720x480 DVD resolution - how does that work?

[sldvd]

I've read that all NTSC DVDs have their video stored in 720x480 resolution with non-square pixels. If the video is 4:3 then the pixels are thin (~0.9 pixel aspect ratio), and if the video is 16:9 then the pixels are thick (~1.2 pixel aspect ratio). The DVD player will output 640x480 with square pixels if the video is 4:3, and 848x480 with square pixels if the video is 16:9.

Now in the case of 4:3, you're going from 720x480 to 640x480, which means you're losing 80 columns of pixels - does that mean the DVD player is horizontally downscaling the video to fit into 640 columns? And in the case of 16:9, you're going from 720x480 to 848x480, which means you're adding 128 columns of pixels - does that mean the DVD player is horizontally upscaling the video to fit into 848 columns?

[DB83]

I'll let someone with more expertise to explain this thouroughly.

But you are very wrong. AFAIK the player will resize 720*480 to 720*544 to get the 4:3 ratio. For 16:9 you start with 720*360 stored as 720*480 and the non-square pixels (16:15) corrects that to a 1:1.78 ratio.

[jagabo]

The video isn't digitally resized to 640x480 or 848x480.

When putting out an analog signal the digital-to-analog sampling rate is set so that 720 pixels are output for the duration of one scan line (analog video has not pixels horizontally, it's a continuous wave form). Each of the 480 scan lines are sent to the TV (no vertical scaling). An analog TV draws this analog signal directly on the face of the CRT. Digital TVs will digitize the incoming analog signal and scale it to their native resolution. If the DVD player is set to letterbox 16:9 to a 4:3 frame it will digitally scale the 720x480 frame to 720x360, add 60 line letterbox bars to the top and bottom, then output as if it was a 4:3 DVD. If it's set to pan-and-scan 16:9 material, 90 colums are cropped from the left and right, digitally scaled back to 720x480, then output as if it was a 4:3 DVD. Both these latter cases the player might not actually perform the digital scaling as a separate step, but rather just adjust the D/A sampling rate on the appropriate sub-frame.

http://www.intersil.com/content/dam/Intersil/documents/tb36/tb368.pdf

For digital output the full 720x480 frame is sent to the TV along with the aspect ratio information, then the TV scales to its native frame size minding the aspect ratio. For upscaled output the 720x480 frame is upscaled to 1280x720 or 1920x1080 (padded with pillarbox bars if necessary) before being sent to the TV, then the TV scales to its native frame size.

[sldvd]

OK, so when the DVD player is outputting an analog signal, then the 720 pixels per line are converted to analog and the DAR information is lost - so you have to tell the TV what the correct display aspect ratio is. But if the DVD player is outputting a digital signal, then the DAR information gets sent and the TV will scale it and display it with the correct aspect ratio. Is that correct?

So another question I have is why do people consider the 720x480 video on a DVD to have non-square pixels? Because on a digital TV it will end up having square pixels, and an analog TV doesn't really have any pixels. So you'd never be able to view those pixels with a non-square shape on any device. I would just consider those pixels to be shapeless, instead of non-square, since you'd never actually see the material at a resolution of 720x480.

[jagabo]

OK, so when the DVD player is outputting an analog signal, then the 720 pixels per line are converted to analog and the DAR information is lost - so you have to tell the TV what the correct display aspect ratio is. But if the DVD player is outputting a digital signal, then the DAR information gets sent and the TV will scale it and display it with the correct aspect ratio. Is that correct?

Basically. In PAL countries the aspect ratio information is included in the data portion of the analog signal. The portion that carries closed caption text, etc. PAL TVs respond to that automatically. That was never standardized in NTSC countries as far as I can tell.

So another question I have is why do people consider the 720x480 video on a DVD to have non-square pixels? Because on a digital TV it will end up having square pixels, and an analog TV doesn't really have any pixels. So you'd never be able to view those pixels with a non-square shape on any device. I would just consider those pixels to be shapeless, instead of non-square, since you'd never actually see the material at a resolution of 720x480.

When you view DVD on a digital display like an LCD monitor the original DVD pixels are scaled for the aspect ratio as well as the native resolution of the LCD. So the 720x480 pixels from the DVD are not seen as square. For example, on a 1920x1080 LCD the 720 pixel width of a 16:9 DVD is scaled to 1920 pixels (2.67x) on the screen. And the 480 pixel height is scaled to 1080 (2.25x) pixels of the screen. So individual pixels are viewed as wider than they are tall. If they were left square the picture would have a 3:2 aspect ratio.

https://forum.videohelp.com/threads/299222-Whats-the-actual-size-of-my-video-miniDV-NTS...=1#post1828745

Of course, the image isn't scaled with a simple nearest neighbor (point resize) filter. But rather filters that generate smooth transitions from pixel to pixel and often some sharpening.

http://www.infognition.com/articles/video_resize_shootout.html

And yes, pixels are often described as dimensionless points. Ie, they have a spacing but not a size. But it's easy to think of pixels as blocks on a piece of graph paper. You can stretch/squish the paper wider or taller to make the blocks non-square and fit any display aspect ratio.

[intracube]

Basically. In PAL countries the aspect ratio information is included in the data portion of the analog signal. The portion that carries closed caption text, etc. PAL TVs respond to that automatically. That was never standardized in NTSC countries as far as I can tell.

http://en.wikipedia.org/wiki/Widescreen_signaling

In addition to the above, widescreen signalling can be sent over SCART (pin 8).
0 to 0.4V = off
5 to 8V = 16:9
9.5 to 12V = 4:3