1. Join Now

    AVForums.com uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

HDTV feed from HTPC over VGA

Discussion in 'TVs' started by av_master, Jul 11, 2005.

  1. av_master

    av_master
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    Does anyone have experience with feeding an HDTV (LCD one, in particular) a 720p signal from the VGA (dsub 15-pin connector) on the graphics card to the same port on the TV? If so, can you get the desktop to display properly, without overscan?

    Apparently this kind of thing is achievable using a digital (dvi or hdmi) connection, but I'm wondering if you can get it to work over VGA.

    Many thanks!
     
  2. Stephen Neal

    Stephen Neal
    Well-known Member

    Joined:
    Mar 29, 2003
    Messages:
    6,080
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    133
    Ratings:
    +681
    Powerstrip is your friend for this. It allows you to create your own resolutions for Windows, and custom monitor timings. (Many of use use it to create a 1024x576 resolution for 16:9 SD TVs fed via VGA->RGB SCART cables)

    To do what you want you have to create a 720p mode, but with only about 690 lines of "Windows resolution" and fewer than 1280 active pixels horizontally. This won't be any use for displaying 720p HDTV clips natively - but will let you see the Windows on-screen furniture (like the task bar)

    The Powerstrip forums or the Home Theater/Theatre forums at AVSForums have loads on this (Entech make Powerstrip)

    What people often forget is that overscan is usually a fixed function of the display - not the PC feeding it. Video signals are designed to be displayed overscanned (well camera operators and directors always estimate that 10% around the edges of a TV picture might be cut off by overscanned delays - so frame for "domestic safe" area)
     
  3. av_master

    av_master
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    Thanks Stephen, that's really helpful.
    So, to get nice and crisp text in a word processor, say, going for a PC signal rather than a TV signal is the way to go, if the TV supports it?
    Curiously enough, many LCD TVs won't let you feed them a PC signal at the native resolution of the panel (e.g. a 1280x720 panel won't accept a signal for that resolution) but only a few "standard" resolutions like XGA etc and then scale them to fit the display. This always gives horrendous text quality. I had hoped to be able to trick it by using a 720p TV signal, but from what you say that won't result in a nice 1:1 pixel mapping either.
    Thanks again for the info!
     
  4. Stephen Neal

    Stephen Neal
    Well-known Member

    Joined:
    Mar 29, 2003
    Messages:
    6,080
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    133
    Ratings:
    +681
    I think I need to explain it again!

    If you use DVI or HDMI and can get your TV to display this in 1:1 mode - then you may well get very sharp pixel mapping.

    However if your TV adds overscan (even in 1:1 mode) then you will need to create a custom mode in Windows that is 1280x720 in video terms, but only containing about 1100x650 ish active pixels. These will still be 1:1 - but the outer pixels will be black! Think of the approx 1100x650 as a smaller window of video within a 1280x720 resolution video signal.

    Flat panel displays usually don't overscan in the display - and instead scale the input to simulate overscan (which is why some have specific 1:1 modes that bypass the scaling - and confusingly why they don't all display 720p on a 720 line panel with a 1:1 mapping...) If this is the case with your display then you may not need to do anything about overscan.

    Projector based displays AIUI DO include display overscanning - so a bespoke Powerstrip mode (720p timing by only about 650p active windows resolution) would be required to compensate for display overscan.
     
  5. spikesmydog

    spikesmydog
    Standard Member

    Joined:
    Jul 12, 2005
    Messages:
    4
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Ratings:
    +0
    I Bought An LCD screen two weeks ago, with just a vga input for the pc. The screen resolution is 1368 x 768, but when I try to use this resolution with the pc, as you say the text is very poor. using Powerstrip with the settimgs below, it all seems to fall in place. it takes a bit of messing around with the different timings, but its worth it.

    PowerStrip timing parameters:
    1280x720=1280,412,48,244,720,5,5,20,74250,272

    Generic timing details for 1280x720:
    HFP=412 HSW=48 HBP=244 kHz=37 VFP=5 VSW=5 VBP=20 Hz=50

    Linux modeline parameters:
    "1280x720" 74.250 1280 1692 1740 1984 720 725 730 750 +hsync +vsync
     
  6. av_master

    av_master
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    thanks for the explanations both of you. I've been looking at these new toshiba 27WL56 and also the 32WL56 panels, and it's difficult to figure out what they will do with which signal. According to the manual they'll accept an XGA PC signal, but I don't know what they'll actually do with it (e.g. the 27" model is a 1280x720 panel, but it will still display XGA, so since that's 768 pixels vertically, there's obviously going to have to be some kind of scaling somewhere). If there is any common sense involved, you'd expect that if you feed it a different PC signal using e.g. powerstrip then it won't start scaling it in funny ways, but then how can you be sure?
    I understand that overscan is a feature of the TV, but I guess that the thing should "realize" that a PC signal (not a signal coming from the PCs TVout port, but rather a proper PC vga signal) does not need to pass through any kind of scaler / electronics that deal with overscan etc. I mean, if an LCD monitor can do it, why does a tv have to make it so complicated?
    The DVI / HDMI solution you mention will work I reckon, but I was thinking of keeping that port free for a PS3.

    One more question about the timings you posted: if these are 1280x720 timings, am I right that they won't give you a 1:1 mapping on your 1368x768 display? Or does it display 1:1 on a subset of the screen (i mean, a 1280x720 rectangle)? Or am I missing something?
     
  7. jcrt

    jcrt
    Standard Member

    Joined:
    Jul 17, 2005
    Messages:
    5
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Location:
    London
    Ratings:
    +0
    I've got a PC connected to a 32WL56 and here's what I've found:

    With PC set for standard HDMI 720p mode output (1280*720), the HDMI image cuts off about 5% or so either side of Windows' display image - ie, we lose parts of the taskbar etc. Curious, given the panel has plenty more pixels to play with for overscan etc. HDMI picture quality is superb, if you don't mind guessing what's in the corners :rolleyes:

    The latest ATI software allows for a custom 720p resolution at 1152*648, which fills about 90% of the panel area, leaving a black border, so I guess we can take our choice from those two, or mess around with powerstrip until we're tired of it. :suicide:

    One thing to note - with the HDMI signal active in 720p, the VGA connection produces rubbish - either a mucky, colour-tinted picture or a blank screen which keeps switching mode. I suspect this is down to the PC not really being capable of properly outputting both together (video card is a ATI 9600 and there are known issues with the things on HDMI). Also can't see anything on HDMI until Windows starts up, but that's definitely an ATI failing and the BIOS screens do show ok on the VGA.

    If you need to, the Tosh will take a 1080i HDMI signal (1920*1080) quite nicely, although text readability isn't great as you'd expect with that much scaling-down going on.

    With the VGA connection alone, TV handles its 1366*768 native resolution happily and the image quality is very good.

    If I can help by trying anything else, feel free to ask - when I get time, I'll play some more. Am also about to contact ATI and Tosh support about the overscanning/black edges situations, so if I get a fix, I'll post it. Or possibly, I'll just buy a NVidia card for the media PC and chuck the ATI! ;)


    Jeff
     
  8. av_master

    av_master
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    Jeff, I've been reading posts and asking members on the lcd forum, and they all find what you say: ...x720 resolutions are a no-go on vga, and pc via dvi and hdmi results in this overscan issue. So it may have to do with the tv rather than the vid card, tho of course everything is possible.
    I guess if you can do 1360x768 on vga with a 1:1 pixel mapping, you get very sharp text in e.g. a word processor, right?
     

Share This Page

Loading...