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Noise problem, but visible, not audible

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  • Noise problem, but visible, not audible

    I posted this in the audio section because the noise I'm seeing is presenting in my video signal from the car computer and is directly tied to how loud I have the stereo turned up. It isn't related to engine rev because on the road at speed the noise goes away completely when I mute the music.

    I did have an alternator whine at first, but only with the CarPC input (not while listening to CDs or my Sirius tuner). I fixed that with an isolator from RadioShack attached to the RCA cable on the CarPC end.

    What I see is a video signal that has a number of small black lines scrolling randomly through the image. Banding of a sort is visible as well.

    One test I haven't done yet for some reason but will do tomorrow is to turn the sub channel off and only deal with cabin speakers to see how that changes the situation. I'll post back on that result.

    My setup:

    Kenwood head unit with sub out
    Fosgate 12 driven by an Eclipse 300W amp
    CarPC is powered by a DC-DC power supply
    Using onboard sound and video of CarPC (geforce onboard)
    Using standard analog audio out with an 1/8" to RCA adapter

    I'm coming into the deck via my sirius tuner which has an aux input pass through (meant for a CD changer really) with which I'm using a retail breakout cable to give me ipod, 1/8", and RCA inputs

    What I'm thinking right now is that my problem stems from the fact that I have a single 4 gauge power wire running from battery to the trunk and my amp and CarPC both split off this power wire. Any logic in that being the problem? Both use the same ground as well that just taps into the body back in the trunk. Remember I only had whine on the PC input and now I only see noise in my VGA signal when my volume is up loud. I do see that video noise though no matter what audio source is selected on my head unit.

    If this is the problem, then what would be a solution? Running a new power wire for the PC? Or tying into some other circuit in the trunk? Not a lot there to tie into really except lights and those are pretty low amperage circuits to be putting a full PC on.

    Thanks in advance!

  • #2
    directly tied to how loud I have the stereo turned up.
    Could be a couple of things.

    -Bad/poor video shielding. Keep your video cable away from your power wires. I have also heard reports of people using ferrites to reduce this.
    -Poor ground
    -Poor voltage regulation on the video screen power.
    -Poor video cable connection.
    System always under construction


    • #3
      I should have my new boxes in another week or so, and my new wire and connectors should be here today. When I re-wire my car I'll take care to address the things you mentioned and then see how things fair.

      So do you think there is any problem with running both my amp and CarPC off the same main power wire run from the battery? And also using the same ground wire?

      I'm running the power and ground into terminal blocks and from them to both the amp and pc along with a 120 inverter. I don't turn the inverter on unless I'm using it and I haven't seen a corilation to it so far with my visible noise.

      I have tried with and without the sub though and while it seems to change the noise a bit, the noise is still very much present without the sub channel turned on in my head unit.

      Oh wait...

      I have the screen's 12V power line hard wired to the head unit's power and ground. Holy crap why didn't I think about that before...

      I'll move that and see if that clears it up.


      • #4
        Noise blockers are sound blockers, don't use them, it's an easy way to solve problems without realy solving them! Throw that [translate]comes out your behind[/translate] away as far as possible.

        Lead ALL grounds in your system to 1 good grounding point. So amps, carpc, head-unit... all to ONE good grounding point.

        Make sure your grounding point is good. That means: sand the grounding point to bare metal. Don't just squeeze, but also solder the connectors to your grounding cables. Bolt the cables very tight to the metal. Melt some solder over the whole thing for optimum contact, and then spray some primer over it to prevent rust.

        Also make sure the grounding cables of your engine/alternator and battery are as short as possible and thick enough, and make optimum contact with the metal of the car.

        Make sure all power cables (power, ground, remote) run at one side of the car and all signal cables (RCA, VGA) at the other side. You might run the speakercables trough the middle of the car, or, if there's no possibility to do that, under the carpet, at the side where your power cables are running.

        If you still have noise problems, it's a sign that your RCA's are interfering with your VGA cable, so that means that your VGA cable is poorly shielded. You can try running the VGA cable outside the car, or through the interior, away from all the other cables. If that helps, you need a better shielded VGA cable, or you can shield it yourself by wrapping it in metal/aluminium foil (no kiddin', this really works most of the time!), and isolate+protect it by wrapping cotton tape around it.


        • #5
          When I rewire things I'll try without the filter again but I have followed all standard suggestions and remember it's ONLY on the pc input that there was audio noise so truly I think that it is coming from the pc itself because of the dc-DC power supply, onboard sound card, or something like that.

          I could have a better ground I guess, but when playing music with my head unit selected as the source, things are pristine, only the pc source had the audio noise.

          My video noise I would bet is solved as well by just getting the screen power from a source other than the head unit power wires.


          • #6
            Noise problems coming from the carpc can be solved by: a descent metal case, wiring a ground cable, coming from the DC-DC power supply to the case (so internal groundings are also connected with the shielding), and wiring the case to your central grounding point (so the internal groundings from the carpc are also connected, through the case, to the central ground point)


            • #7
              That makes a lot of sense. I'll definately make sure to do that when I re-wire. Thanks!