Plagued by noise? Want to brighten up your old master tapes? Gordon Reid, of CEDAR Audio Ltd, explains the different processes that can be used to clean up audio signals that have become contaminated by noise.

Cheap digital audio in the form of CD has made discerning listeners quite intolerant of the noises and distortions present in analogue audio signals. In a perfect digital world, there would be no clicks, crackle, pops, buzzes, hums, or hisses — but we live in a far from perfect world. Even today, the vast majority of mixing desks have all-analogue signal paths, so most DDD-classified CDs are still mastered through numerous analogue stages. And 'vintage' (ie. pre-1982) recordings are necessarily re-mastered from analogue master tapes. As a result, recording engineers are turning to technology to help keep noise to a minimum and to help remove it from existing recordings.

It's easy to remove broadband noise such as white noise, tape hiss, microphone noise, rumbles — simply by turning your master volume control to zero! This is hardly a practical way to improve recordings, but it is a valuable technique for ensuring that mixes start from silence rather than being preceded by a background of hum and hiss. If the background noise isn't too serious, it will be masked by the music once it has started. Indeed, noise gates work on much the same principle, the main difference being that they monitor the incoming signal level and turn on and off automatically. There are many enhancements to the gate idea, such as variable attack and release times, hold time and hysteresis (all added to limit the occurrence of damaging side-effects), but the principle always remains the same: if the device (gate) decides that there is only noise present at its input, it shuts off the signal. While such gates are pretty useless for treating completed mixes, other than to clean up the start and finish, they are incredibly useful at the mixing stage to mute individual tape tracks during pauses. Perhaps the most popular gates are the Drawmer range, but there's no shortage of alternatives.

However, noise can also be heard during a mix whenever the music drops to a level insufficient to mask the background noise or when the high frequency content of the music is low — a bass solo, for example. So what can be done in this instance to get rid of the unwanted noise while keeping the genuine signal absolutely untouched? There are several less destructive methods of noise reduction/removal available, with the more sophisticated systems costing tens of thousands of pounds, but there are also options which fall within the budget of small studios and home recording enthusiasts. Before exploring the solutions, let's find out more about the problem.

WHAT IS NOISE?

This might be a good point to describe what is meant by the term 'broadband' noise. Such noise is, by definition, a random effect which adds (or subtracts) a random amplitude at all times to (or from) all frequencies within the audio spectrum. The catch-all term for this is hiss. The term 'broadband' noise cannot, on the other hand, be used to describe artefacts such as intermittent electrical clicks or microphone 'grounding' buzz. These problems produce clearly identifiable events of limited duration, and may be corrected by quite different methods to those described below.

Broadband noise is constantly present (to a greater or lesser degree) in every signal, and can't be completely separated from that signal by any means yet devised. Such noise is often most intrusive at high frequencies, where the masking effect of loud sounds is least present, hence the use of the term 'hiss'. Unfortunately, whilst noise that lies lower down the audio spectrum, between (say) 100Hz and 1kHz, can't be described as 'hissy', it can still wreck your recordings. So what can be done to combat it?

Firstly, let's set the record straight regarding the Dolby B, Dolby C, and dbx noise reduction systems found on both domestic and semi-pro recorders. These are dual-ended processes, designed to minimise the accumulation of any extra noise attributable to the limitations of analogue recording tape. (Dual-ended processes are commonly called encode/decode systems, because the recording process 'encodes', and the playback process 'decodes', the signal.) Neither the Dolby processes nor dbx enable you to remove noise from within a signal that already contains it — they simply stop you adding too much more when you commit that signal to tape and then replay it. Indeed, both Dolby and dbx help your tape deck to accurately record, and then faithfully reproduce, any noise contained in the original signal!

What is needed to reduce the perceived effect of any noise that is already an integral part of the signal is a 'single-ended' system that can actually remove noise without causing audible side-effects.

FILTERS

The simplest form of filter is the treble control or high frequency equaliser. Applying high frequency cut to a signal is less damaging than the volume control, which removes the signal altogether, because the treble filter only removes a proportion of any signal present above a given frequency (known as the shelf frequency of the filter). In fact, this is exactly how the scratch filters worked on amplifiers designed to be used with record turntables. Unfortunately, if you reduce the amplitude of the noise content of your recording at the given frequency by XdB, you will also reduce the wanted signal at this frequency by the same amount. This is fine if your recording has little or no high frequency content, but natural sounds and modern electronic instruments have frequency responses right up to (and above) the limits of human hearing. The obvious result is that turning down the treble control will reduce the level of hiss but will also cause a dulling of the sound.

"Broadband noise is constantly present (to a greater or lesser degree) in every signal, and can't be completely separated from that signal by any means yet devised."

Even so, there are occasions where judicious use of the treble control can help; for example, by gradually turning down the treble during a fadeout, or by using EQ to remove frequencies above the natural range of a sound or instrument recorded onto a separate track of a multitrack recorder. This technique can be useful to remove the hiss from electric guitar or bass tracks, for these tend to contain very little signal above 3kHz. However, a more precise filter than a treble control is advisable. If you have a Drawmer DS201 noise gate with sidechain filters, try switching this to Key Listen mode and use the high filter as a very selective treble cut equaliser.

DYNAMIC NOISE FILTERS

Simple filtering, as described, can be useful, but it comes nowhere near to providing a universal panacea for noise removal. The next logical step might be to use a variable filter, which removes the high frequencies when there is little or no signal present, but leaves them untouched when the noise is being masked by genuine high frequencies. This type of device is known as a Dynamic Noise Filter or DNF (so called because the shelf frequency of the filter moves dynamically up and down the frequency spectrum, according to information contained in the signal) and there are many of them on the market, most falling within the price range of private studio owners. Typical examples are: Drawmer DF320; Symetrix 511 A; dbx 563 Silencer; Behringer Denoiser and Behringer Professional Denoiser; Crystal. Providing that they are used judiciously, such devices can significantly reduce the audible consequences of noise without too much degradation to the wanted signal.

But DNFs are still far from the perfect answer. They still rely on the music masking the noise at frequencies below that at which the filter is operating, and if more than the bare minimum of filtering is applied, the effect of the filter on the wanted signal can be heard. Typical side-effects are shortening of reverb tails, dulling of low-level signals, and noise pumping — where the background noise can be heard rising and falling in level behind the wanted sound. Although DNFs are designed to track the signal very quickly, they can't respond instantaneously, so they tend to round off fast transients such as snare drums and other percussive sounds. Again, the more filtering you apply, the more noticeable this becomes.

In practical terms, DNFs can produce excellent results where the degree of noise contamination isn't too serious, and treating the individual tracks within a mix is likely to give better results than treating the entire mix at once. In the case of severe noise contamination, the side-effects of using a Dynamic Noise Filter are likely to be too serious, and the background noise pumping may be more obtrusive than a fixed level of noise.

If a DNF is used while mixing, it often helps to route just the noisiest tracks to a stereo subgroup with the dynamic filter connected via the group insert points. Routing reverb returns through the filters is best avoided, due to the risk of 'choking' the reverb decay.

EXPANDERS

The Downwards Expander is one evolutionary step on from the basic gate and its operation is akin to that of a compressor, only in reverse. The Downwards Expander still has a threshold control, but unlike the gate (which shuts the volume down to zero once the threshold is passed) the circuitry applies a progressive gain reduction, the amount of which is determined by the settings selected by the user. For example, if a signal drops 3dB below the threshold, the Downwards Expander may reduce the signal level by 6dB, 12dB, or whatever, depending upon the expansion ratio chosen. Nevertheless, many audio professionals claim that the subjective difference between the true noise gate and the expander are very small, because a gate with a variable release time is almost as subtle. This rules out Downwards Expanders for treating stereo mixes, but — like gates — they are useful for treating individual tracks or sound sources in a mix. A very affordable example of the Downward Expander is the Boss NS50.

Some of the more highly specified semi-pro noise reduction units now feature a combination of dynamic filtering, downwards expansion, and even (in the case of the Rocktron Hush III) compression and excitation — effects which have been included to overcome some of the undesirable side-effects of the noise reduction processes. Perhaps the most popular of these are the Drawmer DF320 and the Symetrix 511A. These combine dynamic filtering to clean up the bulk of the material, a downwards expander for quietening the pauses between tracks (or other such silences), and a 50Hz high-pass filter for reducing rumble and hum. Like the more basic stand-alone DNFs, these units need to be used with caution, especially when treating complete stereo masters. Used 'lightly', they can transform a good recording into a spectacularly clean one, but any attempt to tackle serious noise problems will cause the undesirable side-effects to resurface.

Next month, I'll be looking at digital noise removal systems which can tackle the problem far more effectively — but at a price.

Note: Part 2 of this series can be found over on Sound On Sound's site:
https://www.soundonsound.com/techniques/aural-hygiene-how-keep-noise-bay-part-2