Optimal Screen Resolutions for Playing DVDs on Your PC

Using your PC to play DVDs is an attractive option for a number of reasons. For one thing, you don’t need to double up on hardware (if you already have a monitor, DVD-ROM drive and video adapter, why buy a DVD player and TV set as well?). Most computer monitors have good colour, better clarity than most TV sets, and they can handle higher resolutions. Furthermore, you can use screen refresh rates of 100 Hz or more with no interlacing.

If you want to get the best image quality from playing DVDs on your computer, there are some things to watch out for. Depending on where you live, the DVDs you buy have different resolutions (the number of pixels used to construct the image), to match the number of horizontal visible scanlines used by the broadcast TV signal in your area. This is so you can easily play DVDs on your existing TV set.

In countries that use the NTSC broadcast television standard, DVDs use a resolution of 720 × 480. In PAL regions, the resolution is higher: 720 × 576. Both formats assume that your screen has an aspect ratio of 4:3 (or 16:9 for widescreen sets). This means that, regardless of whether you live in an NTSC or a PAL region, your DVDs are designed to be shown on a screen that has a width-to-height ratio of 4:3 (or 16:9 for widescreen). This also means that both PAL and NTSC DVDs use non-square pixels for displaying DVD material, which makes displaying them on your computer, which is used to dealing only with square pixels, potentially tricky.

If you want the in-depth discussion, read on! If you just want to know what resolutions to use, jump to the summary.

DVDs for PAL regions

4:3 Transfers

DVDs produced for distribution in PAL regions use a resolution of 720 × 576 and are commonly digitised to be played back on a display with an aspect ratio of 4:3. Older TV sets and computer monitors use a 4:3 aspect ratio, which is the ratio of the width of the display to its height.

Now, the interesting thing is that the ratio 720:576 is not the same as the ratio 4:3! 4 / 3 = 1.333…, but 720 / 576 = 1.25.

To look at it another way, consider the pixel density (the “definition” of the display). Imagine a screen that’s 4 units wide and 3 units high, showing a 720 × 576 pixel image (it doesn’t matter what the units are, just the proportions).

Horizontal: 720 pixels / 4 u = 180 pixels/u
Vertical:   576 pixels / 3 u = 192 pixels/u

This means that the pixels on a PAL DVD are not intended to be square on playback: the vertical density is higher than the horizontal. The pixel aspect ratio (the ratio of the pixel width to the pixel height) is 1.0666…:1, slightly wider than tall.

(1/180 u/pixel) / (1/192 u/pixel) = 1.0666...

To display “4:3 transfer” DVDs on your computer, the ideal would simply be to use a screen resolution of 720 × 576, the native resolution of the 4:3 “PAL” DVD. This way, as long as the display area occupies a 4:3 rectangular area, the image will look correctly proportioned. This is essentially the case when you play DVDs through your TV set using a standalone DVD player.

16:9 Transfers

DVDs can also be produced for higher quality playback on a 16:9 (1.777…:1) “widescreen” display. These are identified by terms such as “16:9 transfer”, “anamorphic”, or “widescreen enhanced” on the back of the box. For “PAL” DVDs, the resolution is still 720 × 576, but if you display a 16:9 DVD on a 4:3 set, the image will look compressed or “squashed up” horizontally, with actors appearing unnaturally tall and skinny (even for actors!). It needs to be stretched out horizontally to the full 16:9 in order to look sensible.

16:9 image on a 4:3 display	16:9 image on a 16:9 display

Now, if you’re lucky enough to have a 16:9 TV set, this scaling will probably “just happen”, because the set is designed to fill the screen area with the display image. Again, the pixels will be non-square, only this time they’re quite a lot wider than they are tall (with a pixel aspect ratio of 1.4222…:1)

To calculate the relative pixel density in each dimension:

H: 720 pixels / 16 u = 45 pixels/u
V: 576 pixels / 9 u  = 64 pixels/u

To calculate the pixel aspect ratio:

(1/45 u/pixel) / (1/64 u/pixel) = 1.4222...

However, we’re talking about computer displays, which have an aspect ratio of 4:3. In order to display a 16:9 transfer DVD correctly on a 4:3 screen, you can’t stretch it out, because there’s nowhere to go. Instead, you have to squish it back down vertically.

First, we need to work out how stretched vertically the 16:9 image is when shown on a 4:3 display. This is simply the aspect ratio of the intended display (16:9, or 1.777…) divided by the apect ratio that you’re actually going to show it on (4:3, or 1.333…). As it happens, 1.777… is the square of 1.333… (4:3 * 4:3 = 4*4:3*3 = 16:9), so the stretching factor is also 1.333…, or 4 / 3. To get the right amount of “squash” to cancel out the vertical stretch, you would use a vertical resolution of 4 / 3 * 576 pixels, which is 768 pixels. So, to show 16:9 DVDs on a 4:3 computer screen, you would need to use a resolution of 720 × 768.

16:9 image on a 4:3 display	16:9 image squashed and letterboxed on a 4:3 display

Here’s the math in more detail. First, calculate the vertical “squash factor”, by dividing the aspect ratio of the DVD transfer by the aspect ratio of the display that you want to display it on.

Squash factor = intended aspect ratio / playback aspect ratio
              = 16:9 / 4:3
              = (16 / 4) / (9 / 3)
              = 4 / 3

Or, using decimals:

Squash factor = 1.777... / 1.333...
              = 1.333...
            ( = 4 / 3 )

Then you just multiply the squash factor by the original vertical resolution of the transfer:

	  
Required vertical resolution = Squash factor * original vertical resolution
                             = 4 / 3 * 576
                             = 768

You can also work out the required vertical resolution using pixel aspect ratios and the horizontal playback resolution:

Required pixel aspect ratio = intended pixel aspect ratio / playback pixel aspect ratio
                            = 1.4222... / 1.333...
                            = 1.0666...

Required vertical resolution = required pixel aspect ratio * horizontal resolution
                             = 1.0666... * 720
                             = 768

The Problem

Regardless of whether you want to view a 4:3 or a 16:9 transfer DVD on your computer, there is a problem. You want to use 720 × 576 or 720 × 768, but chances are your operating system gives you a pretty limited choice of screen resolutions, typically:

• 640 × 480
• 800 × 600
• 1024 × 768
• 1280 × 1024
• 1600 × 1200
• …

800 × 600 is often regarded as a suitable resolution for playing DVDs on your computer. Now, 800 × 600 may be pretty close to our ideal 720 × 576 (4:3 PAL DVD), but “pretty close” is loser-speak for “wrong”. :^) Here are the problems:

1. All the “standard” computer resolutions have square pixels on a 4:3 display, so unless the DVD software stretches the image on playback, the aspect ratio will be wrong, even for 4:3 titles. 1.25 is not 1.333…. Don’t ask me why they didn’t just make PAL DVDs 768 × 576!
2. Even if the player does stretch the image vertically, this scaling usually results in an uneven looking picture with noticeable “blockiness” in many places.

3. Most software DVD players insist on scaling the image both horizontally and vertically, so that the aspect ratio is correct and the image occupies the entire area of the screen when in “Full Screen” mode. This again leads to an irregular looking picture. Resampling 720 pixels to fit 800 without any filtering simply looks bad. Even with fancy smoothing filters or line averaging, the image gains a certain fuzziness in the process. How good the scaled image looks probably depends a lot on your graphics card, and your software DVD player, and your monitor.

Original 720 × 576 image

Image resized to 800 × 600 (no filtering)

Image resampled to 800 × 600 using spline filters

Solutions

Fortunately, there are solutions. If you’re using Microsoft Windows, you can use Powerstrip. On modern video cards, you can use this utility to set your screen resolution to a custom size, right down to the pixel. Powerstrip can set your computer’s display to the exact 720 × 576 or 720 × 768 screenmode you need for perfect playback of 4:3 or 16:9 DVDs with no ugly, resource-intensive image scaling. This is a similar approach to the vertical squeeze trick for playing 16:9 DVDs on conventional 4:3 TVs. You can even tell Powerstrip to switch screenmodes automatically when you launch your DVD player. Nice. And if you use CyberLink's PowerDVD player, you can tell it to switch to a specific screen mode when playing a 4:3- or a 16:9-transfer DVD.

Powerstrip is also really handy if you have a TV tuner card and you want to view 4:3 TV broadcasts full-screen on your computer monitor. You can even tweak the video timing parameters so you can feed your computer display into your HDTV set.

If you run Linux or a BSD Unix, chances are you use the XFree86 X-Windows server. This is extremely customisable as well, and you can very probably tweak your video modes to get the exact resolution you need.

What About 1.66:1, 1.85:1, and 2.35:1 Movies?

Well you may ask. As far as the DVD standard is concerned, these do not exist: there are only 4:3 DVDs and 16:9 DVDs. Anything else has to be letterboxed or pan-and-scanned to fit the screen.

It’s unfortunate that these common aspect ratios aren’t supported by DVD players, because the black bars that appear in a letterbox presentation are actually stored on the DVD in order to bring the resolution up to the required 720 × 576. Now, a lot of people will tell you that those bars compress very well using the MPEG-2 video compression scheme that DVDs use, and that’s true enough. However, the hard edges that occur between the black bars and the actual picture compress very poorly indeed, and bandwidth that could be used to give better image quality is simply wasted. You can often see the effect of letterboxing on MPEG-2 encoding as a short series of horizontal alternating dark and light lines next to the edge of the letterboxing mattes. Moreover, without the letterboxing, you would be able to get even higher resolutions than with letterboxed anamorphic discs. However, as most people will be viewing their DVDs on TV sets, this obviously wasn’t considered important.

DVDs for NTSC Regions

For NTSC regions (USA, Canada, Japan?, …), the calculations are similar. DVDs for NTSC areas also use an aspect ratio of 4:3, but the resolution of is 720 × 480.

Horizontal: 720 pixels / 4 u = 180 pixels/u
Vertical: 480 pixels / 3 u = 160 pixels/u

NTSC 4:3 Transfers

In th same way as for PAL 4:3 transfers, you should use the native resolution of 720 × 480 to display your 4:3 NTSC DVDs on a 4:3 screen. Easy.

NTSC 16:9 Transfers

For 16:9 DVDs to display correctly on a 4:3 display, use a screen resolution of 720 × 640. Here’s the math:

4 / 3 * 480 = 640
or
0.888... * 720 = 640

For Microsoft Windows systems, you can use Powerstrip to set these resolutions exactly. Happy viewing!

This page has been accessed 42 times, apparently.
This document last modified and © 2003-01-30 20:30:29 NZDT