PVR-250: correct capture settings when capping mpeg-2

[graysky]

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?

[jagabo]

Anything captured by the PVR-250 will be 4:3 DAR. That's all that exists in NTSC video. Anything else is letterboxed so that the full picture (including the black bars) is 4:3.

In terms of resolution, you will get the highest quality out of the PVR-250 with the full 720x480 frame size and high bitrates.

It's when you want to use lower bitrates that you want to consider smaller frame sizes. You will be giving up resolution in order to get fewer macroblock artifacts. If your source has low resolution to start with, say a VHS tape, and you capture at 352x480, the lower frame size doesn't really lose much detail.

If you want DVD compatability you are limited to certain frame sizes: 720x480, 704x480, 352x480 and 352x240.

[graysky]

Actually, since the source video is analog cable, I can select any AR I want (4:3 or square are both options). That said, the major use I have is to encode to xvid and package in mkv with ogg audio. I'll probably do 640x480. Should I cap. 1:1 AR @ 640x480 or 4:3 AR @ 480x480 in this scenario?

[jagabo]

NTSC analog cable is 4:3 DAR only. All standard definition NTSC video is 4:3 DAR. DAR is Display Aspect Ratio, the final aspect ratio of the frame as seen on the TV screen, not the shape of individual pixels or the the relative pixel dimension of the frame.

The PVR-250 (I have one too) will let you capture at 720x480, 480x480, 352x480, or 352x240, but the are all 4:3 DAR.

If you are encoding as Xvid I suggest you capture at 720x480, resize to 640x480 (or some other 4x3 frame size), crop away any black bars if you want, and then encode as Xvid with square pixels. If you can't afford the bitrate required for 640x480, use a smaller frame -- like 512x384, 448x336, 320x240, or 64x48.

And you'll have to inverse telecine or deinterlace almost anything you capture to get decent Xvid playback. Xvid allows for interlaced encoding but few players do anything sensible with it.

[graysky]

OK, so let's say I captured @ 720x480 but the source has black bars. I cropped them away and am encoding to 640x480. The resulting video looks stretched. I think the source was faked 16:9... should I therefore resize to 640x360 for my avi after cropping out the black bars just like it was a DVD w/ that AR?

[jagabo]

OK, so let's say I captured @ 720x480 but the source has black bars. I cropped them away and am encoding to 640x480. The resulting video looks stretched. I think the source was faked 16:9... should I therefore resize to 640x360 for my avi after cropping out the black bars just like it was a DVD w/ that AR?

The easiest way to get the right frame size is to resize to 640x480 first (or any 4x3 frame size, 720x540, 512x384, 320x240, etc.) and then crop away the black bars.

Note that many compression codecs require the frame dimensions be even multiples of 2, 4, 8 or 16.

[ronnylov]

Take a look here:
http://www.doom9.org/index.html?/capture/par.html

So it depends of both your capture device PAR and your destination PAR. I do not know the source PAR of the PVR-250 but you can findout this yourself by capturing a reference from a DVD player and follow the method in the article here:
http://www.doom9.org/index.html?/capture/capture_window.html

You can use Virtualdubmod to open your captured mpeg-2 files when analyzing.