Hard audio quality problem: Noise / Ground loop?
Noise in audio signal
I built in my CarPC this week. The fact that it is working finally is quite arousing, the sound quality however is rather disappointing.
To put everything into context, first a schema of my installation:
[CarPC] -> grounded on chassis in trunk
[Amplifier] -> grounded directly on battery
There is a noise in the audio signal that follows the rotation speed of the engine. I thought that this would probably be a ground loop, causing the magnetic field of the alternator to produce a little current causing this noise.
The sound of the headunit however is of a very high quality - I really enjoy my CD's ;) - so no problem with a ground loop here.
I tried the following things:
1) I grounded the CarPC on the same (high quality) wire as the Amplifier, directly on the battery. The didn't remove the noise - it actually got worse: I know hear noise when the harddisk moves.
2) I broke the ground-connection in the signal wire from the CarPC to the headunit (3.5 mm <-> ISO Line-in). As I expected, the alternator sound as background noise disappeared. The bad news is that with the sound the same noise appeared - now volume dependent. The louder the sound, the louder the noise.
3) As a test, I connected the CarPC directly to the amplifier. Result: perfect, high quality sound!
1) It has something to do with grounding.
2) The problem is the connection between the CarPC and the headunit. CarPC <-> Amplifier is perfect, Headunit <-> Amplifier is perfect.
How could I solve this? I'm really running out of ideas right now!
BTW: I have a similar problem with the graphics on my Lilliput TFT: The graphics are not very clear. A lot of ghosting and blurriness. I used a double shielded high quality VGA cable.. So I suspect the ground again..