Digital Audio, The Cure for Noise?

The carputer has 12v input. It is a DC-DC power supply.

check and see the difference (voltage) and resistance between the ground from the computer PS and the ground on your car...

you mean measure resitance between battery and car chassis and then between battery and PC ground?

DYohn hit it on the head. You have a classic ground-loop situation. The only way you can get rid of it is to use all digital, as you have suggested, or a ground loop isolator. The way you are describing your situation, and plaese correct me if I am wrong, is that there is no noise when your LCD is not plugged in. Is this right? When you plug it in, the noise returns? If this is the case, neither of the above solutions will be solutions in your case. Where is your monitor? Is it in the front of the car, and the PC in the back? Are they powered from the same place? If the power for one is coming from a distro in the back, and the power for the other is coming from the fuse block in the front, and they are not using the same point of ground, THIS is where your ground loop is happening. It will be extra work, but run power and ground for both devices to the same point in the car. See if this helps. The ground loop is not CAUSING the noise, the noise is an EFFECT OF the groung loop. By introducing noise to the ground plane of the PC, the soundcard is amplifying the noise present. It, in and of itself, is probably NOT CAUSING the noise. Are your PC outputs short, and connected directly to the amp in the back, and you are using your media player and/or Windows volume control in the PC to control the volume of your music? If this answer is yes, I will assume that your PC is, in fact, NOT causing the noise, but amplifying noise already there...It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Fair enough haemphyst, I do tend to agree, I am just at my whit's end with this.. :( I have literally spent the last 6 months trying to fix it. I keep thinking I make breakthroughs, then nothing. Let me share some additional (gory) details of the wiring: It all starts at the front. Plugged into the battery I have a 4awg wire. That goes to the trunk to a "platinum" distro block (I wonder how good these really are or if they're just hype btw). The distro block powers EVERYTHING now.. This includes the AMP and an ITPS device. The ITPS is design to regulate voltage to <~12vdc and also provide a startup/shutdown signal to the motherboard to boot the car and give the pc 45 seconds to cleanly shutdown after the car is shut-off. I've tried w/out this device, no luck, so I doubt it is this device. Off the ITPS I have the LCD and the PC. The PC has native 12vdc input as does the LCD. I used to have the LCD wired to the cigarette lighter (came with the plug) but it didn't make a difference. The LCD has a VGA cable going to the back to the carputer for video. I have also added a 12v noise filter from radio shack. This didn't help a bit and actually buzzed itself for the first few minutes. I grounded it to the chassis and powered it from the ITPS for the LCD. The LCD still works by getting it's filtered power, but I don't think it made a bit of difference on my system. The PC is grounded to a gnd distro block (same type) and that to the sanded part of the chassis. I've tried adding extra wires straight from the amp, computer, lcd, itps, and distro block, to increase the quality of the ground -- again, no dice. The PC had a PC sound card which outputted using 1/8" stereo jacks. I got 1/8" to dual RCA jacks and put my high-quality RCA cables inline (I tried cheapies too btw). These go to my amp. When they are unplugged from the computer, my amp is dead quiet (duh). I never did do the muting plug test. Even if the PC is off, the noise persists. I removed the sound card and it stopped! I was thrilled. That means the sound resonated from the case, since the PCI card is grounded to the case, the motherboard is not. Then I plugged my LCD VGA back in -- doh, noise is still there. So now I realize I need to separate VGA and motherboard, but that is near impossible. Maybe I need to ground my VGA? Will that screw up my video card though? Hmmmm. Oh, I've even unplugged all of the wiring in my computer, even the power supply itself one time.. Still I get the noise, so it IS coming into the VGA cable.. I also tested the usb and keyboard cables.. Nope, it's DEFINITELY coming in from the VGA cable.. So that points to the LCD.. So maybe it needs to be grounded to the ground distro block or the ITPS? Hmmmmm... maybe this is my problem... what do you think? Hmmm, writing this out in the last 10 minutes really has helped me put things in perspective. I bet it's that friggin LCD ground point!! GRRRR!!   *take screwdriver to LCD* I'll post more after I move the LCD ground....

Yes. You have it. Move the LCD ground to the same point as the others. You need to eliminate differing ground points. http://www.epanorama.net/documents/groundloop/basics.html Here's an explanation of your problem. New Project: 2003 Pathfinder

Aaargh.. OK, so I tried grounding the LCD to the amp ground point, and to the computer ground point and to the chassis. No dice! :( Then, I unplugged the LCD power. It was still buzzing.. That was odd, I thought, so I looked and only the VGA cable was plugged in. So I unplugged that, and the noise was still there. My PC was the only thing plugged in. So I unplugged that and it went away. Same if I would have unplugged the RCA cables. So I have one ground consolitation cluster and then those ground to the same place the amp is grounded to. So they're all really grounded to the same place. *sigh* Anyone in NOR*CAL feel like helping me troubleshoot? :) FREE BEER!! :D :D