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noise reduction help wanted

Discussion in 'Hobby Electronics' started by Julian Edgar, Jul 31, 2004.

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  1. Julian Edgar

    Julian Edgar Guest

    I am using an inverter to run a conventional PC in a car. The audio from the
    PC is fed through RCA cables to a FM modulator that connects in-line with
    the car radio aerial, allowing the PC's sound to be heard on a specific FM

    The problem is that there is a great deal of background noise which is
    easily audible, especially when there is no music playing.

    When the FM modulator is connected to (say) a portable CD player, there is
    no noise. When the PC output is directly monitored with headphones, there is
    no noise.

    Using a suppression capacitor (normal car radio type) between the earth of
    the RCA signal leads and the chassis removed a high pitched component of the
    noise. However, there remains a noise which sounds very much like white

    This noise level doesn't change with adjustment of the PC volume output.
    However, it changes in level with adjustment of the FM modulator output
    level or the radio volume control.

    The noise doesn't change in level whether the car's engine is running or

    I have separated the RCA cables from other wiring - originally they were
    bunched with other leads - however the noise as described is still present.
    I have tried a variety of capacitors between the RCA earth and the chassis
    and while I can get rid of the high pitched noise, I cannot reduce the other

    Any ideas?
  2. KLR

    KLR Guest

    I would be careful with these units

    while they do work very well at killing earth hum loops - the ones I
    tried (from WES) did severely cut back the bass to the extent that it
    ruined the sound quality of the system and made it "tinny". The hum
    and interference wasnt good but was more bearable than no bass :)

    I found that by experimenting with disconnecting earth paths one after
    the other - and also experimenting with WHERE you put the earth(s)
    back to the power supply it made a big difference.

    Try taking an earth (and + lead) lead directly back to the battery
    terminals rather than the wiring under the dash, cig lighter socket
    etc (for example) and see what happens.

    If you have a more modern car with a computer controlled management
    system - there may even be separate "digital" and "analog" grounds.
    If you connect a car stereo or audio lead shields in any way to a
    digital ground you will likely get hum or other annoying interference
    through your system.
  3. MC

    MC Guest

    The high-pitched noise was probably the inverter.
    The white noise is probably the PC generating RF.
  4. Phil Allison

    Phil Allison Guest

    "Julian Edgar" <

    ** The output from the PC's sound card is probably infected with lots of
    supersonic hash from the AD conversion - easily seen on a CRO. If this
    is the case then in the FM ( I assume stereo) modulator the supersonic hash
    interacts with the 38 kHz stereo switching frequency and creates all manner
    of alias signals back in the audio band.

    The isolating transformers from Jaycar will act as filters and remove most
    of the hash - or you can build a pair of nice 24dB oct filters with - 3 dB
    point at 15 kHz.

    ............ Phil
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