[opendtv] Re: Spectrum Use for Broadcasting

At 1:19 AM -0500 2/28/08, Cliff Benham wrote:
There is a somewhat noticable improvement in detail, but the major difference between the 480i and 480p outputs is the lack of visible aliasing. Aliasing is far more distracting and annoying to me.
I want to try this same comparison with an HD-DVD player playing an HD DVD as the source to see if there are noticable differences between the HD DVD source and the HDTV program received by the Philips.
[I have noticed the aliasing problem is more visible when upconverting from 480i than when downconverting from 1080i to 480p. When playing any DVD in in a 1080i capable player, the 1080i signal is available only from the HDMI output, and the component output is automatically down rezed to 480p with a screen popup notice that explains the maker of the disk requires this mode of operation.]
There should be a much more visible difference between 480i and 480P. Me thinks that they are not overly concerned about the quality of the 480P output.
By the way, both the 1080i and 480P outputs from a DVD are up-conversions. The original source is already dumbed down for interlaced displays, eliminating excessive vertical detail. The upconversion circuitry may attempt to put back together the original 24P film frames before the up conversions, but these frames have already been filtered to prevent flicker on interlaced displays, then MPEG-2 compressed, which may remove additional detail.
This is just speculation, but when the source is 1080i it is possible that they do a single downsample for both 480i and 480P, push the 480P directly out to the component output, then encode the components to produce the Y/C and composite outputs. IF this is the case they may be limiting the vertical resolution so that there will be no aliasing (small area flicker) on the composite and Y/C outputs. To do 480P properly you can allow nearly double the vertical detail, but this would cause major problems for the interlaced outputs, which would require an additional convolution filter to eliminate aliasing.
There is also an issue with conversions from 1080i, as to produce a good 480P signal you must deinterlace the 1080i first. My guess is that they downsample each 1080i field to 240 lines to produce the composite output. They MAY downsample to 480 lines as well, which will produce a decent 480P image, but not as good an image as downsampled 720P frames.
My guess is that they don't do 480P properly, due to the small number of 480P displays in the marketplace. The conversion to 480P for the component output is a Hollyweird requirement to keep you from having access to an analog component HD output - as if you could record this output at home. Hollywood might be upset if they provided a 480P output that looks almost as good as the 1080i output, which it should. 

Regards
Craig


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