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Tech Stuff - Monitors/Displays: Resolution/Bandwith/Refresh Rates

It all used to be very simple. Analog outputs to lousy display quality on cramped 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!

  1. Overview: Bandwidth
  2. Refresh Rates and Frame Rates
  3. Display Color Depth
  4. Display Aspect Ratio
  5. Standard 4:3 (PC) Resolutions
  6. Standard TV/HDTV Notation
  7. Standard TV/HDTV Resolutions

Overview: Bandwidth Requirement

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 then 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, 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 number of vertical and 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)

Bandwidth Example

The raw (uncompressed) data requirement for an HDTV display of 1080p with True Color (24 bit) running at 24 frames per second (normal movie/TV rate) is:

1920 x 1080 x 24 x 24 = 1,194,393,600 bit/s or ~150 Mbytes/s

To calculate the raw bandwidth required requires that any transmission overheads, such as 8/10, are added to this number.

Refresh Rates and Frame Rates

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.

Just when there is a danger you might even understand this stuff, the industry conspires to complicate things even further. You will occasionally see the term Field Rate. This term is only applicable when used with interlaced display technology and refers to the complete 'package' of even numbered or odd-numbered lines output using this technology. Thus, when dealing with interlaced screens two Fields make a Frame or put another way the Field Rate is twice the Frame Rate. Simple really.

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, it 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).

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Display Color Depth

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 sometimes used to make them 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.

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Display Aspect Ratio (DAR)

The display aspect ratio (but most frequently written as simply '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 used 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. Some hybrid values are illustrated in the table below.

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Standard PC Resolutions

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.

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Standard TV/HDTV Notation

TV standards have 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 ratio this gives a Horizontal resolution of 1080 / 9 * 16 = 1920. The letter 'p' indicates it 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 a 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. When used to describe TV systems the most common value will be 24 indicating 24 frames per second (the normal movie,and increasingly TV, standard). Gaming systems which typically demand very high frame rates to cover very fast screen changes required by the action sequences may have values up to 120 frames per second.

Interlacing: Interlacing was used by broadcast systems to reduce flicker effects especially at lower screen refresh rates (24/25 frames per second). Its effect was to give the visual impression of double the refresh rate by drawing a partial display (odd numbered lines) and then filling in the even numbered lines. Thus, essentially confusing the human eye - in a positive way. Interlacing is mostly irrelevant to modern high speed display technology.

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Standard PC/HDTV Resolutions

These resolutions are common between computer (laptop) displays and TVs using a digital interface such as HDMI, DVI or DisplayPort.

Name Horizontal Vertical Aspect Depth Notes
1080p/1080i 1920 1080 16:9 30, 36, 48 Standard HDTV Resolution. Frequently called Full HD or HD Ready
720p 1280 720 16:9 30, 36, 48 Standard HDTV Resolution.
1366 768 ~16:9 30, 36, 48 WXGA format wrapped for HDTV
1024 768 4:3 30, 36, 48 XGA format wrapped for HDTV.
480p 854 480 ~16:9 Frequently used to carry LCD/LED projector images.
640 480 4:3 Enhanced Definition TV (EDTV). Used where NTSC broadcasts were previously provided (US/Japan) at 480p24 or 480p30 frame rates
1440p 2560 1440 16:9 30, 36, 48 Extreme Definition (XD) or Extreme High Definition (XHD) used by large displays.
2160p 3840 2160 16:9 24, 25, 30, 50, 60, 120 Ultra High Definition TV (4K UHDTV) - current standard UHD-1 (by ITU BT.2020 and SMPTE (US)).
4320p 7680 4320 16:9 24, 25, 30, 50, 60, 120 Ultra High Definition TV (8K UHDTV) - current standard UHD-2 (by ITU BT.2020 and SMPTE (US)).

Notes:

  1. Standard TV: Broadcast Standard Definition TV (SDTV) resolutions may be either 480i (640 x 480) for NTSC (US/Japan notably) with refresh rates of 24 (480i24), 30 (480i30) or 60 (480i60), or 576i (768 x 576) for PAL/SECAM (Europe and RoW) at 24 frames per second (576i24). In both cases the aspect ratio (DAR) is 4:3 and square pixels are assumed. If a pixel ratio of 12:11 (the normal) then 576i would yield a resolution of 704 x 576. Finally, when 576i is broadcast it uses 625 lines of which lines 577 to 625 do not provide image data (historically used for Teletex among other applications).

  2. UHD Naming: In the above table we quote the standard terminology for TVs, in this case 2160p and 4320p. However, unlike HD TV where this nomeclature, for example 1080p, is widely used and known, even among the general public, the term 4K (2160p) and 8K (4320p) tend to be used when describing UHD ready TVs. Retrograde step?

  3. TV Aspect Ratios: Historically TV standards defined the Vertical resolution, for example, 1080p, and aspect ratios are defined as the relationship between the horizontal and the vertical resolutions, for example, 16:9. Thus, to calculate the Horizontal resolution for 720p, given an aspect ratio of 16:9 we would use the formula (720 / 9) * 16 = 1280. We note a trend to define the aspect ratio as single decimal result rather than the two values. In such a scheme 16:9 becomes (16/9) = 1.78 (1.777 rounded up). To calculate the Horizontal resolution the formula now becomes vertical resolution x aspect ratio, for example, 720 x 1.78 = 1281.6 which is an easier calculation and close, but no cigar. Strange.

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