Car Computer/Stereo Noise Feedback

[Sushiman]

Hey guys, Maybe you can help me again...

A while ago i started a project to put a computer in the car, Well the case looks dope (clear plexiglass) and the monitor looks great mounted in the car... Awesome... So then the problem starts. Here is what i have currently.

Monitor - wired into underdash + and - terminals. It has an RCA connector for the monitor.

Computer - wired into a 600w power inverter. The power inverter is wired directly to the car + and - terminals. (power inverter works great)

Stereo - panasonic with AUX input. I also have one 4 channel amp (front/rear speakers) and one 2 channel amp (for subs)

Sound Card - SB128

Here is what happened. the MOMENT i connected the RCA (from monitor) to the video card (ati xpert2k 32meg) i got a lot of feedback/static (very loud) ... then i plugged in the male audio jack into the computer.... and the feedback was horrible. Are there any suggestions as too why this is occuring?? I really would like to figure this out. I'm in need of some serious help.

mark

 

[Hotsauce]

Here is what happened. the moment i connected the RCA (from monitor) to the video card (ati xpert2k 32meg) i got a lot of feedback/static (very loud) ... then i plugged in the male audio jack into the computer.... and the feedback was horrible.

I'll give it a shot here Sushiman.

Lets start with: Are these standard cables that you would normally use for the app.? Are they shielded?

I had a monitor once that had a mic. built in, and speakers that hung on the sides. Couldn't use the mic though because it was in such close proximity to the speakers that I'd get feedback.

Either the mic or the speakers, or both had inadequate shielding, so I just used a mic I plugged into the sound card. As long it stayed a respectable 16 or so inchs away from the speakers I had no problems.

Maybe try moving some stuff around a bit and see if there are any changes.

 

[Sushiman]

Well the cables i am using are a standard RCA cable for the monitor and for the audio, i am using a stardard stereo jack to a Y rca (which goes to the radio).... I dont know if there are shielded cables? But i somehow dont think that would be the problem Thanks though for the suggestion.

 

[RainMaQer]

How far apart are your power wires from the data wires? In stereo installations its recommended that the power be on one side of the car and speaker wires on the opposite... I know from experience that NOT doing this will cause feedback... and thats just crosstalk between power and speaker... here your dealing with power and data... You need to either shield those wires or move them FAR away from each other. Also check your grounds... if you have a multimeter MAKE SURE YOU CAR ISN'T A POWER SOURCE... this will also cause feedback. Could be something that is putting power through its ground... the fuse box... or a power wire hidden somewhere touching. Also make sure your grounds are touching metal that makes GOOD contact with the frame.

 

[Sushiman]

well... i mean... the speaker wires arent that close... a good two feet... but what about the video wire causing static? Would have amplifiers increase feedback? And I dont have a mutlimeter... But i will try to check it out? All my wires are grounded fine. I assume the computer grounds out through the power inverter though.

 

[Sushiman]

I'm not sure how many people are still reading this... But I just figured out its the PSU that is causing the noise. If i have everything plugged in ... and if i unplug from the back of the psu... it stops. A guy at a stereo shop told me its because of all the coils inside? Anybody care to verify this is the reason. If it is, how can I bypass this? can i put a filter of some type somewhere inside the psu? or on the way to the mobo psu connector?

 

[Hotsauce]

I'm not sure how many people are still reading this... But I just figured out its the PSU that is causing the noise. If i have everything plugged in ... and if i unplug from the back of the psu... it stops. A guy at a stereo shop told me its because of all the coils inside? Anybody care to verify this is the reason. If it is, how can I bypass this? can i put a filter of some type somewhere inside the psu? or on the way to the mobo psu connector?

Well Sushiman, if it is the coils in the psu causing it (EMI) I don't really know of anyway to shiel it except distance. EG...psu in the trunk.

My wife is an electrical engineer, when she gets home today, if I'm in my right mind(been up 24hrs) I'll have her read this post and see what she has to say.

 

[Xaotic]

EMI can be shielded against. The easiest way is to get mesh shielding from an electronics specialty house. It looks like screen door mesh, but typically is composed of copper or gold if I remember correctly. To shield the PSU, which will isolate it from the other components. it will need to be wrapped completely in the shielding, including a couple of inches down the wiring harnesses, and grounded. Additionally, there is probably induced noise from the RCA cables. To fix this, use high quality RCA cables like Monster Cables 401XLNs, which are both shielded and grounded. Another way to reduce this interference is to use a high quality balanced line driver for longer cable runs. Make sure that you don't have ground loops, by using good power distribution design with a common ground for all nonautomotive wiring. Low frequency interference in the power lines can be filtered by using a capacitor in series with the load. Finally, amps can make any noise problems much worse, since the noise is also amplified.

 

[Monaco]

is everything grounded at the same point? that was the magic trick the installers used when I was in the car audio biz, as I recall.

you really should get a multimeter to test it and be sure, the RadioShack ones for 10 bucks will get the job done.

please post some pics, your project sounds awesome!

 

[cack01]

I always under the impression that the metal in OEM computer cases (by law<---may be wrong) have to shield against EMI. A Tech guy told my mom this when she asked him if being close to computers would make me go steril. Thankfully I have not tested this, but... if im right couldn't you just hack up an old computer case and block the PSU in??? It may not look pretty, but it may be the only viable solution. Of couse you could put everything in a microwave to protect against EMI but now im just rambling.

P.S. ANyone who knows for sure be sure to post if cases do block EMI. I'm sure they do, but just to what extent.

 

[Sushiman]

Ok, Tommorow im off to radio shack to get a multimeter.

As for the grounds... I have the radio grounded to the underdash ground, I have the monitor grounded to baremetal chasis, the amps are grounded who knows where (i didnt ground those, but they never made the clicking before the new radio... and my old radio had a filter) The ac inverter is grounded directly to the battery terminal ground.

I think for one of the clickings i just need a line filter for the radio. As for the computer, I need to see if it's grounding properly, the static "appeared" to not be there today.... so we'll see tommorow and during the testing phase. Thanks for the help. once i find some more information, i'll bump this back to the top.

Otherwise it's coming along good... just a lot of stuff that goes into this the first time you design this kindof thing. I'll have pics as soon as i'm done. Thanks again for the help guys.

*btw ... will this mesh prevent airflow from the psu case? and is any of the EMI coming off the wires possibly? I know they create a little but not as much as a coil would... Its been so long since i took physics*

 

[Monaco]

my advice would be to re-ground everything. Did the garbage noise change in pitch relative to the vehicle's RPMs?

The only way I was able to get rid of RPM whine in my car was to ground everything at the same place. Head unit, big sub amp, little 4-ch. amp, and Xover are all grounded at one multimeter tested place. Any other way and I got lots of ground noise. I never had much luck with noise filters, a good ground usually works better. Shielded cables are probably a must-have in your setup, too. If your PSU is the source of your noise, try a different PSU! Can't hurt

might want to double-check your invertor, make sure it is a good one and is also grounded perfect. although I've never used one really, so I don't know.

computers really don't produce much EMI...the metal case is supposed to shield things, but it's more of a 'just-in-case' protection the FAA thought up back in the day when people knew less about PCs. The average PC does not release any harmful radiation of any kind. Just ask the tons of folks who run with no case, system spread out on a workbench

 

[Xaotic]

Definitely reground everything to a single location, perferrably using distribution blocks(hopefully fusible to protect the equipment). Make sure that the wire guages on all cable are large enough. I tend to do things with an extreme degree of overkill, 0 guage 7200 strand(I wouldn't have bought it, but I got it for the price of standard 3000 strand 2 guage) on a 150 watt system with positive and negative fusible distribution blocks. The only contact to the car's electronics is at the battery on both positive and negative sides. A more rational guide would be about 4 guage wire for a typical 300 watt audio system. Another way to isolate the noise further is to run a separate feed from the battery to both the computer and the audio system. In your case, the inverter could be adding a tremendous amount of electrical noise ranging from the 60hz frequency in harmonics audible through the 6000hz range. Normally, I don't recommend using power filters, but in your case it may be a good idea. I'm not sure if Alpine still carries them, but they had some failry good filters in the 25-35 amp capacity range about 5 years ago that were not too expensive.

Radiative EMI should be fairly low, if all else fails you might want to try the mesh. It will not significantly impede airflow as it can be a loose mesh. The principle behind it's function is called a gaussian cage, and even though it has holes, it breaks up EMI by disrupting the lines of magnetic force. Some of our EMI/EMC labs have grid patterns up to 1in square. Some emissions could be radiating from the wires, but more likely they are simply causing induced currents in other wires nearby, and they can be shielded as well by using mesh and grounds. The likely culpruits are any wires that have AC current and/or high power. Power to the computer and video output to the monitor come to mind on this issue.

Hope this helps.