Pixel Aspect Ratio

Although Pixels themselves don't have height or width, being simply points in space, they do sometimes belong to frames where a single pixel represents a non-square portion of the screen. These are referred to as non-square or rectangular pixels. If each pixel represents the same amount of height and width it's considered square. In either case, the pixel is said to have a Pixel Aspect ratio (PAR), representing it's width to height ratio. For example, NTSC DVD (4:3) video has a PAR of 10:11, while in Widescreen (16:9) it's 40:33. PAL uses a PAR of 59:54, with 118:81 for widescreen (16:9).

PAR isn't used for display purposes. Display is determined by the video's Display Aspect Ratio (DAR), which tells the decoder what shape an entire Frame is. PAR is instead used for editing purposes, where image editing software often needs to know the shape of each pixel to produce a specific shape in the frame. For example, in order to draw a simple circle it's first necessary to determine how far apart each pixels is both horizontally and vertically, which can be determined by the PAR.

Some video stores a separate PAR value, in addition to a DAR for video decoders to use. Other times it must be calculated from reading the DAR first. You may be using editing software capable of this, but it's generally recommended to know your source video's PAR (or calculate it yourself) in order to double check any applications automatic settings.

Standard computer monitor resolutions use square pixels, as do standard HDTV resolutions. EDTV also uses square pixels, although digital SDTVs use pixels with the same PAR as NTSC or PAL DVDs.

Common PARs

Source	TV System	AR	Resolution	PAR
DVD	NTSC	4:3	720x480	10:11
			704x480	10:11
			352x480	20:11
			352x240	10:11
		16:9	720x480	40:33
			704x480	40:33
	PAL	4:3	720x576	59:54
			704x576	59:54
			352x576	59:27
			352x288	59:54
		16:9	720x576	118:81
			704x576	118:81
DV	NTSC	4:3	720x480	10:11
	PAL	4:3	420x576	59:54
SVCD	NTSC	4:3	480x480	15:11
		16:9	480x480	20:11
	PAL	4:3	480x576	59:36
		16:9	480x576	59:27
VCD	NTSC	4:3	352x240	10:11
	PAL	4:3	352x576	59:54

Related Guides
Resizing DVD-Video To Square Pixels

Digital Video Fundamentals - Resolution and Aspect Ratio

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