This comes up all the time so here is my guide to reducing interference in car audio installations.
Interference usually causes an audiable whine through the speakers. Often the whine will increase in pitch with engine speed. Interference can also represent itself as clicking and hissing. It occurs when the flow of current though sensative signal cables is disturbed by outside factors.
Power supply to audio equiptment in a car isn't clean. This is the primary cause of one form of interference... EMI - ElectroMagnetic Interference. When any cable has a current running through it, it produces a small electromagnetic field around the cable. The field is strongest alongside the cable and weakens exponentially further from the cable. If the power was clean and smooth, the affect of EMI would be minimal (hence why using audio equiptment while the car is off doesn't seem to suffer from interference in the same way as when the car is on.) When the car is running, many components cause the current in power lines to fluctuate. These fluctuations alter the magnetic fields around power cables and this is when EMI causes problems.
- Even in the newest of cars, the current supplied from the alternator is not 'clean,' due to the way contacts are constantly being switch within the alternator as it spins. This causes the current in power lines connected to the same battery to fluctuate very slightly and very rapidly. This is also the reason for a whine increasing in pitch with engine speed.
- Coil packs for sparkplugs are constantly charging and uncharging, there rate dependant on engine speed aswell.
- Indicators are causing a constant load and unload on the entire system when being used, causing the current to fluctuate.
- Electronic PAS
motors are taking large amounts of current in uneven patterns, causing the current to fluctuate.
- Due to the nature of a perminant magnet motor, where contacts are constantly being switched as the amateur rotates, current fluctuates. Hence using wipers, electric windows etc can cause interference.
- Even the audio system itself is drawing power in an uneven fashion to provide current for powerful bass notes etc.
...you get the idea. All in all, the power lines are far from smooth. This causes problems because of two reasons; unstable current to audio equiptment, and induction of fluctuating magnetic fields.
Modern car audio equiptment is designed with unstable current in mind and so this very rarely causes any issues. Most equipment will accept a wide voltage supply and use 'chokes' to help smooth out the current, remove noise and prevent ground loop issues. It is also very important the equiptment has good earthing points as with varying load drawn by the equiptment, any resistance cause by a bad earth will cause undesirable effects.
However, the number one most common cause of interference through induction is running signal cables parrellel to power cables. What happens in these cases is as follows...
The power cable produces an electromagnetic field, and as current fluctuates, the field changes. Any wires running close to this will induce the electromagnetism which will in turn affect the current in that wire very slightly. This slight alteration in a line-level audio cable will affect the signal far more than if it were high-level. When the audio signal is amplified, the interference is amplified aswell.
So far we've talked about EMI - ElectroMagnetic Interference through induction. Motors, relays, alternators and sparkplugs also emit another form of interference... RFI - Radio Frequency Interference. All three produce electric arcing sparks which emit a strong RF noise signal as they operate. The RCA signal cables are most affected by this. Essentially the long cables are acting as an aerial and picking up the RFI. For this reason, it is vital that signal cables are only as long as they need be. It is also strongly recommended that shielded RCA cables are used, as any RFI near the cable will be picked up by the shield and not by the signal wire inside. Also excess cable should not be wound into a coil, as this can worsen the problem. If needs be, the coil can be protected from interference by using a couple of clip-on ferrite rings available from Maplins or similar.
As mentioned in this guide, the alternator of your car produces huge amounts of interference - both as RFI (through sparking) and EMI (through fluctuating current.) Installing a suppression capacitor to the alternator will greatly reduce interference in both forms. Capacitors store a small amount of power -after being charged, they can release the current. So basically as the alternator rotates, the current it produces charges the capacitor. The capacitor supplys a very small amount of current for all the thousands of split second intervals per minute that the alternators brushes switch contacts. This softens the power spike caused when the new contacts come into use and so help to smooth the fluctuations in current. As the spike is softened, the electric arcing spark is also reduced, reducing RFI. The same suppression methods can be used for motors within the car aswell as relay contacts.
So to summarise...
- Run power lines and RCA cables down seperate sides of the car.
- Use shielded RCA cables.
- Only use cables of the length you require - measure up and buy the shortest cable that will reach.
- If avoidable, don't wind up the excess.
- Consider using ferrite rings on your cables.
- Consider fitting a suppression capacitor to the alternator.
- Ensure you have a good earth.
- Ensure the head unit has a power supply choke.
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