[astroboy]

When we record a HD scope movie off the dish, the only way we can watch it CH with our U380 and 7205 is to tell the DVR to send the projector a 480p signal. I accept that limitation, but what I'm curious about is why it looks better than an anamorphic scope DVD. In each case, the projector is seeing a 480p signal. And it seems the former case is undergoing an extra scaling step, since it's starting at 1080i, being scaled by the Dish equipment to 480p, then back up to 720p by the projector--but it still looks sharper. Why?


Thanks,

Jeff

 

[bass addict]

Better source? I would still think a 1080 signal downscaled to 480 would look better than a 480 native.

 

[video_bit_bucket]

A standard DVD is recorded at 480i. As long at the 1080i to 480p conversion is done well you have twice the resolution to work with.


Also if memory serves going from xxxi to xxxp well is not a given, taking a 480p to 720p only requires scaling, the more complicated part of the process is generating a full p frame out a series of i frames. When you introduce the issues of film at 24 frames and video at 30 per second things get dicy. I had a cheap DVD player, have a Dennon 3910, Toshiba A1, Infocus SP 777, and have borrowed a Anthem D2 which has an onboard VP. There is no doubt that some VP's are more equal than others.


FWIW there is a huge variation in quality from disk to disk. I am experimenting with a 130 inch wide scope setup. I have seen HD disks that were no better than a standard DVD upscaled. This with only using a 720 projector.

 

[CAVX]

Another reason it looks better is that a DVD is based on 720 x 540. On a 480 display, there is 854 x 480 pixels, so the 720 doesn't quite cut it. When you down scale a HD soruce from 1920 x 1080 to 480, you end up with 960 pixels wide, and that is why it looks better...


Mark

 

[Erik Garci]



Quote:
Originally Posted by astroboy /forum/post/12951684


I accept that limitation, but what I'm curious about is why it looks better than an anamorphic scope DVD.

Maybe the DVD looks worse because of...


1. Compression before/after downconversion. If a DVD was created from a 1080p source, it was downconverted from 1080p to 480p first, then it was encoded as 480p on the disc (assuming that the disc was encoded with progressive frames). However, in the case of 1080i HDTV, it was encoded as 1080i first, then you downconvert it to 480p. Maybe one downconverter is better than the other, or maybe one encoder is better than the other, and it makes a difference which one is performed first (depending on bitrates and other factors as well).


2. 4:2:0 chroma subsampling. 1080i HDTV has 960x540 chroma resolution, and when you downconvert it to 480p (without recompressing it), the resulting 480p has 720x480 chroma resolution, whereas 480p on DVD has 360x240 chroma resolution. This is related to #1.


3. Filtering. Sometimes a DVD has been filtered to reduce flickering on interlaced displays. 1080i HDTV might also be filtered, but usually to a much lesser extent, so it is still less filtered after you downconvert it.

 

[Dave W]

What you're seeing is the result of the inherenent higher data rate of the HD signal. If both the DVD and HD images were uncompressed, there would be no difference, except possibly due to scaler quality differences.


The decompressed 720x480 image from a DVD is degraded from the original 720x480 image it was created from. The decompressed 1080i image is also degraded from the original 1080i image, but even the degraded 1080i image contains much more of the original image detail (and smaller artifacts) than the degraded 480p image, so when the degraded 1080i image is downscaled to 480p, it will look quite a bit better than the degraded 480p image.


Note that an uncompressed 480p image could look better than a compressed 1080i image, in some circumstances. However, an uncompressed 480p stream would require 330 Mbit/sec, approximately 10-20 times the data rate of compressed HD video! All that compression has consequences, which we sometimes forget. There's a compromise between native image resolution and level of compression to obtain the best image with a given data rate.


Dave