[mythtv] lots of interlacing recently...

David myth at dgreaves.com
Mon Aug 9 04:06:18 EDT 2004



Chris Petersen wrote:

>> What does "interlaced" mean?
>
>
> http://en.wikipedia.org/wiki/Interlaced
>
> basically, every other line belongs to the next frame, which give the 
> picture a very jagged appearance if it's not displayed on the intended 
> screen.
>
> -Chris

I  think BM was asking rhetorically :D

I've seen some problems with PAL interlacing too, also through s-video.
All recoverable nicely but not your normal 50fps interlaced video -> 
25fps full mpeg frames interlacing. This is more of an offset by one, 
25fps film -> 25fps mpeg
I wasn't going to bring it up here but since you did...

Have a look at the ivtv 
archives:http://www.poptix.net/ivtv/current/msg00036.html

My first assumption was (is) an ivtv driver bug but it may also be a 
broadcast artefact.
If the former then it's ivtv - if the latter then I guess it's a Myth 
related issue.

Here's how I described it:
Lets look at an mpeg frame grabbed by a PVR[23]50

It contains all 576 lines from two consecutively broadcast interlaced 
frames (lets call them 1 and 2) Remember this frame I'll call it (I)

Now lets look at the next full frame grabbed 1/25th second later.

It too contains all 576 lines from two consecutively broadcast 
interlaced frames (lets call them 3 and 4) Remember this frame too I'll 
call it (II) - no pun intended.

Now imagine this is a video (50fps) source.
frames 1,2,3 and 4 all belong to different timepoints. (see diagram 1 on 
Jonathan's reference page) There is no way to avoid using deinterlacing 
algorithms - tough, live with it (I know, but luckily this *isn't* what 
I'm on about)

Now imagine this is a film based source (lets call it 25fps because of 
some trickery that broadcasters use.)
Now those 4 frames span either 2 or 3 film frames (call them a,b and c) 
like this:
[remember we progress 1/50th second between each char.]

aabb
1234


or like this:

abbc
1234


In the first situation if you have frames 1&2 (ie film frame a) in your 
mpeg frame (I) and 2&3 (film frame b) in mpeg frame (II) - a wonderful 
reproduction of the original film. This will play back well on a 
progressive display.

In the second situation you have frames 1&2 (ie a blend of film frame a 
and film frame b) in mpeg frame (I) and frames 2&3 (a blend of film 
frames b and c) in mpeg frame (II) - really horrible blurry mpeg that 
will always look bad on any progressive scan player - but which will 
play back well on a PVR350.

What type of de-interlacing fixes this?
I've heard "reverse telecine" - does one of Myths filters provide this?

David



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