How Video Resolution Works

Where the eye meets the screen...

High-definition televisions in a row
Mario Tama / Getty Images

When you shop for a TV, Blu-ray Disc player, DVD player, or camcorder, the salesperson always seems to hype the term resolution. It's lines this and pixels that and so forth... After a while, none of it seems to make sense. Here is what you need to know.

What Video Resolution Is

A video image is made up of scan lines (analog video recording/playback devices and TVs) or pixels (digital recording/playback devices and LCD, Plasma, OLED TVs). The number of scan lines or pixels determines the recorded or displayed resolution. 

Unlike film, in which the whole image is displayed on a screen at once, video images are displayed differently.

How Video Images Are Displayed

A TV image is composed of lines or pixel rows across a screen starting at the top of the screen and moving to the bottom. These lines or rows can be displayed in two ways.

  • The first way that images can be displayed is by splitting lines into two fields in which all of the odd numbered lines are displayed first and then all of the even-numbered lines are displayed interlacing or interlaced scan.
  • The second method, which is used in the digital video environment, is progressive scan. Instead of displaying the lines in two alternate fields, progressive scan allows the lines to displayed sequentially. This means that both the odd and even numbered lines are displayed in numerical sequence.

CRT TVs (TVs that use picture tubes) can be made to display interlaced or progressive generated images, but flat-panel TVs (LCD, Plasma, OLED) can only display images progressively - when faced with an incoming interlaced image signal, a flat panel TV will re-process the interlaced video information so that it can be displayed progressively.

Analog Video - The Starting Point

When it comes to how we look at video resolution, analog video is the starting point. Although most of what we watch on TV is from digital sources, some analog sources and TVs are still in use.

In analog video, the larger the number of vertical scan lines, the more detailed the image. However, the number of vertical scan lines is fixed within a system. Here is a look at how resolution works in the NTSC, PAL, and SECAM analog video systems.

  • NTSC is based on a 525-line, 60 fields/30 frames-per-second, at the 60Hz system for transmission and display of video images. This is an interlaced system in which each frame is scanned in two fields of 262 lines, which is then combined to display a frame of video with 525 scan lines. NTSC-based countries include the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.
  • PAL which was the most widely used format outside of NTSC-based countries for analog television broadcasting and video display, is based on a 625 line, 50 field/25 frames a second, 50HZ system. The signal is interlaced, like NTSC into two fields, composed of 312 lines each.The distinguishing features of PAL are a better overall picture than NTSC because of the increased amount of scan lines, and since the color was part of the standard from the beginning, color consistency between stations and TVs are much better.​ Countries that use the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.
  • SECAM is the "outlaw" of analog video standards. Like PAL, it is a 625 line, 50 field/25 frames per second interlaced system, but the color component is implemented differently than in either PAL or NTSC. Countries that have used the SECAM system include France, Russia, Eastern Europe, and some parts of the Middle East.

The number of scan lines, or vertical resolution, of NTSC/PAL/SECAM, are constant in that all analog video recording and display equipment conforms to the above standards. However, in addition to vertical scan lines, the amount of dots displayed within each line on the screen contributes to a factor known as horizontal resolution which can vary depending on both the ability of a video recording/playback device to record the dots and the ability of a video monitor to display dots on a screen.

Using NTSC as an example, there is 525 scan lines (vertical resolution) total, but only 485 scan lines are used to comprise the basic detail in the image (the remaining lines are encoded with other information, such as closed captioning and other technical information). Most analog TVs with at least composite AV inputs can display up to 450 lines of horizontal resolution, with higher-end monitors capable of much more.

The following is a list of analog video sources and their approximate horizontal resolution specifications. Some variations listed are due to the range of quality of different brands and models of products using each format.

  • VHS/VHS-C - 220 to 240 lines
  • BETA - 250 lines
  • 8mm - 250 to 280 lines
  • SuperBETA - 270 to 280 lines
  • Analog TV Broadcast - 330 lines
  • Analog Cable TV - 330 lines
  • Standard Digital Cable - Up to 480 lines (aks 480i) via analog video output. 720x480 pixels (480i or 480p) via HDMI (digital) video output.
  • S-VHS/S-VHSC - 400 lines
  • DVD-R/-RW/+R/+RW - 250 to 400+ lines (Depends on recording mode and compression used).
  • Laserdisc - 400 to 425 lines
  • Hi8 - 380 to 440 lines
  • Digital 8 - 400 to 500 lines
  • miniDV - 400 to 520 lines
  • microMV - 500 lines
  • ED BETA - 500 lines
  • Commercial DVD - 480 lines (aka 480i) via analog video output. 720x480 pixels (480i or 480p) via HDMI (digital) video output.

As you can see, there is quite a difference in the resolution that different video formats conform to. VHS is on the bottom end, while miniDV and DVD (when using an analog video output) represent the highest analog video resolutions that have been commonly used.

However, another factor that has to be considered is how the resolution is stated for Digital and HDTV.

Just as in analog video there is both a vertical and horizontal component to digital video resolution. However, the total image resolution displayed in DTV and HDTV is referred to in terms of the number of pixels on the screen rather than lines. Each pixel is composed of a red, green, and blue subpixel.

Digital TV Resolution Standards

In current digital TV standards, there are a total of 18 video resolution formats that are approved by the FCC for use in the U.S. TV broadcast system (also used in many cable/satellite specific channels). Fortunately, for the consumer, there are only three that are commonly used by TV broadcasters, but all HDTV tuners are compatible with all 18 formats.

The three resolution formats used in digital and HDTV are:

  • 480p - Represented by 720 pixels running across the screen and 480 pixels running from top to bottom. Each row of pixels is displayed progressively. The total number of pixels displayed on the screen is 345,600.
  • 720p - Represented by 1280 pixels running across the screen and 720 pixels running from top to bottom. Each row of pixels is displayed progressively. The total number of pixels displayed on the screen is 921,600.
  • 1080i - Represented by 1,920 pixels running across the screen and 1,080 pixels running from top to bottom. Each row of pixels is displayed in an interlaced fashion (all odd numbered rows, followed by all even numbered rows). The total number of pixels displayed on the screen is 2,073,600 (half the pixels displayed at a time).

1080p

Although not used in TV broadcasting (up to this point), the Blu-ray disc format, streaming, and some cable/satellite services are able to deliver content in 1080p resolution

1080p represents 1,920 pixels running across the screen, and 1,080 pixels running from top to bottom, Each horizontal pixel row is display progressively. This means that all 2,073,600 pixels are displayed in one action. This is similar to how 720p is displayed but with an increased number of pixels across and down the screen, and although the resolution is the same as 1080i,  not all the pixels are displayed at the same time.

HDTV vs EDTV

Even though you may be inputting an image of specific resolution into your HDTV, your TV may not have the ability to reproduce all the information. In this case, the signal is often reprocessed (scaled) to conform to the number and size of the pixels on the physical screen.

For example, an image with a resolution of 1920x1080 pixels can be scaled to fit 1366x768, 1280x720, 1024x768, 852x480, or another available pixel field per the TV's processing capability. The relative loss of detail actually experienced by the viewer will depend on factors such as screen size and viewing distance from the screen.

When purchasing a TV, it is not only important to make sure that you can input 480p, 720p, 1080i, or other video resolutions you might have access to, but you must also consider the pixel field of the TV (and whether upconversion/downconversion is used).

To go into further detail, a TV that has to downconvert an HDTV signal (such as 720p, 1080i, or 1080p) to a pixel field of 852x480 (480p) for example, are referred to as EDTVs and not HDTVs. EDTV stands for Enhanced Definition Television.

Resolution Requirement For True HD Image Display

If a TV has a native display resolution of at least 720p, it qualifies as an HDTV. Most LCD and Plasma TVs in use, for example, have a native display resolution of 1080p (Full HD). So, when faced with a 480i/p, 720p, or 1080i input signal, the TV will scale the signal to 1080p to display it on the screen.

Upscaling and DVD

Although standard DVD is not a high-resolution format, most DVD players have the ability to output a video signal in  720p, 1080i, or 1080p via upscaling. This allows the DVD player's video output to more closely match the capabilities of an HDTV, with more perceived image detail. However, keep in mind that the result of upscaling is not the same as native 720p, 1080i, or 1080p resolution, it is a mathematical approximation.

Video upscaling works best on fixed pixel displays, such as LCD or Plasma sets, the upscaling may result in harsh images on line-scanned based CRT and CRT-based Projection sets.

Beyond 1080p

Until 2012 1080p video resolution was highest available for use in TVs, and still delivers excellent quality for most TV viewers. However, with the demand for ever larger screen sizes, 4K Resolution (3480 x 2160 pixels or 2160p) was introduced to deliver an even more detailed refined image, especially in combination with other technologies, such as HDR brightness enhancement and WCG (wide color gamut). Also, just as upscaling is used to increase the visible detail for lower resolution sources on HDTVs, a 4K Ultra HD TV can upscale signal sources so that it looks better on its screen.

4K content is currently available from Ultra HD Blu-ray Disc and select streaming services, such as Netflix, Vudu, and Amazon.

Of course, just as millions of consumers are getting used to 4K Ultra HD TVs, 8K Resolution (7840 x 4320 pixels - 4320p) is on the way.

Resolution vs Screen Size

One thing to take into consideration is that with digital and HD flat-panel TVs the number of pixels for a specific display resolution does not change as the screen size changes. In other words, a 32-inch 1080p TV has the same number of pixels on the screen as a 55-inch 1080p TV. There are always 1,920 pixels running across the screen horizontally, per row, and 1,080 pixels running up and down the screen vertically, per column. This means that the pixels on 1080p 55-inch TV will be larger than the pixels on a 32-inch 1080p TV in order to fill the screen surface. This means that as the screen size changes, the number of pixels per inch changes.

The Bottom Line

If you are still a little confused about video resolution, you are not alone. Remember, video resolution can be stated either in lines or pixels and the number of lines or pixels determines the resolution of the source or TV. However, don't get too caught up in all the video resolution numbers. Look at it this way, VHS looks great on a 13-inch TV, but "crappy" on a big screen.

In addition, the resolution isn't the only thing that contributes to a good TV image. Additional factors, such as color accuracy and how we perceive color, contrast ratio, brightness, maximum viewing angle, whether the image is interlaced or progressive, and even room lighting all contribute to the quality of the picture you see on the screen.

You can have a very detailed image, but if the other factors mentioned aren't implemented well, you have a lousy TV. Even with technologies, such as upscaling, the best TVs can't make a poor input source look good. In fact, ordinary broadcast TV and analog video sources (with their low resolution) sometimes look worse on an HDTV than it does on a good, standard, analog set.