[sorry, this question is probably off-topic here, but regularly reading this NG I am confident that this is a place where I can get an answer...]
Just about every laptop has an external video output that is disabled by default. Turning it on when connecting to a projector is often a pain, especially if one uses an "unusual" operating system.
What is the reason for turning it off by default?
- Energy consumption (if no load is connected, it can't consume that much, or can it)?
Op Mon, 09 Mar 2009 11:34:30 +0100 schreef Robert Kaiser :
When it's fully on, it could be performing millions of video buffer reads and hundreds of millions of D/A conversions per second. The energy consumption from this is probably nothing in comparison to that of a long VGA cable, but it's still unneeded energy consumption and (possibly) memory bus load.
Even solid-state electronics wear out eventually.
Heat production?
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I see others have mentioned power, some also commenting on the actions which might cause power consumption.
I discovered another one using an older Thinkpad to drive an LCD projector. When I had both the internal screen AND the external output on, the video update performance went thorough the floor. Whereas I got smooth updates to either/or, it wouldn't with both. Very choppy picture.
- EMI type approval is carried out with shielded cables attached to all ports. An unconnected port might radiate.
- The external video port needn't mirror what is onscreen; it may behave as a separate display surface. In which case, enabling it would steal video RAM for no useful purpose.
- The refresh rate for internal and external displays usually has to be the same when display mirroring is turned on. This usually means a compromise setting.
- Connecting an external display in mirroring mode usually results in the internal LCD changing resolution to match whatever the external display supports.
Thanks for all the answers (I knew I could count > >>
Having different resolutions implies that the external video is driven by a seperate video interface cirquitry which shares the video RAM with the main one. (I hadn't thought of that since I typically use the same resolution on both screens (as I suspect must people do?)) I can imagine that turning off this second interface might save some power, however I would think that most of the power consumption of a video interface is caused by its local processing capacities (3D acceleration and the like), so a secondary, display-only cirquitry should not consume that much, but yes, it probably makes a slight difference (plus it would also explain Bill Leary's observations).
Yes, I had thought of that, too, (especially the possibility to wirelessly spy on someone's screen this way...). However, with practically everybody carrying a mobile phone these days, one wonders, if these FCC/CE regulations are sensible at all.
Apperently (and IMHO unfortunately) so. I wish I could select this in the BIOS settings.
The main reason why I was asking is because one of the laptops I use for presentations (an older Thinkpad) apparently can not be coerced to turn the external display on manually under Linux and the generally accepted solution seems to be to keep it on at all times by configuring the X server suitably.
Since I had not observed a significant impact on battery life time, I was worrying if that might have any other adverse side effects.
As far as the TEMPEST stuff goes, I read a very interesting paper (online, but I can't find it now) some years ago where some students created software to read the backscatter of a monitor off a wall. They could reconstruct an 80x25 text screen very accurately just by taking a picture in the window of a room containing a computer facing away from the window (bouncing off a white wall)
With all of the Thinkpads I've seen you can toggle between internal/external/dual video by pressing Fn-F7. I have a TP 600 here that does that and ISTR the 560 does it as well - both fairly old models now. I don't know exactly how it is implemented but it works fine in NetBSD without specific software support for it.
Often the controllers these days have two outputs specifically for laptop and presentation use. Windows has display controls for driving multiple screens at different resolutions and lots of laptop controllers support this capability.
Having dual output graphics controllers, explains the battery drains.
The regulations from recollection are often based on how much interference from specific distances could be seen on analog TVs! ....
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Well, it doesn't work on mine (T40). I also have an X40 where it doesn't work either. For the X40, I was able to locate a small C program that does the trick. With some 1-2 days of googling and fiddling, I could probably get that program to execute on Fn-F7, but so far, I'm just running it from the command line (as I said: it's a pain..).
For both X40 and T40, external video works if a monitor/projector is connected on bootstrap, so the BIOS apparently detects that there is a load connected and turns the video on.
I also had some (now *very* old) laptops where this used to work out of the box regardless of operating system. Those were pre-ACPI (APM) BIOS models. I believe with APM BIOSes, the switching was handled by the BIOS completely, the OS didn't get involved.
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