Pixel aspect ratio
Encyclopedia
Pixel aspect ratio (often abbreviated PAR) is a mathematical ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...

that describes how the width of a pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

in a digital image
Digital image
A digital image is a numeric representation of a two-dimensional image. Depending on whether or not the image resolution is fixed, it may be of vector or raster type...

compares to the height of that pixel.

Most digital imaging
Digital imaging
Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...

systems describe an image as a grid of very small but nonetheless square pixels. However, some imaging systems, especially those which must maintain compatibility with standard-definition television
Standard-definition television
Sorete-definition television is a television system that uses a resolution that is not considered to be either enhanced-definition television or high-definition television . The term is usually used in reference to digital television, in particular when broadcasting at the same resolution as...

motion pictures, define an image as a grid of rectangular pixels in which the width of the pixel is slightly different from that of its height. Pixel Aspect Ratio describes this difference.

Use of pixel aspect ratio mostly involves pictures pertaining to standard-definition television and some other exceptional cases. Most other imaging systems, including those which comply with SMPTE standards and practices, use square pixels.

## Introduction

The ratio of the width to the height of an image is known as the aspect ratio
Aspect ratio (image)
The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. That is, for an x:y aspect ratio, no matter how big or small the image is, if the width is divided into x units of equal length and the height is measured using this...

, or more precisely the Display Aspect Ratio (DAR) – the aspect ratio of the image as displayed; for TV, DAR is traditionally 4:3 (both analog and digital), with 16:9 (widescreen) now also common. In digital image
Digital image
A digital image is a numeric representation of a two-dimensional image. Depending on whether or not the image resolution is fixed, it may be of vector or raster type...

s, there is a distinction with the Storage Aspect Ratio (SAR), which is the ratio of pixel dimensions. If an image is displayed with square pixels, then these ratios agree; if not, then non-square, "rectangular" pixels are used, and these ratios disagree. The aspect ratio of the pixels themselves is known as the Pixel Aspect Ratio (PAR) – for square pixels this is 1:1 – and these are related by the identity:
SAR × PAR = DAR.

Rearranging (solving for PAR) yields:
PAR = DAR/SAR.

For example, a 640 × 480 VGA image has a SAR of 640/480 = 4:3, and if displayed on a 4:3 display (DAR = 4:3) has square pixels, hence a PAR of 1:1. By contrast, a 720 × 576 D-1 PAL image has a SAR of 720/576 = 5:4, but is displayed on a 4:3 display (DAR = 4:3).

In analog images such as film there is no notion of pixel, nor notion of SAR or PAR, but in the digitization of analog images the resulting digital image has pixels, hence SAR (and accordingly PAR, if displayed at the same aspect ratio as the original).

Non-square pixels arise often in early digital TV standards, related to digitalization of analog TV signals – whose vertical and "effective" horizontal resolutions differ and are thus best described by non-square pixels – and also in some digital videocameras and computer display modes, such as Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....

(CGA). Today they arise also in transcoding between resolutions with different SARs.

Actual displays do not generally have non-square pixels, though digital sensors might; they are rather a mathematical abstraction used in resampling images to convert between resolutions.

There are several complicating factors in understanding PAR, particularly as it pertains to digitization of analog video:
• First, analog video does not have pixels, but rather a raster scan
Raster scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...

, and thus has a well-defined vertical resolution (the lines of the raster), but not a well-defined horizontal resolution, since each line is an analog signal. However, by a standardized sampling rate, the effective horizontal resolution can be determined by the sampling theorem, as is done below.
• Second, due to overscan
Overscan
Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...

, some of the lines at the top and bottom of the raster are not visible, as are some of the possible image on the left and right – see Overscan: Analog to digital resolution issues. Also, the resolution may be rounded (DV NTSC uses 480 lines, rather than the 486 that are possible).
• Third, analog video signals are interlaced – each image (frame) is sent as two "fields", each with half the lines. Thus pixels are either twice as tall as they would be without interlacing, or the image is deinterlaced.

## Background

Video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

is presented as a sequential series of images called video frames. Historically, video frames were created and recorded in analog form. As digital display technology, digital broadcast technology, and digital video compression evolved separately, it resulted in video frame differences that must be addressed using Pixel Aspect Ratio. Digital video frames are generally defined as a grid of pixels used to present each sequential image. The horizontal component is defined by pixels (or samples), and is known as a video line. The vertical component is defined by the number of lines, as in 480 lines.

Non-square pixels do not exist in displays or elsewhere (although a square pixel on a display is not perfectly square, but negligible in this case). A non-square pixel is a mathematical construct used by video engineers to correct the differences between digital video frames and the presentation of those frames in modern digital displays.

Standard-definition television
Standard-definition television
Sorete-definition television is a television system that uses a resolution that is not considered to be either enhanced-definition television or high-definition television . The term is usually used in reference to digital television, in particular when broadcasting at the same resolution as...

standards and practices were developed as broadcast technologies and intended for terrestrial broadcasting, and were therefore not designed for digital video presentation. Such standards define an image as an array of well-defined horizontal "Lines", well-defined vertical "Line Duration" and a well-defined picture center. However, there is not a standard-definition television standard that properly defines image edges or explicitly demands a certain number of picture elements per line. Furthermore, analog video systems such as NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

and PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

, instead of employing progressively displayed frames, employ fields
Field (video)
In video, a field is one of the many still images which are displayed sequentially to create the impression of motion on the screen. Two fields comprise one video frame...

or interlaced half-frames displayed in an interwoven manner to reduce flicker and double the image rate for smoother motion.

### Analog-to-digital conversion

As a result of computers becoming powerful enough to be seriously considered as video editing tools, video Digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

s and Analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

s were made to overcome this incompatibility. In order to convert analog video lines into a series of square pixels, the industry adopted a default sampling rate at which luma
Luma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...

values were extracted into pixels. The luma sampling rate for 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

pictures was 12+3/11 MHz and for 576i pictures was 14.75 MHz.

The term Pixel Aspect Ratio was first coined when ITU-R BT.601 (commonly known as "Rec. 601") specified that standard-definition television pictures are made of lines which contain exactly 720 non-square pixels. ITU-R BT.601 did not define the exact pixel aspect ratio but did provide enough information to calculate the exact pixel aspect ratio based on industry practices: The standard luma sampling rate of precisely 13.5 MHz. Based on this information:
• The Pixel Aspect Ratio for 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

would be 10:11 as:

• The Pixel Aspect Ratio for 576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

would be 59:54 as:

SMPTE RP 187 further attempted to standardize the Pixel Aspect Ratio values for 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

and 576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

. It designated 177:160 for 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

or 1035:1132 for 576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

. However, due to significant difference with practices in effect by industry and the computational load that they imposed upon the involved hardware, SMPTE RP 187 was simply ignored. SMPTE RP 187 information annex A.4 further suggested the use of 10:11 for 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

.

As of this writing, ITU-R BT.601-6, which is the latest edition of ITU-R BT.601, still implies that the Pixel Aspect Ratios mentioned above are correct.

### Digital video processing

As stated above, ITU-R BT.601 specified that standard-definition television pictures are made of lines which contain exactly 720 non-square pixels, sampled with a precisely specified sampling rate. A simple mathematical calculation reveals that a 704 pixel width would be enough to contain a 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

or 576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

standard 4:3 picture:
• A 4:3 480-line picture, digitized with the Rec. 601-recommended sampling rate, would be 704 non-square pixels wide.

• A 4:3 576-line picture, digitized with the Rec. 601-recommended sampling rate, would be 702.915254 non-square pixels wide.

Unfortunately, not all standard TV pictures are exactly 4:3: As mentioned earlier, in analog video, the center of a picture is well-defined but the edges of the picture are not standardized. As a result, some analog devices (mostly PAL devices but also some NTSC devices) generated motion pictures that were horizontally (slightly) wider. This also proportionately applies to anamorphic widescreen
Anamorphic widescreen
Anamorphic widescreen, when applied to DVD manufacture, is a video process that horizontally squeezes a widescreen image so that it can be stored in a standard 4:3 aspect ratio DVD image frame. Compatible playback equipment can then re-expand the horizontal dimension to show the original widescreen...

(16:9) pictures. Therefore, in order to maintain a safe margin of error, ITU-R BT.601 required 16 more non-square pixels per line (8 more at each edge) to be sampled to ensure that all video data near the margins were saved.

This requirement, however, had implications for PAL motion pictures. PAL Pixel Aspect Ratios for standard (4:3) and anamorphic wide screen (16:9), respectively 59:54 and 118:81, were awkward for digital image processing, especially for mixing PAL and NTSC video clips. Therefore, video editing products chose the almost equivalent values, respectively 12:11 and 16:11, which were more elegant and could create PAL digital images at exactly 704 pixels wide, as illustrated:
• For PAL 4:3:

• For PAL 16:9:

## Inconsistency in defined pixel aspect ratio values

Commonly found on the Internet and in various other published media are numerous sources that introduce different and highly incompatible values as the pixel aspect ratios of various video pictures and video systems. (See the Supplementary sources section.)

In order to neutrally judge the accuracy and/or feasibility of these sources, please note that as the digital motion picture was invented years after the traditional motion picture, all video pictures targeted for standard definition television and compatible media, digital or otherwise, have (and must have) specifications compatible with standard definition television. Therefore, the pixel aspect ratio of digital video must be calculated from the specification of common traditional equipment rather than the specifications of digital video. Otherwise, any pixel aspect ratio that is calculated from a digital video source is only usable in certain cases for the same kind of video sources and cannot be considered/used as a general pixel aspect ratio of any standard definition television system.

In addition, unlike digital video that has well-defined picture edges, traditional video systems have never standardized a well-defined edge for the picture. Therefore, the pixel aspect ratio of common standard television systems cannot be calculated based on edges of pictures. Such a calculated aspect ratio value would not be entirely wrong, but also cannot be considered as the general pixel aspect ratio of any specific video system. The use of such values would be restricted only to certain cases.

## Modern standards and practices

In modern digital imaging systems and high-definition television
High-definition television
High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...

s, especially those which comply with SMPTE standards and practices, only square pixels are used for broadcast and display. However, some formats (ex., HDV
HDV
HDV is a format for recording of high-definition video on DV cassette tape. The format was originally developed by JVC and supported by Sony, Canon and Sharp...

, DVCPRO HD) use non-square pixels internally for image storage, as a way to reduce the amount of data that needs to be processed, thus limiting the necessary transfer rates and maintaining compatibility with existing interfaces.

## Issues of non-square pixels

Directly mapping an image with a certain pixel aspect ratio on a device whose pixel aspect ratio is different will cause the image to look unnaturally stretched or squashed in either the horizontal or vertical direction. For example, a circle generated for a computer display with square pixels would look like a vertical ellipse on a standard-definition NTSC television that uses vertically rectangular pixels. This issue is more evident on wide-screen TVs.

Pixel Aspect Ratio must be taken into consideration by video editing software products which edit video files with non-square pixels, especially when mixing video clips with different pixel aspect ratios. This would be the case when creating a video montage from various cameras employing different video standards (a relatively rare situation). Special effects software products must also take the pixel aspect ratio into consideration, since some special effects require calculation of the distances from a certain point so that they look visually correct. An example of such effects would be radial blur, motion blur, or even a simple image rotation.

## Use of pixel aspect ratio

Pixel Aspect Ratio value is used mainly in digital video software, where motion pictures are to be converted or reconditioned so that they can be used in other video systems other than the original. The video player software may use pixel aspect ratio to properly render digital video on screen. Video editing software uses Pixel Aspect Ratio to properly scale
Image scaling
In computer graphics, image scaling is the process of resizing a digital image. Scaling is a non-trivial process that involves a trade-off between efficiency, smoothness and sharpness. As the size of an image is increased, so the pixels which comprise the image become increasingly visible, making...

and render a video into a new format.

## Confusion with Display Aspect Ratio

Pixel Aspect Ratio is often confused with different types of image aspect ratios; the ratio of the image width and height. Due to non-squareness of pixels in Standard-definition TV, there are two types of such aspect ratios: Storage Aspect Ratio (SAR) and Display Aspect Ratio
Display aspect ratio
The Aspect ratio of a display is the fractional relation of the width of the display area compared to its height.The aspect ratio is expressed as two numbers separated by a colon...

(abbreviated DAR, also known as Image Aspect Ratio and Picture Aspect Ratio). Note that both Picture Aspect Ratio and Pixel Aspect Ratio can be abbreviated into PAR. This article reserves the acronym PAR for Pixel Aspect Ratio and instead, uses the term "Display Aspect Ratio" and the acronym "DAR" where appropriate.

Storage Aspect Ratio is the ratio of the image width to height in pixels, and can be easily calculated from the video file. Display Aspect Ratio is the ratio of image width to height (in a unit of length such as centimeters or inches) when displayed on screen, and is calculated from the combination of Pixel Aspect Ratio and Storage Aspect Ratio.

However, users who know the definition of these concepts may get confused as well. Poorly crafted user-interfaces or poorly written documentations can easily cause such confusion: Some video-editing software applications often ask users to specify an "Aspect Ratio" for their video file, presenting him or her with the choices of "4:3" and "16:9". Sometimes, these choices may be "PAL 4:3", "NTSC 4:3", "PAL 16:9" and "NTSC 16:9". In such situations, the video editing program is implicitly asking for the Pixel Aspect Ratio of the video file by asking for information about the video system from which the video file originated. The program will then use a table (similar to the one below) to determine the correct pixel aspect ratio value.

Generally speaking, to avoid confusion, it can be assumed that video editing products never ask for the Storage Aspect Ratio as they can directly retrieve or calculate it. Nonsquare-pixel–aware applications also need only to ask for either Pixel Aspect Ratio or Display Aspect Ratio, from either of which they can calculate the other.

## Pixel aspect ratios of common video formats

Pixel Aspect Ratio values for common standard-definition video formats are listed below. Note that for each video format, three different types of Pixel Aspect Ratio values are listed:
1. Rec.601, a Rec.601-compliant value, which is considered the real Pixel Aspect Ratio of standard-definition video of that type. (Read Explanation)
2. Digital, which is roughly equivalent to Rec.601 and is more suitable to use in Digital Video Editing software. (Read Explanation)

Note that sources differ on PARs for common formats – for example, 576 lines (PAL) displayed at 4:3 (DAR) corresponds to either PAR of 12:11 (if 704×576, SAR = 11:9), or a PAR of 16:15 (if 720×576, SAR = 5:4). See references for sources giving both, and Standard-definition television: Resolution for table of SAR/DAR/PAR.
Video System Picture Dimensions Pixel Aspect Ratio Pixel Aspect Ratio (Decimal)
Rec.601 Digital Rec.601 Digital
Standard (4:3) PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

(e.g. 576i
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...

)
720×576
704×576
352×288
59:54
12:11
Widescreen (16:9) PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

118:81 16:11
Standard (4:3) NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

(e.g. 480i
480i
480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...

)
720×480
704×480
352×240
10:11
Widescreen (16:9) NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

40:33
HDV
HDV
HDV is a format for recording of high-definition video on DV cassette tape. The format was originally developed by JVC and supported by Sony, Canon and Sharp...

1080i / HDCAM
HDCAM
HDCAM, introduced in 1997, is an High-definition video digital recording videocassette version of Digital Betacam, using an 8-bit DCT compressed 3:1:1 recording, in 1080i-compatible downsampled resolution of 1440×1080, and adding 24p and 23.976 PsF modes to later models...

(16:9)
1440 x 1080 4:3

### Main sources

As of the retrieval date, a free membership of ITU Online Bookstore would allow free download of up to three ITU-R Recommendations.
• This standard, which is the basis for HDMI
HDMI
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...

, specifies 16:15 (1.0666) as the Pixel Aspect Ratio of 4:3 576i/p and 8:9 (0.888) as the Pixel Aspect Ratio of 4:3 480i/p.

### Supplementary sources

Adobe Systems Incorporated is an American computer software company founded in 1982 and headquartered in San Jose, California, United States...

web site.
• This source specifies 12:11 (1.09) as the Pixel Aspect Ratio of 576i.