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

[Niels Dybdahl]

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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