7 replies to this topic

[nemdani]

Channel sends 576i50 and output mode is 1080i50.
Is there de-interlacing happening?
If so how is it re-interlaced again?

[Erik Slagter]

It works like this:

- the 576i50 frame (2 fields, actually) is "expanded" to two 576p50 frames (quasi-progressive, as it does not feature full progressive motion data, it is a good as interlaced, but without the interlacing artifacts)
- the resulting frames are scaled up to 1080p50
- two of those frames are interlaced into two 1080i50 fields, which ar combined into one 1080i50 frame.

I hope this is understable?

This way no interlacing artifacts occur and motion data is preserved as good as possible.

If you're connecting to a television set, you should never deinterlace, btw. The television does something similar internally, it does not literally deinterlace, but it produces a full frame from every field, so no motion data gets lost.

[nemdani]

- the 576i50 frame (2 fields, actually) is "expanded" to two 576p50 frames (quasi-progressive, as it does not feature full progressive motion data, it is a good as interlaced, but without the interlacing artifacts)

So bobbing.

- the resulting frames are scaled up to 1080p50
- two of those frames are interlaced into two 1080i50 fields, which ar combined into one 1080i50 frame.

There are no 1080i50 frames. "i" means there are fields. Or is it some kind of fake-interlacing?

How exaclty is the re-interlacing done? Take the odd lines from the first frame and the even lines of the second frame?

If you're connecting to a television set, you should never deinterlace, btw. The television does something similar internally, it does not literally deinterlace, but it produces a full frame from every field, so no motion data gets lost.

That multiple deinterlacing steps is bad, is a given.
Full frame from every field sounds like bobbing which is deinterlacing.
But if the STB is already deinterlacing and then does the re-interlacing like speculated above weaving would be still lossless. Not sure if any TV can do weaving however.

Too bad i can't output 1080p without the help of the Autoresolution plugin. Why is that?

[Erik Slagter]

So bobbing.

Actually bobbing + deinterlacing (most probably), so you get complete, "real" frames from individual fields.

There are no 1080i50 frames. "i" means there are fields. Or is it some kind of fake-interlacing?

This may all sound a bit dazzling, but it really is. Two 1080p50 frames are transformed into two 1080i50 fields (i.e. even lines from the one, odd lines from the other) and these two 1080i50i fields are combined into one 1080i50 frame. And yes a 1080i50 frame is actually built up from two fields, but they are always transmitted as one "pseudo" frame.

Full frame from every field sounds like bobbing which is deinterlacing.

Bobbing in itself is surely no deinterlacing, although it's mostly used in combination with deinterlacing.

But if the STB is already deinterlacing and then does the re-interlacing like speculated above weaving would be still lossless. Not sure if any TV can do weaving however.

The whole process is not lossless, but it has the lowest loss possible. At least it's much better than to deinterlace before scaling and much much better than scaling without any preparation.

Too bad i can't output 1080p without the help of the Autoresolution plugin. Why is that?

Actually you don't want that. The 1080p mode is 1080p25. This means the 1080i50 is deinterlaced -> jerky motion. In theory you'd want this mode for 1080p25 source material, but there is actually no loss in outputting such material as 1080i50.

720p50 is another story, you'd want to use the autoresolution plugin to send it to the television in full p50 frame rate and not in i50.

[nemdani]

Bobbing in itself is surely no deinterlacing, although it's mostly used in combination with deinterlacing.

I disagree. Bobbing by itself ist deinterlacing. See http://en.wikipedia....n_deinterlacing
But yes, today bobbing is often combined with motion adaptive interpolation.

Also i don't see how digital TVs (LCD, Plasma) are able to display anything but full RGB frames, which needs deinterlacing. There's no interlacing mode like for CRT TVs.

The whole process is not lossless, but it has the lowest loss possible. At least it's much better than to deinterlace before scaling and much much better than scaling without any preparation.

Agreed. Lossless was really the wrong wording for it.

Actually you don't want that. The 1080p mode is 1080p25. This means the 1080i50 is deinterlaced -> jerky motion. In theory you'd want this mode for 1080p25 source material, but there is actually no loss in outputting such material as 1080i50.

True, but most content is truly 25 progressive frames and just converted to 50 fields for broadcast. (strictly speaking it's orignally in ntsc film or video format and then transferred to pal, which by itself can be done quite horribly with blend conversions) Not sure if any STB can detect the 2:2 cadence and restore the 25 progressive frames, but that way you wouldn't lose any information. (Autoresolution displays some content as 25 Hz, is that it?)
The only content that usually has really 50 unique temporal fields is sport.

720p50 is another story, you'd want to use the autoresolution plugin to send it to the television in full p50 frame rate and not in i50.

Good point and this seems like the mode you should use if you get annoyed by autoresolution's constant mode switching and if your main channels aren't 1080i50.

[Erik Slagter]

Also i don't see how digital TVs (LCD, Plasma) are able to display anything but full RGB frames, which needs deinterlacing. There's no interlacing mode like for CRT TVs.

No indeed, BUT if you do the trick I've described (bob + interpolate) you will get motion as smooth as on a CRT, as opposed to true deinterlacing, which discards temporal information.

True, but most content is truly 25 progressive frames and just converted to 50 fields for broadcast.

No that is NOT true, unless you only watch movies shot for cinema. Broadcast camera's all shot in 50 fps interlaced.

The only content that usually has really 50 unique temporal fields is sport.

In that case I'd suggest you enable 25p output on your stb (you can arrange that by writing the relevant /proc/stb/video entries) and see how you like that. I think you'll get fed up with the jerky motion soon enough.

Believe me, I've done enough video editing on a frame-by-frame basis to know that fields always differ (= combing effect), even if though it seems there is little motion. And also I know from experience that most TV programs are horrible to watch in 25p.

[nemdani]

No that is NOT true, unless you only watch movies shot for cinema. Broadcast camera's all shot in 50 fps interlaced.

I mostly watch US content. Those went to some nasty NSTC to PAL conversion (especially for NTSC video), where fake spatial resolution gets added and temporal resolution is lost.

In that case I'd suggest you enable 25p output on your stb (you can arrange that by writing the relevant /proc/stb/video entries) and see how you like that. I think you'll get fed up with the jerky motion soon enough.

Believe me, I've done enough video editing on a frame-by-frame basis to know that fields always differ (= combing effect), even if though it seems there is little motion. And also I know from experience that most TV programs are horrible to watch in 25p.

That might be true due to bad interlacing done by the broadcaster. But real temporal resolution of that content is still not 50 fields/second. I usually encode those with Avisynth's TempGaussMC to 25 fps (tho i hardly encode PAL content) and i think that will look better than the fake 50 fps your STB/AVR/TV is doing.


I'm very much interested in messing with /proc/stb/video. What do i need to do?
Also sure that Autoresultion's 720p 50Hz output (i'd rather use multi, if the automatic switching to that 50 Hz mode is reliable) is indeed real 50 progressive frames and OpenPLi's 720p 50 Hz in A/V settings is 50 fake-progressive frames (i.e. fields). How can i check? The only indication i have so far is that Autoresolution labels video content with 50 fields per second as i25.

[Erik Slagter]

That might be true due to bad interlacing done by the broadcaster. But real temporal resolution of that content is still not 50 fields/second.

I don't know about "your" broadcaster but "my" broadcasters send real 50i, not 25p-over-50i. It can be easily seen when the output is switched to 25p.

I usually encode those with Avisynth's TempGaussMC to 25 fps (tho i hardly encode PAL content) and i think that will look better than the fake 50 fps your STB/AVR/TV is doing.

Whatever you like, I prefer to keep as much data (both temporal as spatial) as possible.

I'm very much interested in messing with /proc/stb/video. What do i need to do?

cd /proc/stb/video
cat videomode_choices
# there you can see all possible output modes
echo "1080p25" video_mode

If that doesn't do anything, try the videomode_50hz(_choices)

Also sure that Autoresultion's 720p 50Hz output (i'd rather use multi, if the automatic switching to that 50 Hz mode is reliable) is indeed real 50 progressive frames and OpenPLi's 720p 50 Hz in A/V settings is 50 fake-progressive frames (i.e. fields). How can i check? The only indication i have so far is that Autoresolution labels video content with 50 fields per second as i25.

Autoresolution uses exactly the same mechanism as I described above, it just temporarily switches the output mode, nothing more nothing less. 720p50 is exactly what it is, 720p50. If the source material is 720p50, it will be send as-as when the output mode is 720p50.

You can check by inspecting these pseudo files in /proc using cat, e.g. cat videomode.