A true Progressive output?

[KennyG]

As I understand it todays progressive scan DVD players don't actually output a true progressive signal, it's doublelined, and the reason for this is the signal coming off the disc is actually interlaced during the MPEG decoding process.
If this statement is true, do all MPEG decoders have to interlace the signal during decoding?
If this is true of stand alone DVD players, is it also true of HTPC?

[JonMarsh]

Hi Kenny,

Line doubling means just that- taking an interlaced video signal, doubling each frame by shifting it down, and scanning that out as a progressive signal. Some "doublers" do just that, but not in a "progressive scan" DVD player, and not in an HTPC, normally.

There are two types of "source" material on DVD's: film and video. Film is stored in sets of 24 interlaced frame pairs, with SMPTE codes identifying the frame pairs. A progressive scan DVD player or an HTPC checks the frame codes, assembles the matching frames, producing 24 frames per second of progressive material, and then plays them back using 3/2 pulldown if 60Hz playback is used (as for TV's, with a conventional progressive scan DVD player), the same happens in an HTPC IF 60Hz refresh rates are used; ideally, though, a refresh rate at some multiple of 24 Hz should be ussed, so that a simple integer multiplication of 24 FPS is used. Now, the situation can be complicated by factors such as the incorrect MPEG frame codes existing on the DVD; this DOES occur, from time to time, and the some of the newest players (such as the Sony DVP9000ES) have circuits to watch the vide frames, compare them, and detect and overide "bad" codes.
For video, life is more complicated; on PC's, either bob or weave is offered; bob will effectively halve the vertical resoultion, and weave of course can still have some of the artifacts that result from motion in an interlaced field; the best systems uses some spatial filtering, to smooth and interpolate between the two frames; this lowers vertical resolution slightly, but is perceptually less objectionable than simple BOB.

Have a good weekend,

Jon




Earth First!
_______________________________
We'll screw up the other planets later....

[KennyG]

Alrighty then...one more question...why don't today's "stand alone" progressive DVD players offer switchable frame rates? Something like 72 or 96hz for film based movies, switchable to 60hz for video based stuff?
I'm assuming HTPC does do this...

[JonMarsh]

Because, todays DVD players are designed to the home entertainment specs, for SDTV resolutions, all of which specify a 60Hz field rate; same for 1280X720 HDTV and 1080I HDTV. All run at the same vertical scanning frequency, and only change the horizontal freuqnency. Horizontal frequencies are 15.75 kHz (480i), 31.5 kHz (480p- SDTV and progressive DVD), 37.5 kHz (1080i, 540P), and 48 kHz. Graphics and data grade front projectors are multiscan devices, like computer monitors, and can work at a variety of vertical scan frequencies.
There's a German progressive mod board to DVD players, which does support scaling to higher resoultions, and higher vertical refresh rates, and it will provide a good signal for multi-scan monitors and projectors, but you can't drive a conventional HDTV set at higher than 60Hz.

Regards,

Jon




Earth First!
_______________________________
We'll screw up the other planets later....

[KennyG]

Thanks Jon, I forgot to mention, I was thinking of FPTV when I wrote that last question, hence the "switchable refresh rates" question.
Since I've just sealed a deal on my first projector, I'm putting together thoughts on HTPC, and what I want...or need mine to do.