[mythtv-users] LCD and traditional CRT TVs, overscan and UI size and widget placement
beww at beww.org
Wed Nov 11 14:14:49 UTC 2009
On Wednesday 11 November 2009 06:53:58 Brian J. Murrell wrote:
> Hey everyone,
> I have a traditional CRT TV (am I the only one left with one of
> these? :-). I am driving that with an nvidia 5200 via S-Video.
> As everyone knows, these TVs have an "overscan" feature where they only
> make a portion of the original broadcasted picture visible to the
> viewer. Historically, this is done because the outer edges of the
> broadcast signal are rough and therefore hidden in this "overscan" area.
> But the TV broadcasters know that TVs do this and they compensate by not
> putting anything they want visible too close to the edge of the signal
> they are broadcasting. This becomes obvious if you watch a TV station
> that does have something at the edge of the visible picture, like CNN's
> ticker at the bottom. If you look at CNN on a device that does not hide
> the overscan (i.e. like mythfrontend in a window on your computer) you
> can see how much picture is available between the bottom edge of the
> ticker and the bottom edge of your window.
> Anyway, that's a bit about overscan.
> The problem with this overscan though is that unless you adjust it, the
> MythUI will draw outside the visible area and thus, you won't see some
> of the widgets. But MythUI has adjustments for this, allowing you to
> bring the UI back inside the viewable area. The problem is that it's
> not leaving the UI the same size and just drawing the widgets closer to
> center (i.e. to bring them into the viewable area on all sides), but
> rather it's just reducing the size of the UI.
> The problem with this approach is that it's futile trying to make the
> edge of the UI "canvas" meet the visible portion of the picture as the
> borders of this visible portion of the picture are never straight an
> square. Frequently they are curved and/or have wows in them.
> See this picture I took of my TV:
> You can see a "wow" on the top half of the left border. But this is
> just how CRT TVs are. This is exactly why overscan exists.
> The solution, as I see it is rather than scaling the UI canvas down to
> fit inside the visible window, the canvas needs to stay the same size
> and the positioning of the widgets on the canvas needs to be offset to
> fit them inside the visible area.
> This effect is not limited to CRT TVs though.
> I have an LCD TV that has some overscan as well. It's not as much as
> the TV, but it's still about 20 pixels per side.
> On this LDC TV, I run a gnome desktop and the effect of the overscan is
> that the panels at the top and bottom are only half visible on the
> screen. When I start a mythfrontend on that screen, with it's
> dimensions the full size of the screen, gnome takes this hint and puts
> the window on top of the panels.
> This same problem of the UI widgets being drawn outside the visible area
> exists though, and naturally I want to use the adjustment tool to bring
> them back inside the visible area. But instead of leaving the UI window
> the same size as the screen (1280x720) and just moving the widgets in
> tighter, this adjustment actually makes the mythfrontend window smaller
> and them blam, gnome notices that the window is no longer the same
> dimension as the screen and puts the panels on top of the mythfrontend
> window again.
> So another case to be made for leaving the canvas size alone and simply
> moving the UI widgets so that they are on the visible portion of the
The SMPTE (Society of Motion Picture and Television Engineers) actually
defines three areas in the TV (NTSC) picture (RP-218). I think RP-219 is a
similar RP for HD television.
("RP"s are "recommended practices")
Safe Title: The area where it is safe to put graphics like Chyron titles etc.
Safe Action: The area where "action" is safe.
The rest: The rest of the available picture area, generally the outside areas,
where it is not "safe" to have things happen.
This is similar to the "zone 1, 2 and 3" used to spec things like registration
for cameras and convergence for CRTs. IE: The registration/convergence near
the center of the picture is generally better than it is towards the edges.
I'd include a link to the specifications here, but they are available only
with a subscription or membership. Greedy SMPTE.
Originally TV sets were set up to overscan, so that as the set aged and the
voltages dropped (due to capacitors becoming leaky mainly) the CRT would
still be filled with "stuff". They (probably rightly) figured that viewers
would be more unhappy with black at the edges than they would be with an
overscanned image, especially since directors were careful to pay attention
to safe action and safe title areas. The basic idea was to minimize sets
being returned for repair (or, with console sets, repairmen needing to be
sent to the home).
One more case of the manufacturers driving content. The bottom line drives
beww at beww.org
More information about the mythtv-users