[opendtv] Re: Overscanning on LCD TVs
- From: "Stessen, Jeroen" <jeroen.stessen@xxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 6 Nov 2009 14:26:00 +0100
Hello all,
It's nice to see that you are still on-line here, Alan Roberts !
I missed the opportunity at IBC to buy your book "Circles of confusion".
True, overscan was essential in the times of CRT television, to mask all kinds
of edge problems, mainly due to a less than stable high-voltage power supply,
and imperfect calibration of the scan amplitudes (or drift, or the lack of
such),
not to mention image jitter due to unstable sources (VHS tape recorders).
The nominal amount of overscan in TVs for the USA market was much
larger than for the European market (IIRC almost 11% vs. 5%), because the
USA would get 100-degrees (deflection angle) "raster correction free" CRTs,
which offered no control over the image width and Europe used 110-degrees
CRTs with a diode modulator and an E/W amplifier for image width control.
The 5% overscan that we are used to is not worth fighting about.
Matrix displays do not have any such problems with stability and calibration.
This does not mean that new flat panel TVs can do entirely without overscan.
There's still some things going on in the edges that are best left unseen:
run-in
artefacts from spatial filtering (sharpness enhancement, format conversion)
and from frame rate conversion (the edge is a foreground occlusion object).
Typically the pixels that you see are not the same pixels that you have
received, so it is not always a simple matter of 1:1 pixel mapping.
These things matter more for live dynamic content than for still content, i.e. a
PC desktop. Also, if the content creator assumes a certain standard amount
of overscan (not that such a standard is written, but still) then it is just as
wrong to show too much of an image as it is to show too little. This is why
overscan has persisted much longer in the TV world than in the PC world.
Our TVs offer a number of zoom settings. There is always an "unscaled"
option for the HDMI inputs, but then obviously the source resolution must be
equal to the screen resolution, which is for most screen sizes typically
1920x1080. I'm not so sure if there is "unscaled" for other sources, because
I always keep my TV in the "auto-format" mode to fill up the (21:9) screen.
When stretching from 4:3 to 16:9, or 16:9 to 21:9, some amount of vertical
overscan comes in handy, but then we should call it "cropping".
Speaking of 21:9: I will prepare a presentation on the subject of "21:9 TV"
for The Tech Retreat 2010. Mark is giving me extra time for nasty questions.
Groeten,
-- Jeroen
Jeroen H. Stessen
Specialist Picture Quality
Philips Consumer Lifestyle
Advanced Technology (Eindhoven)
High Tech Campus 37 - room 8.042
5656 AE Eindhoven - Nederland
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