Pixel Aspect Ratio

Posted on January 21, 2012 in Post Production

DV-PAL video has 720×576 pixels. However, these pixels aren’t square pixels, and the aspect ratio is 128/117, about 1.094. Hence DV-PAL video at 720×576 pixels is equivalent to square pixel video at 788×576. If you video record a square with your DV-PAL camera, and export the result to 788×576, the square remains square.

PAL analogue video is 702×576, with the same pixel aspect ratio as DV-PAL. DV-PAL video thus has a few more pixels either side than PAL analogue, to make sure that all analogue pixels are captured in conversion. The square equivalent for PAL analogue video at 702×576 is 768×576. An square filmed with your DV-PAL camera, then 18 pixels cropped, and exported to 768×576 remains a square. (Another way of saying this is that only the centremost 702 pixels within the 720 pixels resolution belong to the 4:3 image.) (This is the ITU standard.) (For 702 vs. 704, see references.)

Some video editing applications use a pixel aspect ratio of 16:15 for DV-PAL timelines, which is about 1.067 (let’s call this non-linear editor ‘NLE-PAR’). Strangely, this is different from the pixel aspect ratio for PAL video. With this pixel aspect ratio, the 720×576 non-square pixels of such a timeline are equivalent to 768×576. A square drawn in such a timeline, and exported to 768×576 remains a square. (Also see list of applications below.)

Conclusions:

(1) It is definitely wrong to export DV-PAL timelines to web movie formats at 720×576 (square pixels), as the image will be distorted. You see this a lot on the web, but it’s plain wrong. (Some video players can set the aspect ratio independently of the encoded pixels, or correct aspect ratio during playback, and so you might be able to play back correctly nevertheless.)

(2) Cropping 18 pixels from DV-PAL video, and then exporting to 768×576 is good, and preserves the aspect ratio of objects within DV-PAL video. Particularly material that has come through analogue equipment (and might have black bars either side) should be cropped, and then exported like this. Similarly, it is correct to export DV-PAL video to 788×576, which for DV video keeps all the pixels available, and gives the correct aspect ratio.

(3) Exporting a DV-PAL timeline (e.g. mp4) at 768×576 (or a fraction of this, e.g. 384×288, or 192×144, 320×240 etc) probably preserves the aspect ratio of graphics created within your video editor, and isn’t too far off for DV-PAL video. It’s probably acceptable, particularly if you want to preserve graphics content (that you created at 16:15 pixel aspect ratio). However, strictly speaking, for PAL video, it’s wrong.

Similar considerations apply for PAL anamorphic. For analog, it is 702×576 pixels that belong to the 16:9 image (same as for 4:3), but now equivalent to 1024×576 square pixels. For DV-PAL anamorphic, at 720×576, the square equivalent is 1050×576. (The pixel aspect ratio for PAL anamorphic is 1024/702, about 1.458. Check: (1024/702) / (128/117) = (16/9) / (4/3).)

Summary for PAL formats (1)
	Full size square equivalent	Other sizes	Pixel Aspect Ratio	Frame aspect ratio
720×576 ITU	788×576		128/117 = about 1.094	about 1.37
702×576 ITU	768×576	384×288, 320×240 (2)	128/117 = about 1.094	4:3 = about 1.33
720×576 ‘NLE’	768×576		16/15 = about 1.067	4:3 = about 1.33
720×576 anamorphic ITU	1050×576		1024/702 = about 1.458	about 1.82
702×576 anamorphic ITU	1024×576	512×288	1024/702 = about 1.458	16:9 = about 1.78
720×576 anamorphic ‘NLE’	1024×576		64/45 = about 1.42	16:9 = about 1.78

Notes:

Similar considerations apply to the equivalent NTSC formats, see references. The difference between PAR and ‘NLE-PAR’ is smaller for NTSC than it is for PAL, and therefore less of an issue.
When resizing to lines other 576/2=288, 576/4=144, i.e. while retaining 576 lines, or going e.g. to 240 lines, you need to deinterlace, see Interlacing.
The above table is just a summary, the whole issue really is more complicated (e.g. 702 vs. 704), and so there can be small deviations from the numbers given here. For more information, see references.
For HDV, the pixel aspect ratio is 4:3. E.g. for 1080i or 1080p you have 1440×1080 equivalent to 1920×1080 square pixels. For 720p you have 960×720 equivalent to 1280×720 square pixels.

Applications: