Why DVD resolutions are not 4:3 or 16:9 compatible?

[alex1234]

Valid DVD-compatible resolutions for NTSC are something like 720x480, 352x480 and something else. All these resolutions do not fit to 4:3 or 16:9 TV screen. In other words, all DVD resolutions do not have 4:3 or 16x9 ratio that means all DVDs that we play on our DVD players and watch on our 4:3 or 16x9 TVs are trancated, i.e. parts of the image are always cut out.

The question: why such resolutions were chosen as DVD standard? Wouldn't a common sense be 640x480 (or something else with 4:3 ratio) for 4:3 TVs and 640x1140 (or something alse with 16:9 ratio) for wide screen TVs?

[Shudder]

?

What do you think is cut out exactly?

[alex1234]

When you watch a 740x480 DVD on PC the picture will be 740 pix wide and 480 pix high. This is a rectangle with ratio 1.51. Now, when you watch the same DVD on a TV (which is a rectangle with ratio 1.33) you can not fit 1.51 rectangle into 1.33 rectangle and the following should happen:

1. left and right parts of the image will be cut off
or
2. the picture will be squeezed horizontally

I think that 1) is happening because the 2) sauses objects look tall.

All this wouldn't accure if the DVD resolution standard was 4:3 or 16:9 or am I missing something??

[andkiich]

You're missing something.

[equinox137]

I think you're confusing aspect ratio with direct aspect ratio.......720x480 or 352x480 (Direct aspect ratio) are the raw dimensions of DVD video. When they're played back on a TV or software DVD player, the picture is resized to whatever the dimensions of the screen are (i.e. 640x480 for 4:3)

[alex1234]

Exactly, resizing. Try to re-size 1.51 ratio rectancle (720x480) to any 1.33 ratio rectangle and to keep the same proportions of the image - it will not fit unless you cut off parts from the left and right (or have gaps on top/bottom). This is exactly what is happening.

Try the following experiment: watch a 4:3 DVD on your computer, find a scene and pause. Now, notice the image on very right and left and memorize it. Then watch the same DVD on TV - the right and left parts will be cut off or (!) image will be slightly squeezed horizontaly or (!) there will be gaps on top/bottom. There is simply no other way to fit a wider rectangle into a narrower one

So the question - why didn't they choose any 1.33 ratio resolution as DVD standards?

[junkmalle]

Exactly, resizing. Try to re-size 1.51 ratio rectancle (720x480) to any 1.33 ratio rectangle and to keep the same proportions of the image - it will not fit unless you cut off parts from the left and right.

No, the displayed pixels are not square. They are taller than they are wide so the 720x480 image fills a 4:3 area.

[LisaB]

The TV's display pixels are not square...that is why the 720x480 pixel frame has 4:3 aspect when displayed on a TV.

Perhaps when you did your experiement, you should have also taken note of the image at the top and the bottom of the screen as well....they are also cut off in the case of a fullscreen picture. This is called "overscan" and it has nothing to do with aspect ratio...your TV is not "chopping" off the left and right - it simply isn't displaying the edges of the image - all 4 edges, in fact. If you do a search of the forums for "overscan" you'll probably get 100's of hits.

[alex1234]

Try the following: capture a TV broadcast to an AVI. In order for this AVI to maintain 4:3 proportions you'll have to use 320x240 or 640x480 resolution. View this AVI on a PC and notice that no squeeze occured, it is a really 4:3 format, just like on your TV. Now, try to convert this AVI to a NTSC DVD MPEG2 - you will not be able to without having black bars on the left and right. This unused space could be avoided if the DVD standard was 1.33 ration compatible.

So, my question still remails, why is it chosen 720x480?

[sacajaweeda]

When is the last time you went to a movie theater and watched a movie on a screen the same shape as your 4:3 television set?

[LisaB]

alex, try doing it instead of just hypothesizing. If you start with a 4:3 AVI (i.e., 640x480) and convert to NTSC DVD (720x480), there will not be any black bars. Both the initial and the final format have 4:3 aspect ratio, so if there is no black bars in the initial AVI, there will be no black bars in the final MPG.

You understand, yes, that aspect ratio has nothing to do with the pixels...it is simply a statement (this information is contained in the sequence headers of an mpg) that such-and-such video needs to be played back with such-and-such aspect ratio. True widescreen DVD has aspect ratio of 16:9, yet still has 720x480 pixels. When the DVD player (software or hardware) sees the 16:9 tag, it knows to appropriately shrink in the horizontal in order to display properly.

There are certain non-standard video arrange methods in TMPGEnc that can lead to black bars...i.e., CENTER will do just what you say, but then the aspect ratio will be off, so you would never, ever, want to do that anyway....CENTER (KEEP ASPECT RATIO) will actually lead to thin black bars on all 4 edges....this is not very popular, but I actually like it since you get to see more of the picture when viewed on a TV screen. The common wisdom is to use FULL SCREEN (KEEP ASPECT RATIO), in this case you get no black bars and the aspect ratio is proper.

[alex1234]

If you start with a 4:3 AVI (i.e., 640x480) and convert to NTSC DVD (720x480), there will not be any black bars. Both the initial and the final format have 4:3 aspect ratio, so if there is no black bars in the initial AVI, there will be no black bars in the final MPG.

Exactly! With one axception - you'll end up with a picture with no black bars but it will be stretched horizontally! To fix this I had to use "custom" size 640x480 so it retained proportions BUT black bars in the right/left appeared. And I still can't understand how to avoid these bars... All this happened while viewing on PC, I didn't try to burn a DVD and view on a TV.

You mentioned that TVs have vertically stretched pixels. Does that mean that my DVD will have right proportions when viewing it on TV? In other words, the same DVD will be stretched wide if viewed on PC and normal on TV?

[lordsmurf]

Alex. Look up. See that thing flying over? It's our information.

Seriously, you're not grasping the concept, and it's been said several times before.

The tv screen has no pixels. The images is displayed at 4:3, no matter the ratio of the stored information. If it is 4:3, it's the same, if it's something else, it's streched to fit. Nothing is cut off, asid from typical outer 7% from overscan, which normally has no image data anyway.

[Shudder]

I think it's time for some screengrabs.

[alex1234]

OK, no more theory! The practice, why I actually posted my question:

I have 640x480 4:3 AVI that I want to convert to a DVD. I converted the AVI to MPEG2 with TMPGEnc. I used 720x480 as a target and "full screen keep aspect ration". When I viewed this DVD on my PC I noticed that objects on the picture are thicker than on the original AVI. Now you are sayng that this how things are supposed to be. Am I supposed to "enjoy" thick people with short legs when I'm watching my DVD on PC?

[LisaB]

no alex, that is not the way things are supposed to be. The resulting video should play back properly on both the computer and TV with no black bars.

When you encode with TMPGEnc, make sure that on the Video tab, Aspect Ratio is set to "4:3 525 line NTSC", and on the Advanced Tab, source aspect ratio should be "1:1 VGA", since your source is an AVI. And, as mentioned before, make sure Video arrange method is set to "Full Screen (keep aspect ratio)"

As long as the source AVI is really 4:3, this will produce DVD MPG with no black bars.

If it still plays back improperly in your software player, then you need to check the settings in your player. For example, with zoom player, if you have it set to "anamorphic" aspect ratio, then it will ignore the aspect ratio tag in the mpg and just display the actual pixels -- this could cause exactly the symptoms you are seeing.

I'm sure everyone out there have their favorite media players, but the Windows Media Player by Microsoft is to be avoided at all costs. Try Media Player Classic or ZoomPlayer is my advice.

[junkmalle]

Alex,

You are confusing display aspect ratio and image dimension aspect ratio. And I may have been further confusing you with pixel aspect ratio! But the three are related.

Lordsmurf is correct when he says there are no pixels from left to right in analog video. It is a continuously varying analog signal. Speaking theoretically, when you capture that analog signal as discreet pixels you can use however many you want. If you play those pixels back at the same rate they were captured you will have a picture that's the same overall dimensions as the original. Obviosly, if you capture with too few pixels, you will lose detail. If you capture with too many pixels you will waste storage space.

I believe the real reason 720 pixels was selected for DVD and D1 studio systems was because that was the approximate resolution of state-of-the-art analog video equipment at the time. Capturing with fewer pixels would have resulted in a noticable degradation of the picture, capturing with more pixels would just have made the files much larger without increasing the fidelity.

The vertical dimension is fixed though. The analog signal traverses the screen from left to right 480 times to create a full image. Of course you could choose to capture only half of them (like on a Video CD at 352x240) but you will be losing detail.

When playing a DVD on your TV, the DVD player adjusts the 720x480 pixels to fit on your 4:3 TV screen. Since 720:480 is a little wider than 4:3 the individual pixels must be slightly taller than they are wide. If you could measure individual pixels of the DVD image on the TV screen, you would see that they are slightly taller than they are wide.

A little aside on overscan: It is difficult to get analog electronics to exactly fit the picture on the screen. As the electronics heat up, or as they age, the picture might move a little up, down, left or right. Or it might grow or shrink. Even the contents of the picture could cause the physical dimensions to change or move. So TV's are designed to "overscan" the image. That is, the image is displayed a little larger than the physical screen. The end result is that you never see the entire image, a little is always (unless you TV gets way out of whack!) clipped at the edges.

Back to pixels: Most computers use pixels that are very nearly square. For example, if you run your video card at 1024x768 on a standard 4:3 monitor, each pixel will be square. This is quite convenient for computer work, but it leads to a problem when dealing with video content. If you display the video pixels 1:1 on the computer display, the image will be wider than the expected 4:3 aspect ratio. An object (captured at 720x480) that is square when viewed on a TV will appear "fat" on the computer monitor.

Some programs compensate for this by automatically resizing the picture during playback. For example, they will take the raw 720x480 pixels from the DVD player and squish them down to 640x480 so the contents look correct, ie, a square now apears to be a square, and people don't look fat. DVD player programs will normally do this when playing from a DVD. They may, or may not, when playing an MPG or AVI file.

Content that is created exclusively for use on computers is usually created with square pixels. Some older low resolution content was created for non-square pixels (especially games from the DOS days) because there was a standard, easy to use, display mode that used 320x200 pixels. Those pixels filled the entire 4:3 screen so they were much thinner than they were tall.

When capturing from video sources you should use dimensions appropriate for your final output device. If you are capturing for use on a computer screen you want to use 640x480, 320x240 (square pixels). When capturing for DVD or Video CD you should use 720x480, or 352x240.

If you don't know the pedegree of your source files you just have to play around with the possible settings to find out what looks right. That's whey there are so many options in TMPGENG.

[LisaB]

Just to add to what junkmalle has said,

Software DVD players *should* look for the aspect ratio tag contained within the sequence headers of an MPG -- this is why a 720x480 MPG with 4:3 aspect ratio tag will play back at the correct aspect ratio on the computer. AVI's, on the other hand, do not have "sequence headers" -- they contain no embedded information about aspect ratio, and so software DVD players will always interpret AVI's as having square pixels.

Junkmalle makes sense when he says that you would be better off capturing to 720x480 if your ultimate goal is to convert to DVD. This way, there will be no resizing during the conversion, and you can maintain the highest quality. However, a 720x480 AVI with 4:3 aspect ratio is not proper, since software will always assume square pixels. It will probably not play back properly in a software player...and when you load the 720x480 AVI into TMPGEnc, you'll need to make sure to manually set the source aspect ratio to 4:3, since "1:1 VGA" is the default for AVI's.

[boolala]

Good Grief, it just isn't all that difficult.

The actual width to height ratio of a TV "pixel" is 8/9.

Therefore: 720 * (8/9) = 640 horizontal resolution.

[alex1234]

LisaB, I will try your suggestions, thanks!

junkmalle, thanks a LOT! Your explanations were excelent and I think I understand the things now. Thanks!!

[junkmalle]

junkmalle, thanks a LOT! Your explanations were excelent and I think I understand the things now. Thanks!!

I'm glad I was able to help out!

[renreq]

Lisa,

I like particularly like the way you explain things. Would you mind looking up my posts and making some suggestions regarding how I can use TMPGEnc and TMPGEnc DVD Author to prepare a QuickTime movie trailer for playback on a 16:9 HDTV? I'm currently having problems getting sound to play. Thanks!

Ren