[mythtv-users] PVR150: Automatic removing black bar(s) from the recording

Brian Wood beww at beww.org
Tue Mar 27 23:53:56 UTC 2007

On Mar 27, 2007, at 5:23 PM, Ma Begaj wrote:

> Hi,
> I have black bars around mpeg recordings. They are not shown if played
> in MythTV, but MPlayer shows these. I read the IVTV mailing list, and
> moste of the people say this:
> ------------
> "This is probably just in the source signal. A TV will overscan the
> signal by 4% or more, so broadcasters can put whatever junk they want
> at the edges of the pictures and most people will
> never see it.
> ------------

Then "most people" are full of crap.

As is usual with faulty statements, there is a kernel of truth there.  
Most TV sets do in fact overscan, the main reason being so that as  
the set ages and capacitors get leaky and scan voltages drop the  
picture will still cover the entire viewable area of the screen,  
resulting in a higher perceived "reliability" of the set. People will  
decide the set is "broken" if they can see the picture edges but, as  
you said, usually don't notice the overscan.

There are actually SMPTE-defined "safe action" and "safe title" areas  
of a TV picture, so that titles and action will not be placed where  
they might be missed on a typical consumer screen.

Unfortunately neophyte directors, editors and other technical types  
are either unaware of these accepted standards or simply don't care.  
This is especially true with non-broadcast networks or even broadcast  
stations in small markets.

Professional monitors actually have an "underscan" button to reduce  
the picture to less than the screen size so that the director is  
certain he can see the entire image.

Information is put into the vertical interval (the black bar at the  
top of the screen) This includes closed captioning on line 21 and  
various test signals used for remote control of transmitters and  
video testing being on lines 15 through 19.

No information is put into the horizontal blanking area (to the right  
and left of the picture), at least in western nations (some Russian  
satellite transmissions encode audio into the horizontal blanking, or  
at least used to).

Too wide blanking is a common problem, it used to result in a large  
fine from the FCC if you got caught, but you can get away with just  
about anything these days.

All this refers to analog video, but even if transmitted digitally  
most video these days originated as analog, very few actual digital  
cameras and program chains are in use. This will of course change  
with time.

> Is it possible to remove these lines during transcoding? I give my
> recordings from time to time to my cousin, and I would like to have
> the video without these black bars.

The usual way to correct wide blanking at a TV station is to use a  
"DVE" (digital video effects) unit like an Ampex ADO, Tektronix  
Kaleidoscope or similar device. Nothing like that is remotely  
affordable by consumers though. Once you get out of the accepted  
blanking intervals video systems consider the signal as "picture" and  
I don't think transcoding would help.

If Myth doesn't have the problem and mplayer does you could probably  
adjust mplayer's parameters to do what Myth does though.

Brian Wood
beww at beww.org

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