Format puzzlement

[Zarkov]

Ignoring HDTV for the moment...

I thought analog video (as delivered off cable) was 525 lines of 400 pixels (two interlaced frames of 262.5x400) at 30 frames per second, corresponding to 6.3 megapixels per second.

I understand recording at 352x240 to reduce bitrate, but why not use a 4:3 format like 320x240?

I gather the selection of (4:3) 640x480 is based upon 13" VGA.

I can see that 704x480 is the logical double of 352x240, so why would the default recording format for my ATi TV Wonder be 352x240 - even with DVD quality selected?

Could one make a 525x400 MPEG-1 recording?

[vitualis]

You have to think beyond the PC.

On computers, you have square pixels. Thus, the resolution of the screen is also pretty much it's aspect ratio (e.g., 800x600 screen is 4:3).

For video, there digital pixel resolution is separate from the display aspect ratio (DAR). VCDs are 352x240/288 but have a fixed DAR of 4:3.

That is, regardless of the actual pixel resolution, the image will be resized to fit in a 4:3 display (e.g., TV).

The ramifications of this are enormous. It means that if you just watch the video on your PC with a media player (e.g., WMP) that doesn't understand the concept of DAR, then your video will be played back with the aspect ratio a bit off.

The is most obvious for SVCD where the pixel resolution is 480x480/576. However, if the media player understood that the display aspect ratio was 4:3 (e.g., most software DVD players like PowerDVD and WinDVD) it will correctly resize the image to the correct aspect ratio.

Indeed, ALL the common digital video formats have the pixel resolution NOT equal to the display aspect ratio:
DVD: 720x480/576
SVCD: 480x480/576
CVD: 352x480/576
VCD: 352x240/288

Regards.