Why can't I make my LCD monitor display interlaced video?

March 13, 2009 10:45 PM

Why can't I make my LCD monitor display interlaced video?

I can't find any filters to display an interlaced video source directly.

All the solutions I've found for displaying interlaced video on a LCD involve deinterlacing it by combining two adjacent 60Hz fields into a single 30Hz frame with artifacts introduced by the deinterlacing process.

I understand the usual explanation given about how an interlaced video source has a half-vertical resolution field at double the frame rate and that a LCD monitor operates on a "display a color until told otherwise" principle instead of a CRT's "display this color until the phosphor fades out".

I don't understand why I can't tell my LCD to update the odd scanlines for one field while retaining the even scanlines (or simulating the phosphors fading), then the even for the next field, then the odd, etc...

At any given point in time, the CRT is displaying a color from each phosphor, so I ought to be able to duplicate that given a fast enough LCD.

It seems like a modern LCD has a response rate fast enough to match a 60Hz CRT refresh rate. TN panels have 2-5ms responses, well below the equivalent of a 60Hz CRT refresh (1/60Hz is about 16ms).

Does anyone know of a filter/player that will do this or have an explanation why this is technically unfeasible?

posted by Diz to technology (6 comments total)

LCDs can and do display interlaced video. For example, 1080i is an interlaced HDTV format that many LCD HDTVs are capable of playing. It's not a limitation of the panel.

It may be a limitation of the video processor (if you're talking about a TV) or software (if you're talking about a computer monitor). Specific info about the device and/or software you're using would probably help answer this. Also, what does it mean to say you "can't tell it" to do something? What are you doing? What behavior are you seeing?
posted by knave at 10:54 PM on March 13

It's not that it's technically unfeasible. It's that it looks lousy. An LCD doesn't fade like the phosphor on a television, so you can see the interlacing.

I have owned DVD-player programs for the PC where deinterlacing was a configuration choice, and could be disabled, which is what you seem to be asking for. PowerDVD can do that, for example. (I'm pretty sure Zoom Player can, too, but I'll be damned if I can find the setting in the options.)

But it doesn't look good. Even at normal playback rate, you can see it when it happens, and it's distracting. Letting the program deinterlace for you is better.
posted by Chocolate Pickle at 10:59 PM on March 13

The root problem I'm trying to solve is pretty display of video from a PS2 on a computer. I'm currently using Descaler's DirectShow filter to access the VIVO function of a GeForce 7800Gt to acquire the video.

Any of Descaler's actual deinterlacing methods gives me obvious mouse teeth/unacceptable delay/icky bob whenever I have a horizontal pan, so I'm resorting to just displaying one set of scanlines and eating the vertical resolution loss. It looks okay, but it's blury compared to deinterlace methods that use all the vertical resolution or displaying on a real CRT and I'd like it to be better.

Why can't I update one set of scanlines with the new field and fade the other set towards a netural grey/black whenever I get a new field? The time to do the set + fade seems doable (640x480x24bit color @ 60Hz = 54MB/s, fading 27MB/s and writing 54MB/s to video ram seems doable for a modern CPU and PCI-E GPU) and it seems like the response time ought to be more than fast enough for the panel to redraw the entire image every field.

If modern LCDs/GPUs/CPUs are fast enough and there's nothing I'm overlooking, it just seems weird that noone's made a video capture/display program that does this already. If noone comes up with any reason why it doesn't make sense, I guess I'll go learn how DirectShow video capture works and try to build it.

@Chocolate Pickle: I don't think ZoomPlayer can display video from a capture source and I think it just punts decoding off to whatever DirectShow filter chain is set up anyway (or at least I have mine set up to do that and don't see any obvious settings to have it to do anything else). PowerDVD may well do what I want, but I don't have a license for it and it seems like the sort of thing that would have been implemented by an open-source widget.
posted by Diz at 12:48 AM on March 14

Some PS2 games support progressive scan. For those games, that is definitely the best option, if your VIVO can do it.
posted by aubilenon at 12:54 AM on March 14

If half the scanlines were displaying material and the other half were faded to gray, the resulting image would look washed out and desaturated.

The guy who owns this blog does what you're talking about. I can't find the posting where he wrote about it, but he has a Wii and a PS2 which feed into a TV card, and he displays the image on his computer monitor using the TV player program that came with the TV card.
posted by Chocolate Pickle at 12:58 AM on March 14

Maybe you should look at this from the other side; ie, getting your PS2 to output progressive video on all games.

The PS2 HDTV Xploder is a boot disc that forces your PS2 to output at various resolutions. It sounds like it doesn't work that well for 720p or 1080i, but that its 480p output is decent.
posted by Oktober at 9:05 AM on March 14