Hack17.Reduce Your Audio System s Noise

Hack 17. Reduce Your Audio System's Noise

There are ways to reduce or eliminate the buzzing and high-frequency noise that the engine adds to your in-car audio.

Despite the fact that we spend so much of our time listening to music in our cars, the automobile is a fairly hostile environment for someone seeking audio perfection. The alternator of the car and the electrical pulses from the spark plugs both create oscillating electronic currents in the audio signal. Really, any moving electrical part can create magnetic waves that show up in electrical wires as audible noise. This is why cables are shielded.

The power from the 12V car battery that powers your car computer fluctuates from 1214V and includes many other little electrical oscillations from the car. While the battery can still successfully power your computer, the little fluctuations pass through the computer and show up in the analog audio signal that it puts out. At other times, the car's electrical noise is picked up directly by audio wires that act as a sort of antenna. This line noise can mess up the audio on its way from the computer to the head unit or the speakers.

Just as there are many potential causes for line noise, there are a number of ways to reduce the noise in a car computer setup. We'll look at those now.

2.7.1. Install Better Wires

You can spend hundreds of dollars on high-quality speaker wire and RCA cables alone. However, the importance of using quality wires is not to be underestimated. Cheap, thin, long RCA cables are like antennas that can pick up noise all along the path they travel through the car. Upgrading to thick, shielded cabling can reduce unwanted noise in the audio signal.

Monster Cable (http://www.monstercable.com) is one of the leading manufacturers of well-shielded cable. They're expensive, but if you use cheap cables the labor of re-running wires and diagnosing hums and buzzes will work out to be more costly in the long run. Just as with computers, using high-quality cables will save you a lot of head-scratching time in the end.

Some cable and interconnect manufacturers will brag about how their wire is hand-made by Swiss engineers and cold-rolled in zero-oxygen chambers, allowing for a superior signal-to-noise ratio. Although all cable is not created equal, you can get superior interconnects without spending a fortune. You don't want to use the super-thin 20" RCA cable you bought at the 99-cent store, but the mid-line Monster Cable Phoenix Gold wires and cables will cover most bases. For places where RF interference will be an issue, seek more heavily shielded cables, or possibly go with a coaxial RCA cable. These provide more shielding from interference, but the cables are very stiff, making tight bends difficult or impossible, and most coax is either black or white, which limits your color-coding options. Canare (http://www.canare.com) is a leading manufacturer of coaxial cable. They make the cables and connectors that TV and radio stations use for broadcast, which will probably be sufficient overkill for your car.

2.7.2. Install Shorter Wires

Long cables waste space and have to be coiled and shoved out of the way in the trunk. Using a 25-foot cable when a 15-foot cable will do also adds more length to the cable to pick up more noise from electromagnetic fields around the car. Well-shielded or not, too much cable is a bad thing. Use the correct length of cable, as short as you can get away with. If you invest in a few tools and some raw materials, you can make your own interconnects. The advantage of this is that they will be the right length, which makes for clean installs. Also, most raw cable is better quality than you would find in an off-the-shelf premade cable.

2.7.3. Use a Better Power Supply

The power supply itself, which should be regulating the input voltage, can be a source of noise. Many small in-car computers use a DC-to-DC power supply, instead of the AC-to-DC power supply used in desktop PCs (i.e., an ATX power supply). While these DC-to-DC power supplies regulate the 5V signal, some of them also pass the car 1214V right through on the 12V line. This also passes along the noise to any device in the computer (e.g., the motherboard) that uses the 12V supply.

A manufacturer called Opus Solutions, Inc. (http://www.opussolutions.com) makes an excellent high-wattage power supply for in-car use [Hack #42]. Not only does it have features for turning the computer on and off with the car, but it regulates the voltage as well and helps eliminate noise from the whole computer.

2.7.4. Upgrade the Sound Card

While this is more in the realm of computers than automotive audio, it's well known in the computer world that the sound card alone can add a good deal of noise, due to the electrical and magnetic fields inside a PC case. If the audio built into the motherboard is cheap or has a built-in amplifier that adds noise to the signal, you should get a new sound card with an output line that doesn't try to amplify the signal to improve the sound quality.

Because sound cards that plug into the computer can pick up noise from the case, an external USB sound "card" may be a better bet. Some of these external sound adapters output 5.1 six-channel DVD sound, or even digital sound over optical cables. The USB sound card runs sound digitally through a USB cable over the USB protocol, allowing long runs of cable that are immune to analog interference. The noisy step, which is the digital to analog conversion, is then done as far away from the computer as possible, and as close as possible to the head unit or amplifier it is connecting to. The sound card can be linked with short (less than 3 feet), shielded, high-quality RCA cables, virtually eliminating the chance of interference between the computer and the amplifier.

Some of the newest six-channel amplifiers (designed for 5.1 surround sound) have an optical or coaxial digital inputall six channels of sound go right from the sound card to the amplifier. For sound cards that have 5.1 digital output, this is a surefire way to eliminate interference problems from the computer to the amplifierbut it won't solve buzzes that are already in the computer due to noisy input power.

2.7.5. Eliminate Short Circuits

Every speaker has two wires, a send wire and a return wire, that connect to the amplifier. If either of these wires shorts (touches) the chassis or any metal part of the vehicle, you will still get sound, but you'll also get any buzzes running through the metal of the vehicle. Remember, in a modern vehicle, the metal of the car is eventually linked to the negative battery cable as a grounding measure. In this case, when you accelerate, you will probably be able to hear the revving of the engine as a "bzzzzzzz!" prominently featured along with your audio entertainment.

If you have this problem, try unplugging all the speakers (either from the amplifier or from the head unit if the amplifier is built in) and then plug them in one at a time, to see when the buzz appears. You shouldn't have to drive the car to test the speakersjust idling and revving the engine should reproduce the problem. If one of them does buzz, it might be that that speaker is shorting with the metal body of the car.

In fact, any device (not just speakers) that contacts the body of the car or ground wires may pick up unwanted signals. If metal parts of the computer (such as its case) are touching the body of the car, this could carry noise as well. Try isolating (through nonconductive insulators such as rubber, plastic, or vinyl) the computer or other device from the body of the car to see if this improves things. You can purchase little rubber feet for less than $1 at Home Depot. The more jelly-like the insulation is, the better, as this also provides vibration damping for the computer.

Another way that ground shorting can occur is if the black ground wire is connected to the body of the vehicle near the device, instead of running all the way back to the negative battery cable. To solve this, make sure the ground is truly connected directly to the battery, such as through a power distribution block, as described in "Gauge Your Wires" [Hack #3].

2.7.6. Use Electronic Noise Filters

Electronic noise filters are small devices that go in-line between the power and the device. They remove fluctuations from the power signal and transmit a cleaner 1214V. This can help the device (radio, amplifier, computer, CD changer, etc.) output a cleaner audio signal. SoundGate (http://www.soundgate.com/products/filters) makes several noise filters.

2.7.7. Eliminate Ground Loops

Ground loops are flows of current caused by different voltage levels in different parts of the car, due to resistance from long wires or the body of the car. When devices on opposite ends of the vehicle are both connected to ground (i.e., the negative battery terminal), one of them is actually very close to the battery, and one is far away. This distance between, say, the head unit and the amplifier can cause hisses and clicking noises in the system.

One way to solve this problem is to make sure that all the devices are connected to a common groundthat is, to the same ground wire or to several ground wires that directly connect. Don't just screw down to the body of the car (a common and easy approach) or whatever black ground wire is handy in the trunk. Instead, make sure that good, thick wires connect the negative terminal to all the audio-related devices.

2.7.8. Use an Audio Isolator/Decoupler

Audio decouplers can also be used to solve ground loops. Audio isolators make sure that only the audio signal gets cleanly through, without the ground loop noise, by completely isolating the voltages between the two devices. SoundGate makes units for this purpose (see http://www.soundgate.com/products/isolators-filters/filters.htm).

2.7.9. Use Better Connectors

Computers tend to have mini-jacks (2.5 mm) coming out of their sound cards. Higher-end sound cards have RCA outputs, which not only stay plugged in better but have more metal contact, and thus less signal resistance. If there are no other options than to use a mini-jack, purchase one with the right length, an angled head, and a thick, shielded cable. Smaller wires are more susceptible to interference, and a straight head makes it easier for the plug to pull out unintentionally.

Gold-plated, professionally mounted plug connectors can do a far better job than poorly soldered, cheap connectors that lack adequate shielding. Preformed connectors are also more tolerant of frequent plugging and unplugging without breaking off.

2.7.10. Use Better Power Inverters

Power inverters that convert DC 12V to AC 120/220V do not always do a perfect job of emulating wall currentthe waveform may not be exactly what the computer's or device's power supply expects. The power supply output may also have unexpected signals, which could cause the device to perform a bit out of specification and create hums or additional noises.

A different power inverter, or a different computer power supply that can deal with a "modified sine wave" power inverter, may solve this problem. Exeltech (http://www.exeltech.com) makes the best inverters with true sine waves, designed to provide power for more sensitive applications such as audio.

2.7.11. Segregate Cabling

A simple way to minimize the amount of power signal interference with the audio lines is to run them as far apart as possible. You can run the power lines down the passenger side of the car and the audio cables down the driver's side, for instance, and then run them to your car PC in the trunk without crossing.

2.7.12. Add Shielding

High-end computer cases actually have two layers of metal surrounding the computer motherboard. This tight metal cage around the computer components actually helps block stray electrical signals, both incoming and outgoing. To maintain this protection, it is even important that all expansion card slots are covered with metal brackets.

Reduce Tape Adapter Noise

If you use a tape adapter, there's another possible source of noise. Turn your car key to the accessory position, insert a tape adapter tape, and listen. You will hear, either faintly or loudly, a rotating, scraping noise. This is the sound of the tape rotators uselessly spinning within your stationary tape adapter, and the corresponding amplification of this noise through the sound system.

When I used to drive a 1983 Cutlass Sierra, I could hear this noise even when drivingan annoying cyclical scrape amplified by my nice sound system and laid on top of whatever I was listening to, in addition to the noise emanating from the tape player itself.

One day, the old tape player was noisier than normal, and it was clear that the rotating gears were stuck on the tape adapter. I could hear a loud "click click click click" as the gears kept sticking. Finally, the noise subsidedthe tape player had broken.

When I put in a normal tape, nothing happened. No sound, just the quietest buzz as the tape player tried to amplify the nonmoving piece of tape that the head happened to be on. But when I used the tape adapter, the sound was quiet and perfect (well, as much as a 1983 sound system can be perfect). The unfortunate event of my tape player breaking ended up showing me that I didn't need the tape rotators to actually spin in order to use a tape adapter. In fact, the sound quality had never been so good.

If you have pretty much stopped using tapes for audio and are using your MP3 player or computer exclusively in an older car, you don't really need the noise generated by the spinning tape player. And if you don't mind a little constructive destruction, you can purposefully "break" your own tape player in order to reduce noise when using your tape adapter. (Of course, this means that you won't actually be able to play tapes anymore.)

The real trick to this hack is getting in and either unplugging the motor or disconnecting the rubber band or gear that goes from the motor to make the tape spin. Some units will let you unplug the motor; in the unit I used in "Reduce Your Audio System's Noise" [Hack #17], however, the tape deck "knew" when the motor wasn't connected and wouldn't play, so I had to disable the spinning mechanically. Experiment and see.

The braided outer wire of well-insulated coaxial cables is another example of a metal "cage" that protects against electrical interference. In any wire or metal device through which current flows, magnetic fields are formed. Tight metal cages can absorb and block these stray signals.

This general principle of shielding can be applied to fully debugging a car computer's buzz problem. If you're certain that you've eliminated all the noise flowing into the computer through the power input, that you've isolated the computer case from the car chassis, and that you've grounded your computer directly to the battery, the source of the noise could be a shielding problem. Many cheaper car PC case manufacturers don't bother to properly insulate their cases from either dust or electromagnetic interference. If the case has open card slots, shut them with metal shields. Sometimes case manufacturers use a tinfoil-like metal shielding in order to pass electromagnetic interference tests. If your case isn't almost airtight, try to get another case that is a fully enclosed metal shell, properly designed to shield against interference (e.g., a small desktop PC case from a leading manufacturer).