noise reduction help wanted

[Julian Edgar]

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
frequency.

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
noise.

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
not.

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
components.

Any ideas?

--

Julian Edgar

 

[Defibrillator]

Sounds like an earth problem.

Either the PC is being earthed to the car only through the RCA leads, or
there are multiple earth paths, and an earth loop.

I'd bond the PC case to the car chassis and, in between the PC and the FM
modulator, try something like this:

http://www1.jaycar.com.au/productView.asp?ID=AA3085&CATID=24&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=563

Defib.

 

[KLR]

On Sat, 31 Jul 2004 10:52:45 GMT, "Defibrillator" <nospam@for.me>
wrote:

Sounds like an earth problem.

Either the PC is being earthed to the car only through the RCA leads, or
there are multiple earth paths, and an earth loop.

I'd bond the PC case to the car chassis and, in between the PC and the FM
modulator, try something like this:

http://www1.jaycar.com.au/productView.asp?ID=AA3085&CATID=24&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=563

Defib.

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.

 

[MC]

Julian Edgar wrote:

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
frequency.

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
noise.

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
not.

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

 

[Phil Allison]

"Julian Edgar" <edgarmednospam@ozemail.com.au

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
frequency.

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

** 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