Computer Audio Problems
Many of the unwanted hums, clicks, and whirrs that compromise the performance of your computer studio are nothing to do with software. We take a look at what causes them, and how to banish them for good!
While there are plenty of possible causes of clicks and pops in digital audio recordings, most are related to the way the computer has been set up, are problems with the soundcard drivers, or are due to incompatibilities with other expansion cards in the same machine. Fundamental hardware issues can cause annoying hums, buzzes, and other background noises. These can occur both inside and outside the computer, but thankfully most can be cured with a little knowledge, a little sleuthing, plus the occasional use of a soldering iron.
Internal Problems
Quite a few musicians still seem to think that choosing a PCI soundcard is more likely to give them audio problems than a USB or Firewire audio interface, but the reverse is sometimes true. PCI's bad reputation largely came about during the early days of soundcard design, when background noises were all too common in low resolution models, and you were lucky to get a dynamic range of more than 60dB with the original cards. All too often you could hear background noises when the CD-ROM or hard drive were accessed, when you moved your mouse, or when your graphics card re-drew any part of the screen display, and there was little you could do to improve the situation.
With the earliest soundcards, you could get digital interference problems just by installing them close to a badly-behaved graphics or SCSI Host Controller card, since these can sometimes push out a lot of radio-frequency (RF) interference ? some nVidia graphic cards have apparently proved troublesome in this respect. However, internal interference from other computer devices rarely causes audio problems nowadays. This is largely due to the huge advances in the design of soundcard ground planes, multi-layer circuit boards, and greatly improved filtering of the incoming power supply signals to remove any digital noise.
Nevertheless, it pays to be cautious, so make sure when installing a new soundcard in a desktop computer that you don't drape other cables over or near it that are carrying digital signals to USB ports, hard drives, and so on. If you can't indulge in some slot-swapping to improve the situation, at least make sure you can see daylight between the two cards, and if in doubt slip a sheet of paper between them to ensure that no component is in electrical contact with one of its neighbours.
While we're discussing such 'proximity' interference, remember that the screening of even high-quality audio cables isn't perfect, so keep external analogue and digital ones at least several inches away from mains cables where possible. Also, try to avoid running such cables in parallel with each other, crossing them at right angles if possible and keeping them well away from CRT monitors, wall-wart power supplies, and particularly the switched-mode PSUs often used by laptops and flat-screen monitors. Analogue cables should also be kept away from digital ones where possible.
If a laptop audio problem is due to indifferent earthing arrangements (laptop audio quality is rarely very high up on the list of manufacturers' priorities, so hefty ground planes and internal shielding are less likely), you can sometimes improve things by creating an earth connection using a cable plugged into an unused laptop serial or parallel port, connecting its screen at the other end to your mixer. This is usually better than relying on the earthing provided by a tiny 3.5mm jack lead connected to the laptop's audio output, and the often tiny earth track connecting it on the circuit board.
Design Problems
Occasionally you may come across a fundamental problem inherent in the computer's design that causes audio interference. In a laptop, for instance, components tend to be very closely packed together, which can result in audio interference on any integral sound chip. Similar problems may also affect the users of designer PCs in elegant but tiny cases with limited expansion potential, where you may have little choice of slots for your soundcard. In such cases the obvious solution is to plug in an external Firewire, PCMCIA, or USB audio interface to sidestep the issue.
Sadly, some laptop switched-mode power supplies seem to add digital noises to your audio even when you're using an external interface. In this instance, unplugging the PSU and relying on battery power during recordings and performance usually solves the problem, which is the way many musicians end up working live. Mind you, many such problems are due to ground loops, which I'll be covering in the next section, and there's an easier solution for these.
Design problems can also affect users of large desktop computers. Some of Apple's dual-processor G5 machines, for instance, have suffered from audio noises that have been tracked down to their power supplies ? they happen when the processor switches between its various performance modes, and can even affect external Firewire audio interfaces.
Ground Loops & Digital Clicks
In essence, any item of audio gear plugged into the mains supply is likely to be earthed via its mains plug, so as soon as you connect two or more such items together via an audio lead you create a loop which acts like a single-turn transformer and can pick up mains hum, circulating noise currents, and other nasties.
Exceptions are those devices that are of double-insulated construction with no ground connection of their own, which you can often recognise by the dummy plastic earth pin on their moulded mains plug, or by the use of a twin-core mains wire which has an oval cross-section (compared to the circular cross-section of three-wire mains cable). Most laptop PSUs also have a two-wire mains cable, relying on an external earth if required, but some more recent models (particularly those with metal casework) do have three-wire cables. Sometimes the fact that an earth wire enters the external power supply doesn't necessarily mean that the laptop itself gets earthed via the PSU's output, but this is often the case.
When you install a PCI soundcard in an adjacent slot to another PCI card, components on the two cards can touch, and this can lead to degradation of critical audio signals. Because it's not always easy to see if components are touching, a simple test is to try slipping a sheet of paper between adjacent cards.
Many musicians already understand what ground loops are, but still consider them a problem that only causes unwanted hum. In fact, you can also run into RF problems, particularly if there's a powerful transmitter nearby, as musicians who have ever heard passing taxi messages coming from their loudspeakers will confirm. Oxidation on jack plugs and the pins of mains plugs aggravates such problems, so keeping your earth connections clean will help in this respect.
However, digital audio gear is another source of possible interference once there's a ground-loop problem, and this creates symptoms that may sound rather different ? imagine the sound of a digital signal, such as the one most of us hear every time we log on to the Internet via a dial-up modem, but leaking into the analogue signal path. Other sounds can be picked up via a ground loop, as well: these include the sounds of your hard drive when its heads move around accessing different files; the scratchy sounds that you sometimes hear when you move your mouse; or those clicks caused by your graphics card re-drawing part of the screen display on your monitor screen.
Even within the computer, there's the potential for ground-loop gremlins as soon as more than one earth connection is made. One classic example of this is soundcards that have a breakout or expansion box that you can fit into a 5.25-inch drive bay just like a CD-ROM drive ? various models from Creative Labs and Terratec, for example. The soundcard already has its main ground connection via the metal backplate, which should be bolted firmly to the metalwork of the computer by tightening down the backplate screw, but then the metalwork of the breakout box gets bolted to the computer's case, the soundcard and breakout box are connected via an internal cable, and a ground loop is born because there are now multiple routes to earth from the breakout box.
One way around such problems is to disconnect the ground connection of the breakout box by bolting it into your computer using nylon bolts and washers. Another option with some soundcards (notably the Terratec range) is to use the breakout box as a stand-alone desktop unit by attaching it via the supplied umbilical cable to the socket on the soundcard's backplate, instead of using the alternative internal ribbon cable and drive-bay mounting.
One of the easiest ways to minimise ground-loop problems is to make sure that your computer is plugged into the same mains supply socket as the remainder of your audio chain, including any mixer and amplifier ? this keeps any loop physically small and therefore less likely to cause problems. Since a single 13A socket in the UK can quite easily power a 3kW electric fire, the vast majority of us should also be able to run our entire studios from distribution boards plugged into a single wall socket, or several wall sockets on the same ring main (a single loop of cable which starts and ends at the fuse box).
Whilst in most houses a single ring main is used for the wall sockets on each floor, with separate rings for individual items with high power requirements, such as electric cookers and shower units, don't assume that all the wiring behind the wall sockets in a particular room will be separated by a metre or two of cable, in many houses you'll find sockets on each side of one room wired through the wall from adjacent rooms, resulting in many metres of cable between them.
