[mythtv-users] Black Bars in Mythtv

[Jonathan Rogers]

On Sat, Feb 9, 2008 at 1:23 AM, Michael T. Dean <mtdean at thirdcontact.com> wrote:
> On 02/08/2008 08:18 PM, K Madman wrote:
>  > I recorded a show and saw on the left and right sides
>  > there are black bars in the actual recordings. I thought at first it
>  > was a problem with my pvr-150.  Now I just put together a completely
>  > different mythtv box with a brand new pvr-150 and when it records it
>  > also has black lines on the side.  The black lines are not from aspect
>  > ratio.  On my old box everything was ok until I ran an update for
>  > mythtv.  What causes them and how do I get rid of them?  Please help.
>  >
>  > This is a snap shot from mythweb where you can clearly see the black
>  > bar on the left side of the recording. (little black bar on the right
>  > also)
>  >
>  > http://www.geocities.com/k7_madman85/SAMPLE.jpg
>
>  You definitely have a problem.  That problem is called NTSC (or,
>  perhaps, PAL).  In other words the problem is you're using an analog
>  format that was defined to have unclean edges.
>
>  If you want to get rid of the black bars:
>  a) overscan the video output so that the black bars are "displayed"
>  beyond the edges of your TV,
>  b) transcode the file and crop while transcoding
>  c) use the crop filter to crop the black bars while playing back the
>  recording

You're absolutely right that the unclean edges that vary with
different video sources is part of the nature of ancient analog TV
design. I didn't really appreciate what overscan is and how much there
is on a typical TV until I started capturing TV and displaying it on a
PC monitor, which I configure to have no overscan and no underscan.

I also have a MythTV machine with a PVR-150 and a PVR-350. The PVR-350
output looks great on a regular analog NTSC TV, since that's what it's
designed for. It's output has a lot of overscan by default, which is
fine for displaying the TV signals it and the PVR-150 record, but is
annoying when the mythfrontend menus aren't completely visible, which
requires configuring mythfrontend to make its window smaller than
fullscreen.

>
>  Note, though,
>
>  So, the only real solution is to go to a format that has clean edges:
>  digital TV.  Note, though, that if you get digital TV signal from
>  satellite/cable to a set top box and output to a PVR-x50, you'll get
>  black bars because the STB will be outputting analog (which is designed
>  to require overscan because of the unclean edges)...  If you have analog
>  to the PVR-x50 (i.e. cable/antenna plugged directly into the capture
>  card), you may not see the black bars.  Instead, you'll likely get
>  green/purple lines and jaggies and rainbows and waviness and other stuff
>  to replace the black bars.

Unfortunately, even moving to all digital TV doesn't necessarily get
rid of junk around the edges. The MythTV system I mostly use has two
digital ATSC receivers and one PVR-150, which is seldom used. Quite a
bit of new programs I record from local digital TV broadcasts are HD
720p or 1080i, and look very nice, with clean edges. However, the
local stations still broadcast a lot of shows at SD 480i resolution
digitally, and those broadcasts often have black bars on top and
bottom, left and right, or both. Quite a few of these shows were
designed to be shown with a 16:9 aspect ratio, but I have to use
MythTV's fill mode to get them to fill the screen of my 16:9 HDTV.
They also often have junk right at the top of the frame, which I think
might be something like CC encoded in the VBI which was incorrectly
included in the video frame when digitizing.

-- 
Jonathan Rogers