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

[James Buckley]

First, sorry that my posts keep appearing at the top and in html, I don't
know how to make MS Outlook (yuk!) do it the normal way.

 

Yeah I'm recording from DVB-T, and my card has hardware MPEG2 compression. I
like Linear deinterlacing also, but again it just doesn't have enough fps to
make snooker or tennis watchable (for example), the inbuilt tuner in my TV
gives perfect picture. I've yet to get any modeline that works (my TV is an
LG 37LP1D), so I'll worry about the framerate problem later

 

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From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Niels Dybdahl
Sent: 01 June 2006 08:31
To: Discussion about mythtv
Subject: Re: [mythtv-users] Having problems / Need info on BOB Deinterlacing

 

On 5/31/06, James Buckley <james at logicland.co.uk> wrote:

Hello, I've been building my Myth box for well over 3 moths now; everything
works except for one major problem, TV playback (lol).

I find that TV playback with either no deinterlacing or non BOB
deinterlacing just doesn't have enough frames a second to be watchable. For 
example, if you compare news tickers you get on sky news or BBC news, over
normal Freeview / DVB-T, they are much smoother than through MythTV. Unless
I can get the tickers to be as smooth as normal TV (or very close to), I'm 
gonna end up scrapping this project, something I really don't want to do.

My setup is as follows, I have a Myth box running latest SVN build,
outputting over DVI (from a Nvidia 6200 chipset) direct to my LCD TV. I 
haven't created a custom modeline, as I can get crisp, no overscanning
output, using the resolution 1360x768, progressive. Now, the solution to a
low frame rate is to use BOB deinterlacing, this makes TV smooth, but has 
some very horrible side effects.

1. The video jumps up and down, not a little bit, but really noticeably
2. The OSD goes horrible, hard to describe, but you no what I'm talking
about
3. Any Interactive overlays, like "push the red button" are given the same 
effect as 2.

Certain scenes on TV go really horrible when using BOB deinterlace, for
example anything with lots of horizontal lines is almost unbearable to
watch, as they jump up and down, also text (as this has horizontal lines 
also) is jumpy, on the news tickers it jumps , and cause it's close to the
edge of the ticker, it's really noticeable.

I'm sure it can't just be me who can't put up with these problems, so does
anyone have an idea what causes these, what can be done to minimise them, 
and if there are any alternatives to BOB deinterlacing, but still doubling
the frame rate.


I think you are running into a combination of several problems:

- Bitrate. Ticker lines seem to be very hard to compress into MPEG2. So to
have soft moving ticker lines you need as high a bitrate as you can get if
you record from an analog signal. If you record from a DVB signal then you
can not do anything at that point. Having MythTV record a DVB signal should
be as good as using any other DVB receiver. 

- Frame rate. If the frame rates in your system do not match, then you will
get some kind of low-frequency disturbance. There are three frame-rates to
consider. You receive a signal with some framerate. In the UK you use PAL-I
which have 50 half-frames per second. Then your videoadapter sends the
videosignal to your monitor. If you do not modify your modeline, it will
probably send 60 or 70 full-frames per second. By deinterlacing you will
convert the 50 half-frames to either 25 full-frames or 50 full-frames per
second. Both will give some low-frequency disturbance when you output with
60 or 70 full-frames per second. You should find a better modeline. Third
your LCD-monitor has an internal refreshrate. The LCD pixels are mounted in
a grid and only one row of pixels are updated at a time. Some older monitors
have a fixed refresh rate at 60 frames per second. If that is the case, it
will never look very good, especially not if your videoboard outputs at 70
frames per second. 

- Deinterlacing. Bob will make OSD and similar jump up and down. I did not
like it, so I chose the linear deinterlacing instead.

Niels Dybdahl

 

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