Return to Zero (Part 4)
Just when you thought noise was a thing of the past... Gremlins are back - in a studio near you.
Last month, we looked at noise reduction systems that contribute greatly to solving the problem of tape hiss, but there are many other sources of noise that lie in wait to pollute your recording. This month we continue our back-to-basics series with a look at these problems and their possible solutions.
It must be stressed that the encode/decode noise reduction systems incorporated into multitrack recorders and cassette based systems can only reduce tape noise and can do nothing to remove noise present in the original signal; indeed, such noise is processed by the electronics just as though it were a piece of wanted information and so it will emerge much the same as it went in. Before looking at ways to minimise this unwelcome addition to our finished recordings, it would make sense to look at the cause.
Electronic noise, which manifests itself as hiss in audio systems is always inherent to some degree even in the most sophisticated and expensive audio equipment. Even the humble resistor generates noise unless the temperature is kept at absolute zero (-273 degrees centigrade) and most working musicians and engineers would find this environment not particularly conducive to stimulating their artistic imagination. Another, and generally more serious source is active circuitry which includes anything using transistors, IC's, diodes or even valves. Transistors tend to generate more noise as more current passes through them, and this noise again increases with temperature. Inside the transistor or IC the temperature is generally somewhat above room temperature. Anyway, without introducing any maths or circuit theory, it is reasonable to state that circuit noise can be minimised, but not eliminated by thoughtful circuit design and that electronics that are well designed in this respect cost more than those that are indifferently designed. It therefore follows that a budget mixer (for example) is a compromise between quality and cost. Of course, the same is true concerning all pieces of ancilliary equipment such as delay lines and other signal processors.
The main job of a mixer is to amplify and mix signals but it is equally capable of amplifying noise, even its own noise. Figure 1 shows how even a simple mixer incorporates three distinct sections of gain or amplification, each of which contributes in some way to the background noise. Section A corresponds to the input gain, the control for which is usually found at the top of each input channel and it is here that the first unwanted noise is generated. The amount of noise for a given design is roughly proportional to the gain of the stage so that the more you turn this up, the greater the noise level created. With line level signals or outputs from electronic instruments, this is not so much of a problem as only a little gain is required to bring the signal up to a level suitable for the following circuitry to handle, but in the case of a low output microphone you will almost certainly need to increase the gain with the result that noise will also increase.
Sections B and C represent the channel gain fader circuit and the main output fader circuit. Although these too produce noise, their contribution is not so significant as that of the input amplifier because their gains are lower.
The March instalment of Return to Zero stressed the importance of setting correct signal levels for recording so it isn't necessary to dwell on this point for long as similar rules apply to both tape recorders and mixers.
Having put the question into perspective, what can be done?
The problem is most acute when using microphones, so your first job is to choose an efficient microphone - one which gives a high electrical output for a given sound pressure level. However, it's quite likely that you have already bought your microphones so a more practical solution might be to buy a standalone, low-noise microphone pre-amplifier which can be patched in between the microphone and the mixer when the occasion demands. Bandive produce one in their Accessit range, or alternatively, if you are practically inclined, you could build the design featured in HSR August 1984.
One subject sometimes overlooked when trying to reduce noise levels is that of cables. The newcomer to the world of home recording may regard them as being rather boring and probably resents the fact that their cost is eating into his budget for some new and exciting signal processor, but their function is vital both from the noise and reliability points of view, not to mention safety.
Any bad connection in the mains wiring is likely to cause arcing and this manifests itself as an intermittent buzz (not hum) which can be very difficult to track down. The wiring of the mains plug itself is often the cause and, as a studio may contain dozens of these, it pays to fit them properly in the first place and then to check them periodically. I'm sure that I don't need to remind you of the importance of the cable clamp in a plug but I'm going to anyway. For this clamp to do its job properly, it must grip the outer sleeve of the cable, not just the inner wires so make sure that this is done properly, otherwise it only takes one careless trip to pull out the wires and blow the fuses or worse. A friend of mine had this very thing happen to him recently and a great deal of damage occurred to his mixer which fortunately (for him) I was able to repair. You can always tell when the mains has run loose inside a piece of gear. Some of the transistors had actually exploded and there were black sooty patches in places where black sooty patches had no right to be.
The other point concerning mains plugs is that the screw terminals clamping the cable ends tend to work loose so these should be checked for tightness every few weeks.
Right, so the plugs are all fine, in fact you've even fitted MK safety plugs with their ingenious V shaped automatic cord grips but the buzz remains. What next?
The type of adaptor that plugs directly into a 13A power socket should be avoided if possible as the internal spring contacts eventually become weak and arcing then starts. On the face of it, the polythene four-way distribution boards that are now so commonly available would seem a better choice, but from my experience, these also suffer from badly engineered contacts and frequently give trouble.
Far better is a home made distribution board that has conventional - or better still industrial power sockets mounted onto a piece of thick plywood. Though this may cost more initially, it could save you a great deal of money in the long run. One last point before leaving mains wiring - fuses. No - I'm not going to warn you about fitting the wrong fuse value, silver paper or (heaven forbid), sawn off six inch nails, I'm sure you all know how fraught that can be. What I do want to point out is that the spring clips that constitute the fuseholders in plugs and cheap distribution boards tend to be rather weak and so you may need to squash these together slightly with a pair of pliers before fitting the fuse in order to ensure a reliable contact.
The screw-in fuseholders on the rear panels of equipment also need to be checked for tightness periodically as prevention is still better than playing hunt the buzz during an important recording session.
All signal leads except those feeding loudspeakers need to be screened and it is a good idea not to waste your money on cheap cable as this is another false economy measure. The screen of the cable should be woven, not just wrapped and although a wrapped screen is easier to wire up, it does tend to offer inferior screening properties and you run the risk of picking up radio interference which is a difficult source of noise to eradicate. (See figure 2.)
The story doesn't end there, however, as even a good woven screen lead may be microphonic and produce noise when bent or moved. It's far better (if you can afford a little extra) to buy professional cable like Musiflex which uses a conductive plastic screen to give a noise free and kink resistant cable. As an added bonus it comes in a full range of jolly colours to make identification easier and is very easy to wire up.
Fitting good metal jackplugs not only improves the screening further, but also makes the leads last for longer. The cheap plastic plugs are mechanically crude and the rivet holding the centre core connector often works loose producing yet another source of noise and crackles. Once the signals and mains cables are given the seal of approval, remember that you can still pick up hum by running the signal leads too close to mains cables or transformers, so it's worth planning your layout carefully.
Other causes of buzz may be traced to sources such as the thermostat in the freezer or central heating system (or even the neighbour trying to burrow through the wall with a poorly suppressed electric drill). Apart from getting the faulty items replaced, there's not much you can do to fend off this type of interference so its usually a matter of turning the heating off before recording.
Hum as opposed to noise is usually caused by faulty or incorrectly wired leads, a faulty piece of equipment or earth loops.
If a process of elimination leads you to believe that you have an earth loop problem, try disconnecting all mains powered equipment except the recorder, mixer and monitoring system; then, by reconnecting the items one at a time, you should be able to locate the source of the trouble. However, solving the problem properly may not be so straightforward. For a more detailed discussion on this topic, see Ben Duncan's articles on Studio Earthing Techniques in HSR September and October 1984.
When you've finally rid yourself of all noise problems and you can record a snail scratching its armpit at twenty paces without a trace of hiss or hum, you are then well on the way to eternal happiness, at least until the guitarist on the next session turns up with a vintage prewar AC 30 that hums like a power station and hisses like a chip pan full of snakes.
Next month we'll make a start on the actual recording process and see how track bouncing works.
Musiflex is available from Turnkey, (Contact Details), or from Keith Hand Musical Supplies, (Contact Details).
Feature by Paul White
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