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

Steve Hodge stevehodge at gmail.com
Fri Jun 2 22:47:19 UTC 2006


On 6/3/06, 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.

You're getting confused between frames and fields. A field is half the
height of a frame, for PAL that's 288 lines per field and 576 lines
per frame. PAL is 50 fields per second. The lines of each pair of
fields are interlaced: a frame is made up of one line from field one
and then one line from field two and so on. You can think of a frame
as being transmitted in two halves (fields), first the odd numbered
lines and then the even numbered lines (or vice versa). That's not
quite accurate though, as the two fields represent slightly different
points in time - that's why you get "combing" on the edges of fast
moving objects when you look at two fields together.

Deinterlacing is the process of converting that interlaced signal into
a non-interlaced signal where each frame has all the lines in order.
There's really no distinction between frames and fields in a
non-interlaced signal, though you think of it as a signal where each
frame is made up of one field.

Now, the MythTV deinterlace algorithms:

"One field" is the simplest. It just makes an output frame out of one
input field and discards the next input field. Since you get a new
frame every second input field, the framerate is 25Hz (for PAL). You
also lose half you're vertical resolution as you're only using the 288
lines from each second input field.

"Linear blend" and "Kernel" both use at two fields to produce one
output frame. "Linear blend" simply blends adjacent lines. This has
the effect of reducing combing but does not eliminate it. "Kernel"
blends several lines via a more complex algorithm and is generally
considered superior. In both cases you get one output frame from two
input fields so the framerate is 25Hz (for PAL). However you maintain
the full vertical resolution are you're using the full set of 576
lines from the two input fields.

"Bob" doesn't use blending at all. What it does is to simply take each
input field and double it's height (by duplicating each line) then
output them at the same rate. So you get one output frame for each
input field. Thus the framerate is 50Hz. For a given output frame your
vertical resolution is only 288 lines (as all the lines came from one
field), but you are using all the available input lines so this
doesn't look too bad.

Hope that rather long description helps you. If you need more info, a
good site is www.100fps.com.

Steve


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