It's true that the information on DVDs is recorded in an interlaced form, so it has to be deinterlaced at some point for display on a plasma or HDTV monitor. The advantage of a progressive output DVD player is that the deinterlacing occurs in the digital domain, before the digital-to-analog converter. This avoids the introduction of some artifacts, and is usually (but not always) better than external deinterlacing.
The situation gets a little muddy, though. Although external deinterlacers have to convert the signal to digital, deinterlace, and then convert back to analog, if their electronics are markedly better than those in the DVD player, they can sometimes exceed the performance of progressive scan players.
So, the bottom line is that the quality of the image you see depends on the relative performance of the DVD's internal deinterlacer vs. the external one, but with the deck stacked in favor of the internal one.
The situation gets a little muddy, though. Although external deinterlacers have to convert the signal to digital, deinterlace, and then convert back to analog, if their electronics are markedly better than those in the DVD player, they can sometimes exceed the performance of progressive scan players.
So, the bottom line is that the quality of the image you see depends on the relative performance of the DVD's internal deinterlacer vs. the external one, but with the deck stacked in favor of the internal one.
