If you've ever installed a car audio system with multiple amplifiers, and signal processors (eqs and or crossovers), you've more than likely had to deal with noise. Noise is generally caused by ground loops (2 or more paths for the ground connection). This is why virtually all audio manufacturers have the audio shield ground (reference) isolated in some way from the chassis (power) ground for that particular device. In this section, I will try to explain why the audio shield ground must be isolated from the power source ground. In following sections I will explain the different ways to isolate the grounds.

OK, the first thing to remember is that all conductors have resistance. This includes the chassis of your car. Also remember that to measure anything, you need a reference and anything that has some sort of output will have a reference as one of it's output terminals. In car audio, the reference is the "shield" of the RCA cables. I know that some audio systems use DIN or mini DIN connectors but in the USA the RCA style connectors are by_far the most popular, so I will refer to the audio transmission cables/connectors as RCA type cables/connectors. You should also remember that amplifiers and other signal processors amplify the audio signal AND amplify any error that may occur in the signal transmission. The error is usually in the ground path.

First, of course, we have a signal source. If it is an indash radio, the case (outer metallic shell) of the radio is electrically connected to the chassis of the vehicle at it's point of mounting. If the radio is mounted in plastic , the radio's ground wire will make the connection to the chassis. Realize that the ground wire of the radio and the shield of RCA cable are connected together inside of the radio. I know that some radios have some sort of isolation between the radio's case and the audio ground but most are connected directly to the case of the radio. The only exceptions that I know of are the head units with balanced outputs through RCA jacks.

Now, let's say that we have to send the signal to the rear of the vehicle (to an amplifier, let's say). Remember that the metal chassis of the vehicle has resistance and if we have current flowing through any conductor with resistance, there must be a difference of potential (voltage) from one end of that conductor (in this case, the chassis of the vehicle) to the other. This means that there is a difference of voltage between the grounding point of the radio (reference for audio signal source), and the mounting/grounding point for the amplifier (probably in the trunk). To complicate things, the current flowing through the chassis, is constantly changing. The chassis provides the return path (to the battery) for turn signals, brake lights, amplifier(s), fans, windshield wipers... Well, you get the idea.

Now you can probably see that the difference of voltage from one end of the chassis to the other, isn't going to be constant because the current from all of those accessories is not going to be constant. This is why the audio signal has to have a dedicated reference connection. If the audio signal is sent to the rear of the vehicle, without the reference, and the amplifier (in the trunk) uses the chassis for the audio reference, the amplifier would not be getting an accurate (clean) signal and would therefore be amplifying a distorted/noisy signal. The actual input signal of the amplifier would be the audio signal plus or minus the voltage difference in the chassis between the mounting positions of the radio and the amplifier. When an amplifier isolates its signal input ground connection from chassis ground, and uses the RCA shield coming from the signal source, it does not have the error which would otherwise distort the signal. Since an amplifier may amplify the signal 100 times its original level, any noise added to the audio signal will be a serious problem.