[mythtv-users] Having problems / Need info on BOB Deinterlacing

[James Buckley]

OOOoooh. That also makes sense (almost.)

 

So, how do I know if linear blend is producing 25 or 50 fps.

 

Basically, I want to emulate what my TV does. With Sky, the pictures is
perfect (at least to my eye), from another thread I'm running apparently sky
does no deinterlacing, and merely passes an interlaced stream (which would
have 50 "half frames" right) to the TV. The TV then deals with it how it
likes (either by deinterlacing the image or just displaying it).

 

I want the same setup; only my problem is it's hard for me to have an
interlaced DVI connection. So the solution must be by deinterlacing the
stream before it's sent). OK, so what refresh rate should I have my TV set
to (I can control this using a ModeLine, I can get as close as 50.01Hz), and
what deinterlacing should I use (don't worry about computing power
limitations, my 64 3500+ should cope with at least SDTV). If I am to use
BOB, I don't (if I can help it), want any "BOBing", ie. Jumping.

 

Thanks for your help.

 

P.S. If I can set up an interlaced link, can I then just pass the video
through without deinterlacing it, therefore allowing the TV to process it?

 

  _____  

From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Niels Dybdahl
Sent: 02 June 2006 22:05
To: Discussion about mythtv
Subject: Re: [mythtv-users] Having problems / Need info on BOB Deinterlacing

 

On 6/2/06, James Buckley <james at logicland.co.uk> wrote:

Actually, I'm wrong. Linear blend, or kernel, (and don't quote me on this,
this is just my understanding), convert [x] interlaced frames into [x]
progressive frames, by guessing at the missing content. Seeing how BOB is 
listed as giving 2x framerate, that must mean 2x the others (IE linear blend
and kernel), therefore BOB deinterlacing must give [2x]. So for example with
PAL TV, BOB must give a 100fps output. Therefore BOB deinterlacing is best 
used with a monitor that can have a 100Hz refresh. The reason the screen
jumps is a side effect, but it's removed when you only see exactly half the
frames BOB is outputting (IE on a 50Hz TV). On a 100Hz refresh monitor or 
TV, every frame will jump, but at a 100 times a second, it probably isn't
very noticeable.

Sorry for the long babble, only just worked it out...


I think it is a little different. Interlaced PAL has 25 frames per second.
Each frame consists of two "fields": A field with the even lines and a field
with the odd lines. So the field rate is 50 fields per second. 
An almost static image will display with the full resolution of 576 lines
while a moving image will have the resolution at 288 lines as the two fields
dont fit, (which creates the combing effect on progressive displays), but
keeps the display updated at 50 Hz. 


The description of the linear blend filter says that it averages each line
with the value of the next line (which is taken from the opposite field).
This calculation require two fields, so it could be done once per frame or
once per field, which would give either 25 frames or 50 frames per second. I
have not been able of finding exact information on this. However it is
impossible to create information that is not there, so it can not create a
resolution of 576 lines at 50 Hz, so the real resolution is equivalent to
288 lines. You might imagine an example where all the odd lines are black
and all the even lines are white. Averaging the lines as described will give
a uniform gray image, so the linear blend filter reduces the resolution by a
factor of 2 for still images and also might reduce the update rate by a
factor of 2 depending upon how often the calculation is done. 

My understanding of Bob is the lines in each field are doubled, so that each
field makes a frame. The frame frequency for PAL will then be 50 Hz which is
twice that of the interlaced signal. But the resolution is reduced to 288
lines. Again imagine the example of even lines being white and odd lines
being black. This should blink between completely black and completely white
frames. So it can not show a resolution of 576 lines. 

In the description of the "transcode" tool on
http://www.transcoding.org/cgi-bin/transcode?Transcode, there are mentioned
deinterlace functions named smartdeinter and smartyuv, which only reduces
the resolution on moving parts. Why doesnt MythTV have such a filter? Does
it take too much processing power? 

Niels Dybdahl

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