I always thought DVDs were 480p, which is considered SD and thats why 720p material looked so much better.

Yesterday I went to www.apple.com/trailers and saw that not only do they categorize their 480p trailers as HD, but when I watched a 480p trailer on my Panny 50PV80, it looked pretty damn good, and definitely better than a standard DVD.
What is going on here?

Try it for yourself:

http://movies.apple.com/movies/universal/the_incredible_hulk/the_incredible_hulk-tlr2_h480p.mov

http://movies.apple.com/movies/universal/the_incredible_hulk/the_incredible_hulk-tlr2_h720p.mov

Did you watch it in full screen mode or a window? It's a newer compression codec than DVD but likely a lower bitrate. I had a look and don't believe it looks better than a well mastered DVD. The mastering quality does vary substantially however between DVD's.

I watched it full screen on my 50" plasma.
720p version looked better, but I was really impressed by the 480p version - so much so that I thought I was watching the 720p one first....

I think DVDs are Interlaced not Progressive, hence the need for deinterlacers, a good deinterlacer with a film sourced DVD should give more picture detail equal to Progressive most of the time. While a video sourced DVD deinterlaced relys on bobbing and weaving all the time. A dvd player with a poor de-interlacer may bob and weave film sources reducing the visible fine detail.

A 480p source may have color encoded at the same resolution as the black and white image. Color resolution on DVDs is normally half the vertical and half the horizontal resolution of the black and white image. The dvd player interpolats this up to the black and white images resolution. Not all dvd players are good at doing this interpolation.

Film and video is sampled then filtered at half the frequency of the sampling to prevent aliasing effects. So detail resolution maybe less than detail resolution of computer graphics, or film/video from sources that have been downscaled to the same resolution.

On DVDs pre-smoothing - reducing the fine detail, is often done before encoding to reduce the mpeg bit rate. Too low a bit rate for too much information leads to mosquito noise and blocking noise. Noise reduction in the dvd player may automatically reduce mosquito and blocking noise, but on discs that are properly encoded this is not needed and when done reduces image detail.

In the UK PAL dvds are often claimed to be 576p, if NTSC dvd claims are like PALs then NTSC is probably not equivelent to 480p.

PAL DVD claim equivelent to 576 progressive x 720 and calibration - reference dvds are that good but in reality most dvds were designed for CRT displays on which they typically give 500+ (very rarely 540) interlaced x 711 resolution so are usually not mastered better than this.

Dvd is mastered up to 6.75MHz the higher the MHz the finer the details - more lines of resolution. To retain this detail they must have a flat response all the way up to 6.75MHz. However I believe to save bit rate, frequency response is usually tailed off before 6.75MHz. All dvds are filtered vertically to reduce flickering. Superbit dvds are not filtered horizontally.

DVD was developed when CRT was the main type of display and was over spec for it.

On a 4:3 tv a dvd has at most 540 lines of resolution because in this case each MHz = 80 lines of resolution. So I believe most dvds are mastered with less than this, very few with more.

The 720 lines of resolution comes from the sampling rate. But CRT has at best 711 pixels of resolution, it does not have time for more. it takes 52.65 µs (microseconds) to draw a scan line and aprox 0.07407 µs per pixel.

PAL Dvd players typically claim 500+ lines of resolution which is more accurate to DVDs in reality than the maximum DVD spec 576. It is unfortunate that the DVD specs do not include a minimum quality - resolution of discs.

You then get to reduce the resolution of the display by the Kell factor.

I think DVDs are Interlaced not Progressive
The video is meerly stored on the disc in interlaced format. The content itself can be either.

The standard DVD resolution is 480i. What that means is there are two "fields" of 240 lines by 720 pixels stored on the disk in digital format. A standard DVD player will output these frames unchanged in a 480i format, via "composite video", S-Video, or "component video". A 480p DVD player will first "de-interlace" or recombine the two "odd" and "even" fields into a single 480p frame, and output this as a "480p frame" (resolution 480 lines X 720 pixels), via "component video". This output requires a high definition display - either an HDTV or a computer type display, or there will be no video quality difference from the standard 480i TV. The 480p display requires THREE seperate video cables, a "component video" display.

The ultimate way to display a standard DVD on an HD display is via an "upscaling DVD player", which reads the 480i signal, converts to 480p, then "scales" the 480p output to 720p, 1080i, or 1080p. However, due to the copy protection rules, the upscaling DVD player must use an HDMI interface.

gary has a great post, but in short i woud like to say, dvds are SD l and so therefore as the screen size gets bigger the worse dvds are to look, with HD being 720p you have more lines more resolution to work with therefore doesnt look as bad as 480 i/p

so say a 480i/p video on a small screen then watch it on a bigger screen it will look diffrent.

Keep in mind of the resolution/spec numbers are like 720x480 i think, so therefore put a screen of that size and thats the best it will look, think for a moment how small is the amount of pixels of a screen of 720x480.

darn i made this complicated,

HD is better then SD so higher resolution better

"The video is meerly stored on the disc in interlaced format. The content itself can be either"

Thats why I said film sources can be de-interlaced to be as good as progressive. Players with progressive output also benefit from having faster DACs with more oversampling. But the player needs to correctly detect the film source and switch between methods of de-interlacing. Often dvds have incorrect flags as to how they should be de-ineterlaced and films often have poor edits which need to be de-interlaced with bob and weaving. The player may also have the color upsampling bug where it is assigning color incorrectly.

DVDs are recorded in MPEG2 at 5mbps while Apple encodes all the trailers carefully at H.264 which is a MUCH more advanced codec and retains MUCH more detail at lower bitrates. The loss of quality over the master is much higher on the DVD. That's the reason.

It's like comparing a 1080p BD picture with a 1080i TV broadcast. It simply has better encoding.