Audio Extermination

David Navone offers a number of tips and techniques for debugging multichannel systems.

The 2002 car audio components include many new decks, processors, and amps capable of reproducing multichannel surround sound. With video displays, games, VCRs, navigation, MP3 audio, e-mail, DVD-A, DVD-V, etc., the possibilities for in-car entertainment and business are incredible. The problem is that multiple channels can lead to increased system noise that’s very difficult to eliminate. This is because the noise lies in the same low frequency bandwidth as the signal. Simply laying the signal cables near the car’s chassis is enough to permit inductive coupling of the interference into the signal.

Loop Area Noise
There’s a decade’s old myth in car audio that running the signal cables down one side of the car and the power wires down the other side of the car will prevent noise from coupling into the system. The problem with this concept is that the car’s chassis is the largest wire in the car’s electrical system. All electrical accessories, including the car stereo components, use the car’s chassis as the ground return to the case of the alternator or the battery’s negative post. As soon as the signal cables are laid next to the car’s chassis, noise can easily couple into the signal path. How does the noise couple into the stereo system? The answer is by way of the loop area created by the signal cables.

Loop area can be defined as the separation of the conductors multiplied by the length of the cable run. Since the channels are connected together at the source (i.e., the deck) and again at the input of the next stage (i.e., the amp), a loop forms in the signal path. At any one instant in time, this loop causes a different noise in the (+) and (-) conductors due to their physical separation. This difference noise is added to the original signal and manifests itself as alternator whine, ignition ticks, accessory pops, etc.

With a simple mono signal cable, the width of the loop is the distance between the conductors times the length of the cable run. With small UTP (Unshielded Twisted Pair) cables, the separation between conductors can be less than 1 mm — the thickness of the insulation. With larger co-axial designs, the separation distance can be as much as 7 mm. This is one reason why double and triple-shielded cables have major problems for low frequency noise.

With a stereo pair of cables, a second loop is created between the left and right channel pairs. The loop is defined as the distance between the channels multiplied by the length of the cable run. With some side-by-side cables, the separation of the left and right pairs can be 10 mm. This is why signal cables with control leads separating the left and right conductors pick up more noise than cables without the added separation. With UTP designs, the distances between the left and right channel pairs is minimized again.

When four-channel decks became popular, there weren’t very many four-channel signal cables. This meant that two pairs of two-channel cables were often used. Many installers called with complaints such as, “The system was pretty quiet on two channels, but as soon as we plugged one or both of the rear channels, alternator whine appeared.”

The reason that the alternator whine appeared in the four-channel system was that yet another loop area is formed between the front channel pairs and the rear channel pairs. This loop was not complete until one, or both, of the rear channels were connected into the system. Four-channel loop area is defined as the separation of the front and rear channels, multiplied by the length of the cable run.

The mono cable has a simple loop between the two conductors. A stereo cable has loops in each channel and a loop between the two channels. A four-channel cable has loops in each channel, a loop between the front channels, another loop between the rear channels, another loop between the front and rear channels, etc. The point here is that when low frequency noise is a concern, then minimizing the loop area presented by the signal cables is of paramount importance.

A properly designed signal cable will optimize the individual conductors, the paired conductors, and the front and rear conductors. Although it may be possible to hand-wind a mono cable, it’s not possible to use a Makita drill to make a stereo pair of signal cables. Uniformly hand-winding the eight conductors required for four-channel cables would be nearly impossible.

Surround Systems
Whenever a new technology emerges, we notice that many of the specifications that define a quality component are conspicuously missing. For instance, when the first DSP decks arrived on the scene, their output level was in the 2-volt range. Also, the source impedance, dynamic range, etc. were not particularly impressive. A component with a high source impedance and a low signal level will be more susceptible to low frequency inductive noise than a component with 10 volts of signal in series with a low source impedance. Many of the new mobile surround components suffer from this problem. In many instances, the new technology makes the sale, not the specs.

Our editorial director, Rob Hephner, mentioned that he was implementing a multichannel surround system in a new install. This means that the signal cable requirement will be at least 5.1 channels (six channels of audio). Multi channels mean more loops and more chances for noise to become a problem. Rob was looking at using a four-channel cable and a regular stereo cable for this system; however, there would always be a huge loop between the two cable runs. So, Rob asked Rusty Everett at Monster Cable to supply a six-channel cable in their new XLN-PRO design. This cable design is the quietest we’ve ever measured.

Checking for Cable-Induced Noise
It’s possible that cable-induced noise is creeping into your regular stereo system, your four-channel system, or your new multichannel surround system. To find out if your system is picking up noise in the signal cables, one method is to simply eliminate the signal cables and “move the deck.” Temporarily relocate the deck to the vicinity of the amp and use very short signal cables or male-male RCA gender changers to connect the output of the deck to the input of the amp. If the noise goes away, then the cables are the problem.

If this test seems too difficult, it may be possible to temporarily reroute the signal cables outside the vehicle for the tests. If the noise lessens when the signal cables are moved, then there’s a good possibility that the cables are picking up the noise. Be sure to turn on the vehicle’s headlights and listen to the stereo system on a quiet passage.

The solution to the problem of cable-induced noise is to use signal cables designed for high quality audio and extremely low noise. And, for noise-free surround systems, using the right signal cables takes on even greater importance.