[mythtv-users] Garbage at top of recording.

Michael T. Dean mtdean at thirdcontact.com
Fri Jun 16 14:19:24 UTC 2006

On 06/16/2006 07:06 AM, A JM wrote:
>  "If you're not overscanning, you can crop it at playback."
>  I was able to get rid of the garbage in playback by setting overscan
>  to 1%, is that normal?


>  Is that preferred to cropping?

That's up to the user.  ;)  (It doesn't matter which you do--you can 
make arguments all three ways.)

Cropping in Myth can only be done in 16-pixel increments.  So, if you 
want to crop 5 pixels from the top and nothing from the other sides, 
you'd specify "=1:0:0:0" and actually crop 16 pixels from the top (the 5 
garbage pixels you want to crop and 11 "picture" pixels).  So, there's 
not a lot of fine-grained control for cropping.  However, because TV's 
are designed to overscan, the "picture" pixels you're cropping probably 
aren't meant to be seen, anyway.

Software overscanning (i.e. in Myth), on the other hand, requires 
scaling the image.  This may cause image degradation.

Hardware overscanning (i.e. where the display actually shows parts of 
the image outside the visible area) works extremely well and allows you 
to play back the image without scaling, but generally cuts off /a lot/ 
of the image--but it cuts off parts you weren't supposed to see, anyway.

Signal overscanning (i.e. using an X modeline) works pretty well for 
analog displays, but for fixed-pixel displays may mean that the image 
has to be re-sampled by the TV (even if played back 
pixel-for-pixel)--unless you have the signal perfectly sync'ed with the 
hardware design.

In other words, which is best is a matter of user preference tempered 
with consideration of the display device.

>  What happens to the recording if I was going to archive it with
>  MythBurn I assume this setting is only for playback within Myth?

Yep.  You'd burn the VBI data to the disk.  However, you can re-encode 
the video (don't know how to do it in MythBurn, but it's possible) and 
remove the portion of the video with the garbage lines.  Doing so, 
though, means a very computationally-expensive decode/recode process 
(i.e. takes a lot longer than just using the video you've recorded).

>  Is
>  there a way to set the overscan when the recording actually is taking
>  place?

Nope.  Overscan basically plays the video so part of it is off screen 
(thus, you don't see the garbage).  Note that normal TV's overscan _a 
lot_ more than you would expect (making the 16-pixels/crop look like 
nothing), so if you play back the DVD on a normal TV with a normal DVD 
player, you won't see it.  If you use xine on a digital monitor, you can 
always zoom out so you don't see it.  (I do this all the time when 
traveling--I copy TV to my laptop and watch it in xine with an 
appropriate zoom.)


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