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Thread: video interference

  1. #1
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    video interference

    Guys i have:
    Zotac 9300 duel core
    M4 atx
    Cartft 8 inch transreflective lcd.
    Audio amp
    Gps mouse

    Issue: i get horizontal distortion/interference lines whilst pc is on and these lines increase with hdd/cpu activity.

    My tests:
    Groundloop
    I initially thought ground loop. I checked earth point on the car and it ' looked good'. There seems to be good metal to metal contact between nut and surface. This is also the point which te car earths some componets. I sanded the surface some more and also the nut.
    No diffeence

    I then ran a lead dirct to neg terminal on batt. No difference

    This leads me to beleive - not ground loops (i may be worng here, feel free to comment)

    I then removed each component connected one by one, including gps mouse and external hdd, no change
    Then thouggt vga cable in car may be picking something up, used another monitor and cable - same lines

    This then lead me to psu and is where i am confused:
    Same psu powering same pc via a bench top dc source is fine on bech, but when pc goes in car, and i use same benchtop dc source
    these distortion lines are there.

    But when i power pc in the car with a deskto psu powered by wall outlet, lines are dramatically reduced (some are still there but not enougt to annoy).

    How can i test the psu to see if this is the cuprit? In this situation, would one opt to buy another psu (fork out more cash) to test?

    Not sure which way to turn next

  2. #2
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    Hi
    Im no expert but sounds like an ground loop issue, have you tried earthing the psu and the monitor directly to the cars battery earth.
    Im assuming the monitor is in the front of the car and the psu is in the boot, you may still have issues as the run to the boot will be longer.....I had the same issue and this improved it (in my case) I also got some ferrite beads http://www.jaycar.com.au/productView.asp?ID=LF1294
    but not sure they helped never had much luck with them
    maybe this will help http://www.foresight-cctv.com/Ground%20Loop.htm .....it just gave me a headache ......

  3. #3
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    hi,

    Well the LCD is powered from 12v rail of m4. This means the LCD is earthed to m4 and m4 earthed at a point in the boot. And yes, I have earthed m4 (and LCD which is connected to it) directly to car neg, as stated - no difference.

    edit: now that I mentioned, I have not powered the screen via wall power, whilst connected to car pc powered by m4. If the distortion is removed, I will need to power the screen from a 'cleaner' source. If the lines are still there, than it is coming from the video signal itself - which means what?
    Last edited by mos33y; 09-23-2012 at 11:25 PM.

  4. #4
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    Hi
    Hmmm could be a noisy 12 v rail on the psu (you might be drawing to much current of the 12v rail) ....you could rule that out by powering the LCD via a long extension cord and a power pack whilst still in the car...If still no luck maybe the monitor vga cable or video card isnt earthing sufficiently...As I said no expert maybe hold off pulling all your hair out till one of the more knowing ones drop in
    Last edited by Wozza; 09-24-2012 at 01:07 AM.

  5. #5
    Maximum Bitrate Mickz's Avatar
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    Firstly, Iím assuming youíre using a good quality extension cable IE its ľĒ thick or more and has an inbuilt Toroid filter at each end of the cable. Noise coming up the M4 +12v line should not be a problem with that monitor. I have a similar installation and I think you said that powering the M4 on the bench did not cause a problem.

    The things that changes in the vehicle are:
    1. As the +12v supply to the M4 varies, the amount of Noise the M4 produces can increase.
    2. That long VGA cable has noise induced into the outer braid and Toroid suppressors only do so much, and sometimes little at all.
    3. The noise that is generated by the M4 and PC may be getting back into the Video output stages in the PC and not at the Monitor.

    When I carried out my initial testing of the M4 and all the Noise this device is capable of producing, it never once produced one speck of noise on the Cartft 8 inch transreflective lcd monitor I use. I ran it with no grounding, full grounding and everything in between when investigation noise and interference paths using some elaborate equipment and the monitor was faultless with the M4. Video levels going to the monitor are usually above what the M4 can induce into the braid of the cable, Iíd also be looking at grounding around the PC/Video card allowing noise to get into the low level video circuitry. Try another video card if possible for a test. Powering the PC via a standard Mains ATX supply will always produce far less noise than almost any 12V SMPS. The fact that you still had some residual interference is a concern as there should be absolutely none, and powering it with the M4 or other 12v PSU will only be worse if itís visible with a Mains unit.

    To simplify: The difference between Bench powering and in Vehicle powering is that Noise from Digital equipment is now wrapped around the PC and PC cables by the Vehicle chassis and every other cable that runs through the vehicle. The vehicle chassis is highly resistive at RF frequencies and Digital noise also has very high RF component frequencies. I realise that nothing Iíve said is going to help much, wish I had a straight answer but itís difficult to fault-find without actually being there.
    GA-Z77N-WIFI, i5-2400S, 8GB, Intel 520 128GB SSD, M4-ATX Modified, 2 Rev Cams, 2 Web Cams
    8" 16:9 TRANSFLECTIVE, Win8-64, Dual GPS RX and Garmin PC + Odyssey Nav, FM-DAB+, BB-Rec
    T-Screen HVAC control, custom microcontrollers, code and FE. CarPC Project

  6. #6
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    Thanks for your comments guys - always appreciated.

    1 more component that I forgot to mention, (which may or may not be of relevance in this case) is the PAC C2R-CHY4 Radio Replacement Interface I have. This interfaces with the car's canbus and all speaker wires are connected to it from amp (along with various other inputs/outputs). As my amp and pc are earthed at same point, this device is the only other device that has access to earth.

    having done some more reading here http://www.epiphan.com/products/othe...FYchpQodpl0AIw as well as links provided by wazza, I am being drawn back to the notion of groundloop. Dont forget, this mobo/m4/LCD all worked fine whilst on bench. The added components that could cause issue was car earth (and less filtered power from car batt).

    I think it is ground loop because when then external desktop psu was connected, these lines were considerably reduces. My thinking is the reason the lines were not eliminated was because my amp and pac harness were still connected to car earth (eventhough the amp was not powered at the time). The pac and amp are earthed at different ends of the car, so if there is going to be a potential difference, this would be an ideal scenario.

    however, another point that points me away from GL is the fact that NO distortion exists during the ' no vga signal' screen. Again, I dont know if this fact is significant, but it may mean that this distortion originates not from its power source or earth, but from vga signal itself?

    O.k, ill stop before i confuse myself even more. I have a few tests in mind to try and narrow it down:

    1. power the screen from wall socket and pc from car bat -
    .if, no distortion;
    need cleaner LCD power
    else:

    2. connect another mobo to m4 and remaining items in car (amp etc)
    if, no distortion;
    Mobo/m4/car/graphics card/ combinations do not work and another mobo is required
    else:

    3. buy a DC filter and connect to input of pc
    if no distortion;
    great!
    else: Throw a grenade at the car. Mind you its a Jeep, so it will probably be left with a slight scratch.

  7. #7
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    If a grenade wont do it Ive heard good things about C4
    If it looks like the Mobo/m4/car/graphics card/ combinations is the issue you may just need to upgrade the earth somewhere
    Ive had issues running modded desktop perspex/wood cases where the earth wasn't up to spec due to no metal case bridging the gaps.
    Might be as simple as running a wire from your enclosure to the cars earth
    Last edited by Wozza; 09-25-2012 at 07:17 PM.

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