At some time or another you may well be called upon to salvage the work of a less experienced predecessor, so this month we look at a few signal processing tricks to help you along.

Remixing a multitrack tape recorded by someone else (or one of your old recordings made when you were less experienced) can be an uphill struggle but it's surprising just what can be achieved with a bit of imagination and a few appropriate toys. Some of these tricks are also applicable to cleaning up old master tapes.

One of the main problems that you are likely to come up against is noise, but other problems include mains hum, dull or even totally inappropriate sounds. We'll take a look at noise first.

The term 'noise' need not only refer to tape noise. Noise builds up due to track bouncing (and this includes contributions from the tape itself), the mixing desk and any signal processors that were used at the time of recording. If used, guitar amps are likely to have made their presence felt, being notoriously noisy. So what can be done?

As you are probably aware, nothing can be done to remove recorded noise without affecting in some way the signal that you are trying to preserve, so the skill is in finding the best compromise which most effectively reduces the noise without unduly affecting the sound of your recording?

Before proceeding, you need to be aware that noise is generally a broad band phenomena in that it is present right across the audio frequency band. However, because of the way in which we perceive different frequencies, high frequency noise is the most noticeable, and so the most dramatic improvement can be made by tackling this. The first thing that most people will think of trying is EQ; after all, when you cut the treble on the desk, the noise also gets softer so this seems to be a good place to start. Also nearly everyone has access to some sort of equalisation, however rudimentary.

Filtering

You might assume that as most audio signals occupy less than the entire audio band some improvement could be effected by using a graphic or parametric equaliser to attenuate the unused part of the spectrum. In practice this can help, but the problem is that the sound of the programme still alters, the reason being that its harmonics and overtones almost certainly spread right across the audio range, even if only in very small amounts and the human ear soon notices when they aren't there. For example, a bass drum might have most of its acoustic energy below 200Hz but if you start to cut the high frequency end, even up as far up as 10kHz or more, you will notice that it loses its attack and starts to sound lifeless. The same is true of bass guitars. It therefore follows that only slight improvements in high frequency noise can be made using EQ, usually to the detriment of the sound quality.

EQ is however useful for removing or reducing low frequency interference noise such as mains hum, especially if the wanted signal contains little in the way of low frequency energy. The most effective way of dealing with hum is to use a parametric equaliser set to a narrow bandwidth and tuned to 50Hz (60Hz in US). If maximum cut is applied, the worst of the hum should disappear and the filter frequency can be fine tuned by ear for best results. Unfortunately, even mains hum has its harmonics and conventional rectifier circuits generate a lot of second harmonic, so ideally, you'll need another filter in series with the first, tuned one octave higher (twice the frequency). This type of EQ treatment is known as notch filtering because it effectively cuts a notch in the audio band. You could try using a graphic, a sweep equaliser or even the low frequency EQ control on your desk, but this will affect the signal you wish to preserve more drastically, particularly if it contains much in the way of bass. On the other hand, you could probably apply maximum low end EQ cut to a hi-hat track without altering the sound in a significant way.

Dynamic Filtering

This very effective technique in effect allows loud signals to pass unprocessed, where the noise will be hidden by the music. However, when presented with lower signal levels, the machine progressively attenuates the high frequencies so that when a sound has died away altogether, the unit may well be heavily cutting all signals in excess of 1 kHz, for example. This may sound drastic but providing that the vital attack part of a sound contains all its harmonics, the ear will accept a less bright sound during the decay portion and still perceive the result as being natural. So this yields, in subjective terms, a vast improvement in the apparent signal to noise ratio but without seriously degrading the wanted signal. Most units of this kind have a user-variable control which alters the severity of the filter's action. The result is far less obtrusive than the conventional noise gate. The Symetrix 511 is a good example of such a dynamic filter. This particular model contains two channels of filtering and two low level expander gates which really ensure silence during breaks in the music. Because of the subtle way in which this system works, it can be used to great effect for cleaning up old masters or cassette tapes as well as individual tracks of a multitrack recording.

One popular salvage package is the Symetrix 511 followed by a psychoacoustic enhancer such as the Aphex C or Scintillator which restores some of the lost high frequency information. This is a particularly useful combination for treating tapes that have become dull through time or bad storage and it is even used to process old 78s for re-recording onto tape.

Digressing slightly, these units are invaluable when recording guitar by DIing one of the Rockman-style effects boxes now so popular. Many of these, including the original, give a great sound but are a bit too noisy for serious mastering work. By connecting the output via a Symetrix 511, you can say goodbye to noise and still end up with a good, bright sound. I'd go so far as to say that if you're thinking of buying a gate purely for cleaning up purposes, you should seriously consider getting one of these instead, you'll find it far easier to set up, less obtrusive in use and the noise pumping problem of conventional gates is substantially reduced.

For those of you working on a tight budget who would still like to experiment with this technique, Tantek do a stereo dynamic noise filter and it does work very well though it doesn't have the built-in expander gate.

Gating

Gating is a way of making sure that the silences between passages of music or even between notes are truly silent. This is achieved by switching off low level signals altogether. Simple switching however would be too obtrusive, so all studio gates have attack and release controls which the user can set to complement the signal being processed. The trick is to set the release time such that the end of the note (or its reverb) is not cut off abruptly as it dies away and that is fairly easy to do, but a lot of users still don't really understand what the variable attack time is for. Well, if a gate always switched on instantly, the first cycle of a sound may be cut in half and the resulting waveform would cause a slight click. Whilst this goes unnoticed on drum sounds, it would ruin a synth string sound with a slow attack and so a slower attack time must be set. Conversely a slow attack time on the gate would rob a drum sound of its impact so the skill here comes in choosing the best settings for particular sounds. Percussive sounds that have had too much reverb added can be tamed a little using either type of gate to speed up the decay at the end of each beat, thus effectively shortening the reverb.

Again, once you know what you're looking for, setting up by ear becomes fairly straightforward, though gates are seldom useful on complete mixes (except for cleaning up the very start) as there are usually no periods of true silence other than at the beginning and at the end.

In the section on dynamic filtering I mentioned expander gates. These differ from conventional gates in that they don't switch off when the input falls below a preset threshold level but merely turn down the gain. The more the input falls below this threshold, the more the gain is turned down so although the overall effect and operation is similar to that of a gate, the action is less obtrusive. The expander gate in the Symetrix 511 only has a threshold control, the attack and release time is fixed. In practice this works well and combined with dynamic filtering it creates a powerful tool for cleaning up sounds.

Drums

One engineer's idea of a good drum sound may be another's idea of a cardboard box and so you will probably find that you want to change the drum sound quite drastically. If the kit is still on separate tracks you're in with a chance but if it's all mixed onto one or two tracks, all you can do is to use a psychoacoustic enhancer to restore some of the missing crispness and perhaps tweak the EQ a little. As a bonus, enhancement will also increase the apparent level and brightness of any cymbals in the mix which often helps. However, if the bass drum and snare drum are still on separate tracks, you can get up to some interesting tricks.

If the sound is too bad to salvage you can replace it with a sampled sound providing you can get the drum sound on tape to trigger your sampler. One way of doing this is to use one of the noise gates that outputs a trigger pulse when the gate is open, and this is effected by setting it up to open in the usual way when the beat comes along and then using its trigger output to drive a sampler or even a digital drum machine voice via a pad-to-MIDI converter such as a Roland Pad 8. If you have an old analogue monosynth, you may be able to use this to create some interesting bass and snare sounds and trigger these instead.

It's also possible to take the sound straight off tape and feed it into the pad input of an electronic drum kit such as one of the Simmons range. Depending on the off-tape levels, you may find the use of a pre-amp necessary to get the system to trigger properly.

If, however, the sound is not too far away from what you want, you may be able to salvage it without replacement. A noise gate will let you tighten up any drum sound and it has the added bonus of letting you lose any noise caused by applying excessive EQ which you may need to employ in order to tailor the sound to your liking. If you can fit the gate after any EQ device you have patched in, so much the better on this score. Indeed, noise generated in the EQ may actually complement the drum sound; it can add to the effect of impact and will be gated out when the beat ends. Indeed you could mix in a little noise on purpose, say from a synth, to add a bit more bite to a snare drum.

Again the old faithful psychoacoustic enhancer will work wonders with muddy or dull bass and snare drum sounds and if you add noise by over zealous use, that won't matter much as you can gate it out as before.

Guitars

Another area where subjectivity reigns supreme is in the area of guitar sounds. Of course the first line of defence is EQ but what can you try next? If a guitar is overdistorted there is little you can do about it but if it's too dry, you could play the whole guitar track back through a good guitar amp and mic up the results. Any noise penalty incurred could be dealt with using a dynamic noise filter, though you may get away with using a standard gate if the noise problem isn't serious.

An acoustic guitar is a different problem and a badly miked one may sound both boomy and lacking in top. You can put back the top easily enough using an enhancer again but what of the boominess? Well the best bet is to use a parametric EQ and start by applying full boost, then sweeping the frequency control until the booming is at its most obnoxious. When this point has been found, turn the gain control to cut and then experiment with the bandwidth for the best tonal quality. A good compressor may help to even up the sound but this will of course bring up any residual noise so you would be well advised to connect a dynamic noise filter directly after it.

Bass guitar may be treated similarly but it's more difficult to hide noise due to the fact that the high frequency content of a bass is usually far less than that of a standard guitar. More punch can be added to a dull bass by using a compressor with a long attack time to emphasise the leading transients and sloppy playing can be tightened up by gating the bass track using a gate keyed to the bass drum track. In this way, bass guitar will only ever appear when played simultaneously with the bass drum.

Vocals

Problems here include popping, sibilance and fluctuating levels; out of tune singing I can't help you with though it has been known for people to correct the odd flat note using a harmoniser.

Popping can be improved by patching an equaliser into the side chain of a compressor and boosting the bass end. This will cause the compressor to reduce the gain in the presence of a very bassy sound and so it will attenuate pops quite effectively, pops having a very substantial low frequency content. Attack and release times need to be fairly short but some fine tuning will be necessary for the best result. Likewise, if the EQ is set to boost the high end, sibilant sounds will turn down the gain in much the same way and that is how most de-essers work. It's possible to do both jobs at once if your equaliser allows you to set up the desired response and a dual sweep or parametric unit would be most appropriate.

Wandering levels can also be corrected using a compressor but you may need to take the usual steps to curtail unwanted noise as any hiss present in the original recording will be increased by the same number of dBs as you are compressing by. Clarity of diction or presence can be improved using an enhancer but again, any noise penalty thus incurred must be taken into consideration. Gating can be effective on vocals but you must take great care to avoid cutting out bits of wanted material and a gate with a side chain filter might help here, especially if the vocal track contains any spill from other instruments.

Summary

From what you've read so far you have probably inferred that it is easier to do the job properly in the first place than it is to patch it up later, but there are inevitably occasions on which a valuable old recording must be retrieved. By applying some or all of the techniques outlined here, you should be able to get the feel for what is and what is not possible and I think you will be surprised at just what you can achieve with a little tenacity.

Finally, there must be lots of readers who have already developed salvage techniques of their own so please write in and we'll pass them on; the exchange of information and ideas can only be to the benefit of everyone.