In an ideal world, studios would function without hum, hiss, glitches, or mysteriously absent signals, but as Paul White has discovered through experience — they don't!
If I had a penny for every time I've heard a sound engineer ask, "Why can't I hear any signal?", I'd have £5.39 by now! While music is most definitely an art, the studio itself remains rigidly bound by the laws of physics, but occasionally, the gremlins from beyond the nameless dimensions break through into what we laughingly call reality, and disrupt our smooth‑running, mechanistic world. Signals disappear, distortions creep in, and otherwise pristine master tapes exhibit inexplicable clicks and pops. The aim of this short piece is to show you how to identify the more common gremlins, and to banish them to their distant domain.
Silent Signals
The most common cause of a disappearing signal is through console routing (or being switched to 2‑Track Monitor when you're trying to listen to the stereo mix). Before you check anything, make sure everything is switched on, and that the power lights are glowing, indicating that the mains fuses are intact. I know this sounds obvious, but nearly everyone, myself included, falls for it at some time or other. A particular favourite is to have a mixing console that's switched on, but a device plugged into the master insert points that's switched off.
When trying to track down a missing signal, solo the mixer channel to confirm that something is definitely getting into the desk. Then, check your routing, including anything that might be plugged into the channel insert point — getting the inputs and outputs swapped around on a piece of outboard gear is a great way to lose your signal. By using the Solo buttons, and checking any metering along the signal path, it shouldn't be too difficult to identify where the signal stops — but don't start panicking and unplugging lots of leads.
If you feel you must change something, then take it a step at a time, and if you suspect a lead, check it out with a cable tester first. If you don't have a cable tester, buy one now — they're not expensive. (I have one of the EMO models which costs under £50, and also doubles as a fuse tester and junction box.) All dubious leads should be clearly labelled and put to one side, so that when that elusive spare five minutes finally shows up, you can sit down and fix them.
Always retain a healthy suspicion of insert points, or normalised patchbay connections, and ensure that the socket hasn't gone intermittent, by temporarily linking the patchbay send and return points using a patch cable.
If a mic is producing an inexplicably quiet signal, double check the mic/line selector, and make sure the mic has phantom power if required. Also, check to see if the mic is fitted with a switch, and if this is the case, turn it on! If you're not sure whether the problem lies with the mic or the wiring, simply plug in a spare mic and check the circuit that way. Only this week I spent several minutes scratching my head at the non‑appearance of a drum mic signal, only to find that the drummer had knocked the head off the otherwise neat and effective clip‑on mic. Fortunately, it was his mic!
Spurious Signals
So much for tracking down a missing signal, but what about signals that were never intended to be there in the first place? Undesired audio signals can usually be traced to mains hum, circuit hiss, or radio frequency interference, which may either be airborne, or carried on a dirty mains supply. Hum is often due to ground loops, although it can also be caused by using cables where the screen has been wrongly wired, or become disconnected (see 'Overcoming Ground Loops' box). It may also be caused by inadequate mains power adaptors, so if you are using a mains PSU that didn't come with the piece of equipment in question, try to eliminate it from your enquires by using a different one. Yet another source of mains hum is electromagnetic radiation from transformers or mains cables, so make sure that mains cables don't run alongside signal cables, and if they must cross, try to arrange it so they do so at right angles. Keep sensitive equipment, such as mic preamps, away from power amplifiers or mixer PSUs, and also be aware that the neon lamps in distribution boards can cause a buzzy kind of hum if brought near to sensitive cabling.
Fluorescent lamps, light dimmers, and computer monitors all radiate significant amounts of interference. Electronic dimmers should never be used in studios, and fluorescent lights should really only be used for maintenance work. The new generation of energy saving bulbs don't seem to cause any problems, but I can't guarantee them to be safe in all situations. Interference from fluorescent lamps and dimmers can usually be differentiated from that of a ground loop, as their brand of hum contains a large element of buzz due to the high harmonic content of the interference source.
Crackles And Pops
Crackles and pops are often caused by mains‑borne interference, such as thermostats and heating systems switching on or off. Running your studio from a separate ring main will help reduce the risk of these glitches getting onto your recordings, but if the problem is severe, the best answer is to get a qualified electrician to fit suppressor components to the offending devices. It can also help to use mains filters, but from my experience, these seldom offer a complete solution.
A related source of crackles and bangs can often be traced to loose wires, fuse holders in plugs, or cheap extension boards where the spring socket contacts have lost their spring. It's always good practice to check your mains plugs on a regular basis, as wires have a habit of working loose, even if they've never been disturbed. Personally, I blame it on those gremlins.
Undesired audio signals can usually be traced to mains hum, circuit hiss, or radio frequency interference...
Other areas to watch out for are poor connections in signal leads, and crackly pots or faders. Once a fader has gone crackly, you can try to clean it with spray cleaner, but in my experience, the only long‑term cure is to replace it.
You may also experience radio frequency interference, but an improperly grounded studio system, or badly designed equipment is the usual cause — unless you happen to have a radio ham living next door, in which case both the law, and the inverse square law are on his side, not yours! Of course, you could always charge a capacitor up to a thousand Volts or so, ground one end, and discharge the other onto his aerial using very long piece of wire and a fishing rod — I'm told this does the trick, but you didn't hear it from me!
Hiss
Finally comes hiss. Excessive hiss is nearly always due to a lack of attention to gain structure (see my article in last month's SOS). Aside from making sure that the gains are optimised, it's also important to ensure that nothing is being routed into the mix that isn't needed. Unused channels should be de‑routed from the mix buss, and all unused sends turned down, or routed to an unused aux send output. If recording electronic instruments, you should also be aware of the amount of noise that's being produced at the source. Electric guitar amps, for example, tend to be noisy, so gate the track while mixing if you can. Similarly, a lot of synth modules are quite noisy, so to make the best of them, work with the MIDI volume set high, and a reasonably high note velocity level. The loudest notes should be reaching the maximum velocity level of 127 — if they're not, then you're not achieving the best possible signal‑to‑noise ratio.
The other major cause of excessive hiss comes about when using insensitive microphones to capture low level, or distant sounds. There's no real cure for this problem, other than using the right mics for the job, or getting the mic closer to the sound source. If you try to record something like remote birdsong with a basic dynamic mic, then it's no good blaming the mixer mic amp for being too noisy.
I hope these few simple spells and incantations will help you banish your own studio gremlins back to the twilight dimension from which they came. If you don't make the effort, I can guarantee that they'll breed, and become even more difficult to remove!
Overcoming Ground Loops
Don't try to cure ground loops by removing mains grounds, because this is obviously dangerous — instead, ensure that all your mains leads are plugged into one distribution board, rather than spread right around the room, and follow the wiring recommendations that came with your mixer as regards external connections. A full investigation of ground loop problems is beyond the scope of this article, but the general rule is that if a loop does cause problems, try to make sure the offending piece of equipment is grounded via one path only (see my article in SOS August 1994 for further information). If the piece of gear runs from an external power supply, the chances are that it is not grounded via the mains, so you should use a connecting cable where the screen is connected at both ends — in other words, a conventional instrument lead. However, the casework of the unit might also be grounded to another unit via the rack metalwork, so try unbolting it from the rack to see if the hum situation changes. If removing it from the rack cures the hum problem, use plastic screws and washers to isolate the unit from the rack metalwork. Studiospares market a special kit for this very purpose.
If the unit is grounded via the mains, it often helps to disconnect the screen from one end of the connecting cables. I favour leaving the screen connected at the mixer end of the lead, but I know that others prefer to do the exact opposite