Audio noise on AV diplay in motor vehicle

Discussion in 'Wireless & RF Design' started by Andrew Leigh, Dec 11, 2014.

  1. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    Was not too sure where to post this.

    I have fitted a 12V TV to the rear of my vehicle for my grandkids, there is a whine that increases and decreases as the rev's fluctuate. Much like the old unsuppressed car radio' in the 70's.

    1. The TV is driven by an aftermarket Radio / DVD / Sat Nav combo specifically made for my Toyota.
    2. The DVD player connects to the TV via a 5m RCA cable that runs from the front of the vehicle, down the centre of the vehicle to the rear.
    3. The power is taken form the factory fitted 12V plug in the rear of the vehicle.
    4. There is no whine when the motor is not running and with full ignition on.

    With the TV on full volume there is not whine when in any mode other than DVD. The minute the mode goes to DVD the whine begins, even before the DVD initialises. As the whining sound is not present in other modes (radio, iPod etc.) I have made the assumption that it is not power related as then the whining would always be there.

    Now the cords are you standard el cheapo's that one would get at any consumer electronics store. All three RCA plugs are connected at both ends, DVD and TV.

    So the questions would be;
    a) Am I thinking in the right direction in that it is not power related.
    b) How do I fix the problem?

    Cheers
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It could be power-related, if all modules except the DVD have adequate filtering of the supply.
    Can you power the system temporarily from an alternative external 12V supply and see if the whine vanishes in DVD mode? If it does, that points to the car's 12V power supply being the noise source. If it doesn't vanish, then the DVD module could be picking up radiated noise.
     
  3. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    Thanks will power the player as instructed and report back.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    One more point.
    The RCA cable should be of good quality i.e the ground shield should be completely around the signal cable. Not just running next to it.
    If noise is there with a good cable try disconnecting the Signal ground from the DVD side. That the RCA cable should be grounded from one end. This will prevent ground loops. Something I noticed during my car audio days
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    alternator whine. there are a lot of methods to get rid of it on line.
     
  6. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    The independent battery solves the problem. I suppose now just to remove the earth's on the RCA's at one end.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You have a ground loop between the front of the vehicle and the rear of the vehicle. You are violating the "single point ground" principle by feeding DC power to two different audio components from different locations in the car . Try a stereo audio isolation transformer like one of these.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That would be the next thing to try.
     
  9. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    ;) I have violated many things in my life, alcohol, traffic laws, speed limits but this is a first. I have violated a "single point ground". :D

    I get what you are saying but the comment tickled me somewhat.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You violated a principle. Actually, I make money solving this problem, especially in aircraft.
     
  11. bwilliams60

    Active Member

    Nov 18, 2012
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    Not sure what you are meaning by this? I have worked on automotive electrical all my life and this sounds interesting. Most grounds in a vehicle are tied to the body and frame and find their way back to the battery. Mys first thought would be whether or not the braided cable that is used to suppress RFI has been replaced with regular cable or not. My second thought is where this 12V outlet is sourced and where do the wires run in relation to the ignition, alternator or any other device producing AC voltage. I might be inclined to disconnect the alternator and run it to see if this is the source and find the actual source. Also check your ground circuit for voltage drop.
    Mike can you enlighten me more on this principle. Always open to learn more.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You do not hear alternator whine in most factory-installed automotive audio/entertainment systems. When people install add-on equipment, especially trunk-mounted amplifiers, is when the troubles start. The problem usually is that the existing "head-end radio/tuner/cd player" takes its negative power lead from some given location near the front of the car, typically on or near the firewall. Now someone wants to add a billion watt amplifier that they mount in the trunk. If they connect the negative power lead of the amplifier to the car body/frame at the rear of the car, expecting that the metal of the car will provide a current path to the negative lead of the battery, they have just created the "groundloop".

    The problem is caused by the fact that the car body is already used as the current return path for all of the other stuff, like headlights, DRLs, fuel pump, tail lights, heater fans and blowers, etc. Because the car body/frame is steel, and has a finite resistance, the return currents cause a voltage drop. Even worse, because the alternator puts out audible a.c. ripple, the current flowing in the car body can be plainly heard if it gets into audio devices, especially amplifiers.

    Simply put, if you have an audio source (powered by 12Vdc, but grounded near the front of the car) connected to an amplifier( also powered by 12Vdc, but grounded to the car someplace different), you have just created a "ground loop", where the current flowing in the car frame between the two disjoint ground connections creates a voltage drop between the two ground points and now appears as a common-mode noise signal added to whatever low-level audio signal passes from source to amplifier. The root cause is the two disjoint power feeds, the resistive nature of the car body, and the preexisting currents that naturally flow along the car body/frame.

    This has absolutely nothing to do with "shielding", inductive or capacitive coupling, or "filtering", which is what most newbies think is going to cure it...

    Note that the TS's problem is sort of the reverse of the scenario I layed out here. He is using the amplifier inside the car's audio system and his audio source is a DVD player in the rear of the car. Chances are that the DVD player's negative side of its power feed is connected to the car frame 10 feet from where the front-mounted audio system gets its power. The voltage (in the car body) between these two grounds is what is causing his problem. Hence my comment about him violating the "single point ground" principle.
     
    Last edited: Dec 14, 2014
  13. bwilliams60

    Active Member

    Nov 18, 2012
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    You are correct in your assessment but my question comes from the grounding side of this "phenomenon" The problem is not necessarily in the car body but in the cabling between the battery negative post and the body or frame of the vehicle. We ran into this a lot as youngsters would put big amplifiers in their trunk and tie the ground cable to the body. In our auto electric shop, we either increased the size of the ground cable or better scenario was to run a second battery in the trunk just for the stereo and provide proper size cabling for it. We then used a magnetic switch to isolate it from the vehicle and it would charge when it was running. This worked out really well and we always use braided cable. The problem that occurs in these situations is as you said, there is a ground loop created because the ground system is insufficient for the high energy consumers that have been added.
    I figured I knew what you were saying, just wanted clarification, thank you.
     
  14. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    OK some feed back.

    I have a dual battery system in my car. The secondary battery is used to run a camping fridge in the boot and to run the invertor in my caravan. The power comes from two 16mm2 leads from the battery to the rear of the tow bar, two 4mm2 leads then are wired from this into the boot as an independent supply to the factory fitted 12V outlet.

    Thus far the power had come from the factory fitted 12V socket. So I decided to try the power source from my dual battery as this had a common earth with the cranking battery. This did not help and the whine continued.

    I then disconnected the shield at the TV end of the RCA cables and the problem got worse. I did notice that the shield is poor, it is one of those cables with the strands of wire wrapped around the signal cable in a spiral fashion.

    Now having read MikeML's latest post it would appear as if changing the RCA cable will make little difference as the problem exists in the grounding. Should I attempt to take a ground from the battery ground directly to the ground on the radio / CD unit?

    A bit stumped now.
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    First thing I would try is the transformer-based stereo ground-loop isolator I showed you in the earlier post. That may cure it (90% probability). There is no Ohmic connection between the barrels of the RCA phono jacks when you use the isolator.

    Second thing would be to rewire the DVD player. Isolate it from the car body by mounting it on plastic (easy in modern cars). Run the DVD's +12V wire to the dashboard area and tap into the +12V wire that is already feeding the dash-mounted radio. Run the DVD's -12V (gnd) wire to the dashboard area and tap into the -12V (gnd) wire that is already feeding the dash-mounted radio. The RCA cables should be long enough..

    To get the audio clean you may have to do both steps....

    A temporary test would be to isolate the DVD player from the car body (both mounting screws and gnd wire), and temporarily power it with a small 12V SLA battery. If that cures it, you know you are on the right track.
     
    Last edited: Dec 16, 2014
  16. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
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    2
    Thanks, will see what I can accomplish.
     
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