Deinterlace for a regular (interlaced) TV?

[ed6612]

Hello,

I am using GBPVR with PVR500 card on a system that is only connected via TV-Out to regular projection TV set. In this setup does it make any sense at all to deinterlace? The TV itself is interlaced so why go through a process of deinterlacing playback. I have tried chaning the deinterlace setting from encoder-passthrough to none and it did not seem to have any effect on picture quality. Wondering what the settings be in this scenario for optimal quality?

Thanks,

-Ed

[groover km]

Nope, it will have no effect on an interlaced display such as a SDTV.

[ed6612]

Thanks, thats what I figured.

I am also using Intervideo codec for MPEG2 decoding and following some advice switched codec deinterlaving to weave via the registry value:

"BOBWEAVE"=dword:00000002

I believe this has improved the tv-out display quality somewhat.

[csy]

I have also been wanting to pass the interlaced video straight to my SDTV without deinterlacing.

IMO deinterlacing is a very very bad thing. It takes a pure picture (interlaced) and modifies (distorts and adds artifacts) to make it presentable on a progressive-format screen. Additionally it halves the temporal sampling rate. The picture quality without deinterlacing is much sharper and clear compared to deinterlacing and does equal the the same quality as a standalone TV. So it makes sence not to deinterlace if you don't have to.

One thing to be cautious of when experimenting with transparent interlacing (no deinterlacing) is that the broadcast TV may be transmitting progressive-format programmes/shows in the interlaced signal. This can trick you into thinking that your "no deinterlacing" is working fine when it actually isn't. You need to view interlaced-camera-recorded shows, eg live sports and lookout for the jaggied edges (jaggies). Interlaced-recorded ticker tapes are also a good source to view how well it is working. As a double-check, deliberately try watching on a computer monitor, - you should see all the interlace jaggies as movement occurs (if you don't then deinterlacing is occuring or the programme/show is progressive format).

You stated that you changed the LiveTV deinterlace setting from Encoder-passthrough to None. In "none" mode you are bypassing the MPEG2 encoder and decoder and instead directing the raw captured video (YUV stream) directly to the renderer. This means there are no video decoders in the circuit, ie you are not using the Intervideo decoder. With that said however the raw YUV stream is feeding interlaced video directly to the renderer and hence to the video card.

If you have the LiveTV deinterlace setting configured to encoder-passthrough then you maybe using the Intervideo decoder, and I agree "BOBWEAVE"=dword:00000002 in the GBPVR sub-directory of the Intervideo Video decoder registry settings will force Weave (no deinterlacing).

I recommend you use graphedit and open lastgraph.grf in the GBPVR program directory to confirm exactly what video decoder you are actually using.


There are several steps you must follow to ensure the captured interlaced raster lines are exactly 1-for-1 mapped to the same raster lines on your SDTV. By default the mapping is not 1:1. If they do not map 1:1 then some raster lines will be presented in reverse interlace-field order resulting in double-difference interlacing jaggies seen on the TV when motion occurs.
Starting from the source capture resolution which is nominally 720x480 for NTSC and 720x576 for PAL.
1) Set the screen resolution to be the same as the capture resolution.
2) Disable any aspect ratio correction (use stretch to full screen) (not applicable for "none" mode and the Intervideo decoder does not have this setting when used in GBPVR).
3) Disable video card scaling/overscan correction on the TV-out. On ATI cards enable TV "video mode" (no action required for Nvidia cards).
4) Disable (meaning zero) GBPVR scaling and overscan correction in config.xml
Note: GBPVR v.0.96.12 appears to have an internal scaling problem requiring a tweak to correct this as per thread:
http://forums.nextpvr.com/showthread.php?t=14475

The next problem is to get the interlaced field order presented in the correct order to the SDTV. This is where everything starts falling apart. Interlaced video has 2 fields, one containing the odd numbered raster lines (odd-field), and the other containing the even raster lines (even-field), with the fields running at double frame-rate but only half the vertical resolution. For broadcast TV the odd-field is recorded and played-back first, but Digital Video (eg handycams) has the even-field first. In my experience you have a 50/50 chance of striking the correct field order (with either raw-YUV or MPEG2), resulting in double-difference interlacing jaggies seen on the TV when motion occurs.

Finally the last problem is the field-rate synchronisation. What should happen is the broadcast frame-rate should be used to clock all the downstream frame-rate generators, and therefore it is all synchronisted and no field slips (drops or gains) occur. For the most part this all works correctly, however in my experience the TV-out encoder chips on Nvidia and ATI video cards appear to free-run, resulting in periodic dropping or gaining a field. The problem is each field slip results in swapping the field order presentation to the SDTV, therefore resulting in double-difference interlacing jaggies seen on the TV when motion occurs. So this shows up as say 2 minutes of smooth motion then 2 minutes of juddering motion, repeating.

So in conclusion it hit the too-hard basket for me and I am now deinterlacing.

Anyway I wish you the best if you have successfully got transparent interlacing to work correctly.

Edit: corrected some typos and missed words to make it read better.

[ed6612]

Thanks for the in-depth explanation Csy. I have experimented a little more and saw no noticeable quality difference between LiveTV and Timeshifting modes despite having LiveTV deinterlace setting set to 'None' in the config.

This makes me belive that interlacing is not a factor in my case since Timeshifting forces the use of the MPEG2 decoder in order to playback the buffered stream from the hard drive?

I will use GraphEdit as soon as I figure out how this utility works (still a newbie )

What exactly do you mean by interlacing jaggies as viewed on the screen?

-My resolution is already set to 720x480 (NTSC)
-My tv-out is via S3 UniChrome with no scaling or aspect ratio correction and an enabled deflicker filter (no way to disable it in the UI).
-GBPVR overscan is all 0, VerticalZoom at 1.0 and HorizontalZoom at 1.05. AspectRatio is 0,0,1,1

The main visual differences between Tv-Out quality vs. the TV's Tunner are:

1. TV-Out causes horizontal scan lines to stand out more and be discerned more easily
2. The overall picture is more blury with Tv-Out
3. Like you mentioned about field-rate synchronization, by observing a channel with a horizontal stock tickler it does seem that the motion is more jugged or smooth at times without CPU spikes.

I will keep playing with this some more and see what else I can find out.

Thanks again,

-Ed

[csy]

It's good to see you having a play

The key advantage of no deinterlacing is a very sharp picture, near equal to a standalone TV. Because you have a more blury picture, then I suspect sub-optimal configuration or poor TV-out capability on your video card (I don't have any experience with the S3 video cards).

Jaggies refer to sawtooth effect on vertical edges when the edge moves on the screen, which is seen on progressive-scan devices (computer monitors), but should not be seen on interlace-scan devices (SDTV). This is because the odd lines record and playback movement ahead of the even lines, hence there is a time displacement between each horizontal line. This should not be seen on the SDTV because it displays all the odd lines first followed by the even lines at the correct time displacement.
Check out the examples on this link: http://www.100fps.com/

If however you have an incorrect mapping of scan lines then some odd numbered lines will become alligned with the SDTV even lines and be displayed on the screen with the wrong time displacement. This time displacement is double what it should be and will show up as either very bad jaggies or badly blured vertical edges. The fact that you are seeing scan lines, strongly indicates that the mapping is very wrong. You should not see any difference between standalone-TV and the TV-out.

Something else to consider is that there maybe some form of default interlacing occuring that you are unaware of which is causing a blury picture ("bob" deinterlacing typically gives a blury image compared to standalone TV). To prove this, connect a computer monitor to the video card and if there is no deinterlacing then you should clearly see jaggies on moving objects on the monitor. If you do not see jaggies on the computer monitor then some form of deinterlacing is occuring.

Edit:
I forgot to mention that the deflicker filter will definately make the picture blury. You need to find a way of disabling it.