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

[Niels Dybdahl]

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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