVD player maintains a screen or display buffer. This buffer is continuously output at the refresh rate (typically 60Hz = 60 times per second). Independently, applications write data to the screen buffer, usually via some form of graphic interface (API) or library, at a rate determined by the application. For example, a movie application will typically update the screen buffer at 24 frames per second (FPS), while a spreadsheet application might do so only when a cell's content changes, say, once per second or less. As long as the rate of change to the screen buffer (FPS) is below the refresh rate of the screen (Hz) all is well and no visible data is lost. If, however, a gaming application writes data at, say, 85 frames per second (FPS) to cover fast moving action while the screen refresh is 60Hz (60 times per second) then some visual data will be lost - some screen buffer updates will be overwritten by the application before the display refresh occurs. This results in what is typically called screen 'tearing' or other more exotic, and confusing, terms.
Note: When CRT's were the dominant display technology the term Refresh Rate was literally the time necessary to refresh the display based on the decay time of the screen phosphor. In modern LCD/LED screens this is no longer a requirement. Instead, is has become widely used to describe the rate at which the display content is re-written or updated from an external buffer (in a PC, DVD player, set-top box). The term Frame Rate more correctly should be used but this can lead to some confusion especially when discussing computer applications. We stick with the term Refresh Rate 'cos we remember CRTs and because, other than embedded TV applications, there is only a vague connection between the rate at which the display is updated (what we call the Refresh Rate) and the rate software writes to the internal screen or display buffers (what we call the Frame Rate).
The color depth defines the number of bits used to define the color of each pixel. The color depth normally contains Red, Green and Blue (RGB)components though sneaky games are some times used to make then fit exactly. Some of us can remember when 8 bit color (giving 256 colors) was the norm for PC screens. The industry rapidly moved through 16 bit color (giving 65K colors) to what is known as True color using 24 bits (giving 16M colors). Modern systems and displays (including HDTV displays and the HDMI interface) now support Deep Color using either 30, 36 or 48 bit colors and indeed some graphics cards can support up to 64 bit colors.
The aspect ratio defines the relationship between the number of horizontal pixels and the number of vertical pixels. Historically, the TV standard (standard definition TV) used an aspect ratio of 4:3. Thus, for compatability reasons the computer standard (and most digital point-and-shoot cameras) also uses 4:3. To illustrate; if the number of horizontal pixels is 1024 then the number of vertical pixels would be 1024 / 4 * 3 = 768; if the number of horizontal pixels is 800 then there will always be 600 vertical pixels.
Note: Most PC display resolutions typically give either both the Horizontal and Vertical resolutions, for example, 800 x 600 or occasionally only the Horizontal resolution from which the Vertical resolution can be computed using the 4:3 aspect ratio. In the case of TV resolutions, for historical analog reasons, only the Vertical resolution is defined and the Horizontal resolution is computed from it.
A significant change ocurred when the High Definition TV (HDTV) aspect ratio was set at 16:9 to accomodate wide-screen formatted film material. In this standard if there are 1080 Vertical pixels (recall that TV standards always use the Vertical pixel as the dominant value) there will be 1080 / 9 * 16 = 1920 Horizontal pixels. Increasingly, modern PCs/laptop displays are now moving to either 16:9 aspect ratios to align with HDTV standards or hybrid values. These hybrid values are illustrated in the tables below.
The following table shows a variety of PC display resolutions. In most cases these may be carried across a 15 pin VGA (analog), DVI (digital or analog) or DisplayPort (digital) connector.
| Name | Horizontal | Vertical | Aspect | Depth | Notes |
| VGA | 640 | 480 | 4:3 | 8, 16, 24 | |
| WVGA | 800 | 480 | 15:9 | 8, 16, 24 | Common with LCD or LED projectors as is a 854 x 480 (a 16:9 aspect ratio) format called FWVGA or 480p |
| SVGA | 800 | 600 | 4:3 | 8, 16, 24 | |
| XGA | 1024 | 768 | 4:3 | 8, 16, 24 | |
| SXGA | 1280 | 1024 | 4:3 | 8, 16, 24 | WXGA+ | 1366 | 768 | ~16:9 | Frequently found in newer 14/15" laptops and also used by 720p. | SXGA+ | 1400 | 1050 | 4:3 | 8, 16, 24 | Frequently found in older 14/15" laptops, mostly displaced by WXGA+ or 720p or higher resolutions. |
| UXGA | 1600 | 1200 | 4:3 | 8, 16, 24 |
Other standards do exist but are mostly attempts to create wide screen (16:9 aspect ratio) and are not common.
TV standards have a evolved their own notation to describe certain characteristics. Thus, today the most common HDTV (and increasingly computer) display will have its resolution defined as 1080p or 1080p60 or even 1080/60p. Because it is a TV based defintion (see note above) the 1080 defines the Vertical resolution. Given that modern TVs have a 16:9 aspect ration this gives a Horizontal resolution of 1080 / 9 * 16 = 1920. The letter 'p' indicates is supports a progressive scan (one complete frame at a time starting with the first line and progressing sequentially to the last line) and is always the case for displays connected to PC or laptops (and increasingly HDTV systems). This letter can also be 'i' indicating the display is interlaced, a TV only system which indicates that the frame is output in two parts or fields, first the odd numbered lines then the even numbered ones. Finally, when present, the value 60 defines the Refresh Rate (frequently, and more correctly, referred to as the frame Rate - see note.). With computer displays this value is most frequently 60 but can be 24, 25, 30 or 50 for HDTVs.
These resolutions are common between computer (laptop) displays and TVs using a digital interface such as HDMI, DVI or DisplayPort.
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