Thread: conflict on obd-II and audio

Originally Posted by mcpc

I have a computer using ODB-II interface for grabbing data (using dashcommand). I also want to use the computer for mp3, by connecting it to the ausiliary port. However, everytime i connected the ODB-II cable...the aux line-in in my headunit doesnt work, but everything is fine if the ODB-II and aux line-in are not using at the same time. Anyone knows why?

It could be several different things, but it sounds like a ground problem. How is your OBD-II interface connected to your computer, and what are you plugging into your aux line-in?

-jjf

I use a auxilary cable connecting my sound card to the aux line-in of the head unit. As for the odb cable, it is just a usb to odb-II interface.

I guess its not a grounding problem, as i tried to use the standard ac-dc atx, so its not somthing relating to my m4-atx...weird.

I agree with Joe, it sounds like a grounding problem. When you plug in the OBD-II interface into your PC, it connects PC ground to vehicle chassis.

Where does the head unit get its power from? Can you connect its ground to the vehicle chassis?

Originally Posted by mcpc

I use a auxilary cable connecting my sound card to the aux line-in of the head unit. As for the odb cable, it is just a usb to odb-II interface.

I guess its not a grounding problem, as i tried to use the standard ac-dc atx, so its not somthing relating to my m4-atx...weird.

USB uses a chassis ground on most laptops. So, if you are using a DC->DC converter, you can get some weird cases of ECU/Chassis -> noisy DC offset chassis.

But, generally the symptom would be loud buzzing on an audio input. When you say 'doesn't work', what do you mean?

-jjf

Thanks mate. Now I sorted out that it is a grounding problem. After twisted my audio input cable, I can hear the songs now. But as you said there is loud buzzing noise from speakers. Any quick way to eliminate those annoying buzzing noise?

Another issue is the dashcommand gauge reading. When connecting my PC using the OBD-II cable, the readings (e.g. RPM, airflow ratio) in Dashcommand are not stable even when my car is idle, the reading is moving up and down all the time. It happens with my 2 years old PC (with the motherboard "ECS RC410-M"), but it works just fine with my 10-years old legacy computer (at least 7 years old). Any clues?

Originally Posted by mcpc

Thanks mate. Now I sorted out that it is a grounding problem. After twisted my audio input cable, I can hear the songs now. But as you said there is loud buzzing noise from speakers. Any quick way to eliminate those annoying buzzing noise?

Another issue is the dashcommand gauge reading. When connecting my PC using the OBD-II cable, the readings (e.g. RPM, airflow ratio) in Dashcommand are not stable even when my car is idle, the reading is moving up and down all the time. It happens with my 2 years old PC (with the motherboard "ECS RC410-M"), but it works just fine with my 10-years old legacy computer (at least 7 years old). Any clues?

I'm not certain on the OBD-II side of things, but isolating the ground on your audio is really easy. You are going to want one isolation transformer for each audio line (2 for stereo operation, 4 for surround, 5 if you have a dedicated subwoofer). I anticipate you are just using a stereo connection. You may be able to get these parts at your local Radio Shack, local car audio supplier, home theater supplier.... (more common isue incars than home theaters, though happens in both). If you go the raw parts route (like buying isolation transformers from RS, or a component supplier) you will generally have 4 wires coming out of the transformer. There should be a datasheet saying which two are on which coil. You simply connect one coil to your computer, the other coil to your car's aux-in. A standard TRS (Tip-Ring-Sleeve) connector will normally be wired with the audio for the left channel at the tip, the right channel on the ring, and the common ground on the sleeve. It is possible you have multiple ground loops, I would try to make sure you only have your computer connected to ground at one point.

The reason ground loops cause problems is that ground potential isn't exactly the same at all points, and these differences cause current to start flowing, and the high and low points to change. This causes interference to many things. On the audio, it is easy to notice as a buzzing noise. On a digital line (such as the OBD bus, USB bus...) it causes weird things to happen increasing the BER (bit error rate). Many protocols intended intended to be used by wire (generally if it is intended for wireless use, there are error detection and/or correction mechanisms) assume a BER of 0.