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correct capture settings for standard TV (4:3)
http://forums.linhes.org/viewtopic.php?f=17&t=13306		 Page 1 of 1
Author:  graysky [ Thu Jan 04, 2007 6:13 pm ]
Post subject:  correct capture settings for standard TV (4:3)
I'm using a PVR-250 on my standard 4:3 TV. I know that MPEG-2 streams are unique in that they use two different aspect ratios to generate a "final" video size:

1) DAR (display aspect ratio which is either 4:3 for regular TV or 16:9 for HDTV). This is a multiplier used to give the resulting width of a video.
2) VAR (video aspect ratio which can be whatever). Like 2.35:1 or 1.78:1, 1.33:1 etc.

As I understand it, players take the DAR times the source's height to get the final width displayed. An SVCD for example:

DAR = 4:3 (=1.333)
source size = 480x480

The displayed window is the product of the DAR and the height of the source or 480*1.333 = 640. Therefore, the "final" video displayed is 640x480.

My question is will I get higher quality by capturing to 480x480 with a 4:3 AR (thus giving an output of 640x480), OR by capturing to 640x480 with a "square" (1:1) AR? I know more bits/pixel will result from the 480x480 example since it gives a smaller area, BUT how does the AR affect the video quality since it's scaling the original file?
Author:  ryanpatterson [ Thu Jan 04, 2007 9:59 pm ]
Post subject: 
You are best off maxing out the pvr-250 at 720x480.
Author:  graysky [ Fri Jan 05, 2007 3:53 am ]
Post subject: 
If I capture @ 720x480, what AR is recommended, square (1:1) or (4:3)?
Author:  jmairs [ Fri Jan 05, 2007 7:07 am ]
Post subject: 
I have a PVR-250 also and have been disappointed by the PQ during LiveTV compared to my DirectTV receiver that has TIVO built in. Let me clarify...while watching a hockey game (via Directtv s-video) a full rink shot has a blur or softness to it. The zoom ins look much better.

I'm using an analog SUN CRT for display.

I'll be glad to sacrifice disk space for pq.
Author:  ryanpatterson [ Sat Jan 06, 2007 9:53 pm ]
Post subject: 
graysky wrote:
If I capture @ 720x480, what AR is recommended, square (1:1) or (4:3)?

What are you asking? Just change the recording profile in myth frontend and increase the resolution. No other changes are required.
Author:  graysky [ Sun Jan 07, 2007 5:59 am ]
Post subject: 
AR is confusing in MPEG-2 since there are to of them (see my original post). According to this guide, the true display width is the product of the source HEIGHT and the DAR which is a bit confusing at first. For example, if you capture 480x480 with an AR of 1:1 (square), the final video frame is simply 480 times 1 or 480x480.

If you use an AR of 4:3, the final size becomes 480 times (4/3) = 640 giving you a final video output of 640x480 -- again, the source is 480x480. It seems like the source (what you tell myth to capture) width is more or less irrelevant since the output width is a function of the DAR and the source height.

The math on a 720x480 @ DAR 4:3 only gives a 640x480 image. Seems like capturing to that resolution is only helpful resolution-wise if your DAR is 16:9 which makes the math: 480 times (16/9) = 853x480. Do you understand my question now?
Author:  ceenvee703 [ Sun Jan 07, 2007 11:16 am ]
Post subject: 
graysky wrote:
The math on a 720x480 @ DAR 4:3 only gives a 640x480 image. Seems like capturing to that resolution is only helpful resolution-wise if your DAR is 16:9 which makes the math


That's not correct. I didn't read the article you linked to closely, but they seemed to be correct in their info, although a bit confusing in how it was presented. Hopefully I'll present it a little more clearly.

Let's limit our discussion of MPEG2 video to what you get on your DVDs. Basically, almost all the DVDs you get commercially come in a SINGLE resolution, and that resolution is 720x480. That is an industry-standard video resolution; it's what your DV camcorder shoots.

The DVD player (whether a set-top box or a computer-based player) then looks at flags present on the DVD, and decides whether to display that 720x480 video in a 4:3 aspect ratio (some TV shows), or a 16:9 aspect ratio (most movies). The aspect ratio of the image INSIDE of those 720x480 pixels is irrelevant... it might be letterboxed, windowboxed, or neither.

On a 4:3 TV set, with 4:3 flags set, the 720x480 is displayed as-is and fills the screen. TV set "pixels" are non-square. With the 16:9 flags set, and if the DVD player has been properly "told" it's connected to a 4:3 set, you get letterboxing... the 720x480 pixels are squished vertically and things look correct.

On a 16:9 TV set, with 16:9 flags set on a played DVD, and with a properly-configured DVD player, the material is stretched horizontally to fill the screen, and things look correct. 4:3 flagged material gets "windowboxed" (bars left and right) so it looks correct.

On a computer, with 4:3 programming, the 720x480 pixels are usually squeezed to 640x480 to give square pixels, since a computer deals in square pixels UNLIKE a TV set. Likewise, 16:9 programming is stretched to provide a 16:9 aspect ratio in square pixels.

There are other supported resolutions in DVD authoring other than 720x480:

http://www.dvddemystified.com/dvdfaq.html#3.4

(scroll to allowable resolutions)

I often use 352x480 as my cable TV quality is so poor I don't feel it's worth using 720x480.

MythTV defaults to 480x480, which is not on the list. However, as long as the recorder can record at that resolution, and MythTV knows to play it back sized appropriately, it's fine as long as you stay within MythTV. If you take that recording, however, and try to use it in some DVD authoring programs, they will reject it as an invalid resolution.

Hope this helps.
Author:  graysky [ Sun Jan 07, 2007 12:41 pm ]
Post subject: 
ceenvee703 wrote:
On a 4:3 TV set, with 4:3 flags set, the 720x480 is displayed as-is and fills the screen. TV set "pixels" are non-square. With the 16:9 flags set, and if the DVD player has been properly "told" it's connected to a 4:3 set, you get letterboxing... the 720x480 pixels are squished vertically and things look correct.


I don't think 720x480 w/ a 4:3 AR is displayed as is because that ratio isn't 4:3, it's 3:2. If we use mplayer as a judge, the conclusion is a bit more confusing: I captured a program to 720x480 w/ 4:3 aspect ratio setup in my recording profile; then played the mpeg-2 stream w/ mplayer. Here's the output:
Code:
VDec: vo config request - 720 x 480 (preferred colorspace: Planar YV12)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.33:1 - prescaling to correct movie aspect.
VO: [directx] 720x480 => 720x540 Planar YV12


That 720x540 makes no sense if I'm understanding the article I referenced correctly because they're teaching that source height times DAR = vertical display resolution; in this case, source height is 480 and DAR is (4/3) and the math is 480 times (4/3) = 640.

If I back-calculate what mplayer is doing in this case, I think it's (720/x) = 1.333 -- Solving for x gives 541 which rounded gives 540. Again, this contradicts the article I referenced and looks wrong when viewed. If I add the -aspect 3:2 switch to mplayer it leaves the video 720x480 and it looks fine (although it chose a different colorspace for some reason):
Code:
Movie-Aspect is 1.50:1 - prescaling to correct movie aspect.
SwScaler: using unscaled yuv420p -> rgb32 special converter
VO: [directx] 720x480 => 720x480 BGRA


I have no idea which end is up with all this mpeg-2 DAR and SAR bullsh*t -- if anyone smarted than I wants to take a crack at explaining it to me and either confirming or debunking that article I referenced, I'll all ears!

Thanks for trying :)
Author:  ceenvee703 [ Sun Jan 07, 2007 5:03 pm ]
Post subject: 
graysky wrote:
I don't think 720x480 w/ a 4:3 AR is displayed as is because that ratio isn't 4:3, it's 3:2.


You are leaving out a factor, called DISPLAY aspect ratio. Since video pixels are not square, you have to multiply by an additional factor (a.k.a. the display aspect ratio) to convert 720x480 video on a computer, which displays square pixels.

Photoshop has supported non-square display aspect ratio files for a few revisions now. Before that, you had to work in square pixels (for instance, 720x540, which IS 4:3), then resize to non-square pixels (720x480).
Author:  ceenvee703 [ Sun Jan 07, 2007 5:58 pm ]
Post subject: 
Here is a different page for you to read:

http://www.mir.com/DMG/aspect.html
Author:  psicard [ Wed Feb 14, 2007 11:34 am ]
Post subject: 
I ran across a pretty decent article on pixel aspect ratio that helped clear up some of my confusion why 720x480 is 4:3. It's here:
http://www.artbeats.com/pub/articles/aspect_ratio_1.pdf
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