DV aspect ratio and resolution demystified

S

skippyfetus

Carbonite Member
Ok, a while back I posted a message called digital video confusion and Slapdragon and iamloser posted some pretty useful comments. However, there's still a couple things I don't understand. First, what I do understand (or think I do):

The DV format is 720x480. The pixels are smaller horizontally and when you take that into consideration, you get at 4:3 image. Well, almost: If you multiply 720 by 0.9091, you get 654. 654 divided by 480 is 1.36 which is close enough to 1.33 to count as 4:3 I guess. Like Slapdragon said in an earlier post, D-1 was 486 pixels high which was closer to a 4:3 aspect ratio, but they decided to drop the 6 lines because 480 is a multiple of 16 and easier to encode without them.

When I capture video into Vegas, it looks fatter than it's supposed to. This is because it is showing those thin pixels as square pixels. If I select "simulate device aspect ratio" it squeezes the video horizontally to give me a 327x240 preview image which is also 1.36. Also, if I go into preferences and choose "square pixels" it squeezes the video like it did before, but this time it leaves the overall frame the same size and uses black space on the left and right sides. I have no idea why it does this. I also have no idea why Vegas doesn't "simulate the device aspect ratio" by default. Who wants to work with a distorted image?

I'm in the process of reading the American Cinematographer Video manual (about 25% done) so I may figure this out on my own anyway, but here's a couple more things that I haven't quite figured out. It may seem stupid, but I'm relatively new to video. If DV resolution is 720X480, then how can some DV cameras like the DVX claim 500 lines and others like the XL1s claim 530? Is this 530 lines before it is compressed into DV format? I could see where starting with a higher rez image before compression could give you more resolution.

Feel free to call me out on something I'm wrong about and add your own observations. Let's completely demystify DV. This is the kind of stuff you should put in your guide to video slapdragon.
 
Re: DV aspect ratio and resolution demystified

Yes, that's how they do it. The CCD image is capturing a higher than SD resolution image, but recording the regular SD image to tape.
 
Re: DV aspect ratio and resolution demystified

[quote author=Tony_H link=board=Events;num=1090303010;start=0#0 date=07/19/04 at 22:56:49]Also, if I go into preferences and choose "square pixels" it squeezes the video like it did before, but this time it leaves the overall frame the same size and uses black space on the left and right sides. I have no idea why it does this.[/quote]
Very bad thing to do.  Don't do that.  When you tell it the aspect ratio is other than what it really is (i.e., telling it it's 1.000:1 instead of .9091:1) then vegas will digitally scale the video to "fit".

Only use "square pixels" (i.e., 1.000:1) for items that are truly square pixel aspect ratio, such as scanned-in photographs.

I also have no idea why Vegas doesn't "simulate the device aspect ratio" by default. Who wants to work with a distorted image?
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Because when you choose "simulate", you're telling Vegas to scale it (although in the preview only, the actual innards are still working at the raw resolution).  Scaling introduces artifacts and doesn't show you the true raw image.  If for some reason you needed to see pixel accuracy, you definitely would NOT want "simulate aspect ratio" selected.  For almost all the stuff I do, I DO choose "simulate aspect ratio", but when you're studying resolution charts or something like that, you need to disable that so you can see what the images really look like on a pixel-for-pixel basis.

If DV resolution is 720X480, then how can some DV cameras like the DVX claim 500 lines and others like the XL1s claim 530?
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You need to read up on "TV Lines" of resolution.  TV Lines is not a count of the 480 pixel height, it's a count of the vertical lines that can be packed together on a horizontal scan line IN A SQUARE THE SHAPE OF THE HEIGHT OF THE PICTURE.  (don't worry, it'll take a while for that one to sink in).  So because DV is 4:3, a square would be a 3:3 patch extracted from the 4:3 frame, which means you can only count the alternating black/white pixels over 3/4 of the width of the frame.  And 3/4 of the 720-pixel line width is 540.  The maximum lines of resolution possible to be recorded on the DV format is therefore 540 lines of resolution, expressed as "TV Lines" (which means lines per horizontal measurement that is equivalent to the maximum vertical measurement).

See, it really has nothing to do with the 480-pixel height at all.  Odd, huh?
 
Re: DV aspect ratio and resolution demystified

Very bad thing to do. Don't do that. When you tell it the aspect ratio is other than what it really is (i.e., telling it it's 1.000:1 instead of .9091:1) then vegas will digitally scale the video to "fit".
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You mean it will resample the footage? I could see where it could loose some of the signal this way. I wasn't planning on editing this way, I was just messing around with preferences. Also, I just noticed that if you import footage like a WMV or image, Vegas automatically gives it square pixels, cool.

Because when you choose "simulate", you're telling Vegas to scale it (although in the preview only, the actual innards are still working at the raw resolution). Scaling introduces artifacts and doesn't show you the true raw image. If for some reason you needed to see pixel accuracy, you definitely would NOT want "simulate aspect ratio" selected. For almost all the stuff I do, I DO choose "simulate aspect ratio", but when you're studying resolution charts or something like that, you need to disable that so you can see what the images really look like on a pixel-for-pixel basis.
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Awesome, that makes sense.

You need to read up on "TV Lines" of resolution. TV Lines is not a count of the 480 pixel height, it's a count of the vertical lines that can be packed together IN A SQUARE THE SHAPE OF THE HEIGHT OF THE PICTURE. (don't worry, it'll take a while for that one to sink in). So because DV is 4:3 and the image has 480 lines, a square would be 3:3, which means you can only measure the alternating black/white pixels over 3/4 of the width of the frame. And 3/4 of the 720-pixel line width is 540. The maximum lines of resolution possible to be recorded on the DV format is therefore 540 lines of resolution, expressed as "TV Lines" (which means lines per horizontal measurement that is equivalent to the maximum vertical measurement).
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WTF? I think I'm going to have to read this when I get up in the morning. I didn't understand ****. ;)

Ok, I just re-read it and it made a little bit more sense, but what about what Mike said:

Yes, that's how they do it. The CCD image is capturing a higher than SD resolution image, but recording the regular SD image to tape.
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That's not how it works?

So, when a camera manufacturer is talking about it's resolution, it's speaking of TV lines... but that's completely different from DV lines? TV lines are measured in the horizontal direction for 3/4 of the picture and DV lines (so to speak) are measured in a vertical direction for the whole picture? Ok, I think I'm starting to get it... can you point me to a webpage or something where I can read more about it?
 
Re: DV aspect ratio and resolution demystified

Ok, it has sunk in. I also found a web pate on TV resolution... it's crazy that my video manual doesn't seem to mention TV lines. I'll come back after I've edjamucated myself.
 
Re: DV aspect ratio and resolution demystified

[quote author=Tony_H link=board=Events;num=1090303010;start=0#3 date=07/19/04 at 23:48:17]So, when a camera manufacturer is talking about it's resolution, it's speaking of TV lines... but that's completely different from DV lines?[/quote]
Yep.

TV lines are measured in the horizontal direction for 3/4 of the picture
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Only for 4:3 systems. For 16:9 systems they'll sample a patch as wide as 9/16 of the height.
... and DV lines (so to speak) are measured in a vertical direction for the whole picture?
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Er, well, there's not really such a thing as "DV lines", but if by that you mean "the image is 720x480, so my resolution is 480 lines, right?" then I guess that's right. But it's not a standard definition.

Ok, I think I'm starting to get it... can you point me to a webpage or something where I can read more about it?
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Search through the archives here, I seem to remember writing an encyclopedia on it a few times. Also, you can try this page:
http://www.dvfreak.com/res.htm
I don't agree with some of it, but overall it'll give you a good head start on wrapping your head around the difficult concept of "TV Lines of Resolution." Just skip all the opening paragraphs and page down to the Andre De Clerq stuff.
 
Re: DV aspect ratio and resolution demystified

Ok, Barry, I understand the horizontal resolution and TV lines/scanning lines thing. The reason that a camera cannot have more than 540 scanning lines is that it cannot have more than one color or line per pixel. This initially confused me. However, a camera can have less than 540 lines of resolution because a "line" can span horizontal pixels. Therefore a camera like the DVX has horizontal lines that span it's horizontal pixels.

One last thing... when I'm looking at production monitors and it says: 800 lines, what are they talking about? Why 800 lines... and isn't that way too much for a device that can't capture more than 540? Is this for editing higher rez stuff? I know that when we're talking about how many lines a camera is capable of, we're talking horizontal... but what about monitors?

Thanks Barry, I'm really glad I know this stuff now.
 
Re: DV aspect ratio and resolution demystified

Monitors don't have pixels. They're analog devices capable of displaying much higher resolution. Monitors are analog creatures, and they derive their resolution from megahertz and bandwidth and frequencies and all sorts of other analog terms...
 
Re: DV aspect ratio and resolution demystified

Ok, I know that CRT monitors are analog... but what is the advertised resolution referring to? Obviously if your camera is capable of 500 (tv) lines, it would be unnessasary to use a monitor that has 800 (tv) lines of resolution right?
 
Re: DV aspect ratio and resolution demystified

It would be overkill to use such a high-res monitor, yes. Although more is always better.
 
Re: DV aspect ratio and resolution demystified

So I have a question then...is that just a marketing gimmick? No SD signal can be 800 lines, right? So for an SD monitor to boast it can have that many lines of resolution is just for the sake of sounding cool?
 
Re: DV aspect ratio and resolution demystified

Another thing that I find confusing is that people are always throwing around the word "lines"... "Oh, it's got 800 lines", "that camera can do 530 lines", "DV only uses 480 lines", and "that format records "500 lines". Now, it's not clear what lines they're talking about. Well, at least I know that they're probably talking about tv lines when they're talking about monitor or camera resolution.
 
Re: DV aspect ratio and resolution demystified

Ok, I think Barry's out of answers... let's see if I can figure this out.

Scan lines are the 525 horizontal lines in NTSC video of which DV uses only 480. So, 480 lines is the maximum vertical resolution in DV because a pixel cannot sharply display more than one color at a time.

TV lines is the measure of how many black and white stripes or dots can be sharply displayed 3/4 of the picture horizontally. The maximum horizontal resolution is 540.

So, when people say that there's 525 lines in NTSC and 625 in PAL, they're talking about scan lines

Whey people say that DV is 480 lines, they're talking about scan lines

When people say my DVX has 500 lines of resolution, they're talking about TV lines... because it's impossible for DV to have 500 lines of vertical resolution if it doesn't even use that many.

When they say that this monitor has 800 lines, they must be talking about TV lines because that seems to be what people are talking about when they're talking about resolution. Plus, it's a big number and how could you possibly need that much vertical resolution.

In the American Cinematographer video manual, there's a list of digital formats and their resolutions. It's apparent that they're talking about scan lines since they list the HDcam format as 1080i and 720p. However they list DV as having 500 lines of resolution... that matches more closely with the TV lines of the DVX than the 480 of DV. I'm sure that they're talking about scan lines though.

This is what I mean. Why are we expected to just *know* what kind of lines they're talking about... especially when it could go either way. One thing that I've noticed is that when speaking of a device, it's usually TV lines, or the vertical resolution... and when they're speaking of a format or a broadcast, they usually mean the number of horizontal scan lines.

I'm just thinking out loud, has this been helpful to anybody?
 
Re: DV aspect ratio and resolution demystified

I'm not totally out of answers, just been busy shipping anamorphic adapter guides... and I was hoping SlapDragon would step in and fill in the gaps when things started turning analog in the discussion! :)
 
Re: DV aspect ratio and resolution demystified

[quote author=Barry_Green link=board=Events;num=1090303010;start=0#7 date=07/20/04 at 16:59:33]Monitors don't have pixels.  They're analog devices capable of displaying much higher resolution.  Monitors are analog creatures, and they derive their resolution from megahertz and bandwidth and frequencies and all sorts of other analog terms...[/quote]

Color monitors do have pixels, sorta. There are generally either vertical columns of RGB, or a some type of pattern. If bandwidth and other constraints are sufficient, the monitor will be limited in resolution to these points. Since there is nothing that is going to cause the phosphor "pixels" to line up with the DV horizontal pixels, having a higher resolution monitor (finer pitch) will still make for a clearer picture.
 
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