| Author | Message | ||
     Steve Rating: N/A |
Hi, I am trying to make a home-brew hmd, and I am trying to figure out what kind of chip i could use to take the 120hz interlaced signal from an nvidia graphics card to separate the signals. Thanks! Steve | ||
| BOPrey Rating: N/A |
What are your targeted spec of this HMD Steve? | ||
| steve Rating: N/A |
Ideally it will be 1280x1024, 60 hz. Perhaps the chip which is used in the current low-end stereoscpoic hmds (i-glasses, etc) cannot accomidate the 1280x1024 signal. | ||
| Xander Rating: N/A |
Wow, 1280x1024. Where are you going to get the displays? What kind of FoV are you aiming for? | ||
| BOPrey Rating: N/A |
Steve, Why don't you start with a none-stereo version first and then build on it. | ||
| reverent Rating: N/A |
I currently am working on such a thing for my thesis. Although I am working on a non-proprietary version, (any stereo capable 3D video card) lets just assume that you have that 120Hz interlaced signal that has been produced by the over/under method and has the vertical sync effectively doubled. If all the above is correct, you can try a few things: First you can take the output of the vertical sync (pin 14 on the VGA 15 pin connector) and use this signal to trigger high speed video OP-AMPS that would switch the video source to and from the appropriate LCD screen based on the vertical sync. There are many OP-AMPS that can do this much faster then 120Hz. You would just use a high-speed low power schottky IC, usually a flip-flop and would change its output state on the rising edge of the vertical refresh. Although this does work, it does take some time and research (what doesn't though) and unfortunately, you need the input to be a VGA signal, which a lot of small LCD screens will not accept as an input. Another drawback is that although a lot of LCD screens refresh at 60Hz, you will be changing from left to right at 60Hz also. This will effectively cause some timing errors, as the weakest link is that your video source is only 120Hz, and ringing from the OPAMPS can and will manifest themselves as lines rolling on your screen. Also, on some LCD's there can be some missed vertical refreshes on the LCD that WILL be noticed, and will ruin the stereo effect. This ultimately depends on the LCD screen, as some LCD / OPAMP combinations work better then others. There are many other drawbacks that will take to long to explain, but I am just helping to point you in the right direction. Second you can improve on the above and take the split outputs and convert both of them to composite video. This remedies the above VGA only input problem but can completely ruin the sync between the two separate outputs. Again, trial and error comes into play, but scan converters are not cheap, and trial and error becomes very costly (I should know). If you want to use this method I suggest using a PLL to sync the video switching from your circuit to the same phase, frequency, and pulse width as the scan converter by cracking open the scan converters and finding their internal clock. This has only worked on the scan converter I bought from radio shack for 99.95 and, although it allowed the composite video signal to still be "stereo correct" (as far as the correct switching between LCD's), it is a 200-dollar solution and imposes other problems: Composite video is itself interlaced. What I have experienced is that by feeding an interlaced VGA signal into this scan converter at any resolution less then 1280X1024 will cause the scan converter not to display correctly (i.e. it shows the default color bars...no display at all). While 1280X1024 is very nice and all, it is much too large a resolution for my LCD's. So, either I get bigger LCD's or I allow my LCD to "try" to display it correctly. None of the small (2.5" or less) LCD will display this resolution to my satisfaction, often either cutting off part of the picture or because it has such a high vertical resolution, there are often some timing problems between the VGA card, scan doubler, scan converter, and LCD screen that cause the display to either flicker or miss many vertical refreshes. Remember you are taking a quite sophisticated signal and, for the lack of more appropriate language, screwing with it a Hell of a lot. This brings me to the only other solution I have come up so far. Use the driver that produces the over/under method but throw out the scan doubler (pigtail box that glasses plug into) and run the VGA output into a scan converter (VGA to composite). If you have composite out on your video card, you can use that too. If you hooked an LCD up to this you would see an over/under image. Use a LM1881 sync separator IC in conjunction with a HFA1115 sync stripper IC to effectively strip out the vertical sync of the composite video, us a PLL frequency doubler circuit to double that stripped vertical sync, and then combine the new, doubled vertical sync with the original composite video signal. Then, using the even/odd output of the lm1881 IC, you can use video OPAMPS to switch the appropriate composite signal to the appropriate LCD at the correct time. AND AS A BONUS…if you have an LCD that will refresh at 120Hz, you MIGHT just be able to display the sync-doubled composite signal on 1 LCD and use a shutter glasses driver synced off the even/odd output of the LM1881 to view stereo on a SINGLE LCD screen. This is obviously the best method, and what I employ now. Why didn’t I just tell you this? Because I have spent 3 years working on this, and spent thousands of dollars (understanding wife) to figure this out, only to have these God forsaken 120Hz LCD displays to come down in price and totally make my life so much easier. Dang them for that. There are undoubtedly some spelling errors and maybe a sentence or two will not make sense; however I am very tired and really wanted to help, so I am trying the best I can. If you have any questions, you can always email me as I only visit this board once a week. I find that by explaining myself to others, I make my understanding of what I am explaining better. | ||
| Anonymous Rating: N/A |
Our site is cool, but also yours is very nice too |
