Macintosh: Solutions for noise in the audio signal
Learn how to reduce buzzing or humming noises that are caused by ground loops.
When you connect audio equipment to computer-based sound systems, you want to be careful not to accidentally create ground loops.
Ground loops are unwanted additional paths to ground which allow non-audio currents to mix with audio signals. This creates audio interference, sometimes referred to as "AC hum"?. When you add a computer to the sound system, though, you may hear more complex noises related to the computer's processor activity such as beeping, whining, scratching, or other noises.
Ground loops are created when multiple pieces of equipment using differing grounding methodologies are connected together. Eliminating them generally focuses on the interconnection methods. While ground loops have always been present, new higher performance computer systems may make ground loops more noticeable.
There are several techniques for minimizing or eliminating ground loop-based interference:
- Reduce the amplification of the noise relative to the signal through appropriate "gain staging." This technique involves setting the gain as low as possible in the section where the noise is coupling in, and compensating with higher gains in other sections. For instance, if the ground loop is created by a pair of powered monitor speakers, the interference can be minimized by lowering the gain control on the speakers themselves, and raising the gain in any previous stages. Remember not to turn any stage up too far, so it doesn't induce clipping.
- Reduce the potential difference between currents by plugging the computer and all attached peripherals into outlets which are as close together as possible.
- Eliminate the path(s) of the current by using isolation transformers or by breaking the shield connections on the audio cables going into the computer. The most dramatic improvements are generally seen with this technique, as it essentially eliminates the offending currents, which are carried on the shields of cables connecting to analog audio equipment. If you use a balanced (XLR) interconnect, isolate the current by breaking the shield (pin 1) connection. Use a cable designed without this connection or a shield isolator adapter. If you use an unbalanced (RCA or 3.5mm) audio connection, use an isolation transformer. The next section of the article lists resources for these cables.
The goal in reducing ground loop noise is to break any connections between the shields of cables connected to the computer and any analog audio equipment which in turn shorts the shield to the "green wire" ground of its power cord. You can use a simple ohmmeter, available from Radio Shack or other electronics stores, to identify where the unwanted ground connections exist.
For instance, in a configuration where the computer is connected to a FireWire audio device which is in turn connected to an analog mixer, an ohmmeter might show that the cable shields connect the ground from the computer chassis through the FireWire audio device and from there to the chassis of the mixer, which is in turn connected to the "green wire" ground of its power cord. (Make these measurements with all AC cords unplugged.) To break this connection, incorporate isolation in the audio cable connecting the FireWire audio device to the mixer.
If an ohmmeter is not available, a methodical approach of adding one device at a time can indicate when the unwanted electrical connection is completed.
Isolation Transformers for Balanced and Unbalanced Inputs and Outputs
There are two types of inputs and outputs. Professional equipment uses balanced inputs and outputs. Consumer equipment uses unbalanced inputs.
Professional multimedia equipment, with balanced inputs, uses either XLR or 1/4-inch TRS (tip-ring-sleeve) connectors. These consist of three wires or pins for each channel. Two of the wires contain an equal-voltage audio signal, while the third wire contains the ground. These specialized connectors are most often found on professional USB audio interfaces, FireWire audio interfaces, mixers, and amplifiers.
Most consumer audio and video equipment has unbalanced inputs that use either coaxial (RCA) connectors or 3.5 mm connectors. A coaxial cable has two wires. One wire carries the audio signal, and the other is the shield or ground wire, which carries the ground. This means that a stereo signal requires two cables, one for right and one for left. 3.5 mm connectors usually contain a pair of wires and one shield, providing left and right stereo plus ground.
Apple computers are designed to integrate with existing consumer multimedia equipment, including most stereo equipment, televisions, VCRs, and portable speakers. Consequently, Apple computers use unbalanced inputs and outputs. If you are connecting your computer's sound output port to another piece of equipment with unbalanced inputs, consider using an unbalanced isolation transformer.
Unbalanced to Balanced
For connecting unbalanced output (such as your computer sound output port) to a balanced professional input, a third type of isolation transformer is used. This isolation transformer has unbalanced inputs and balanced outputs.