Mixing could be the subject of a whole book in it's own right. Unfortunately though, space is limited, so here are a few guidelines and thought provoking observations on the subject.

Creating music, particularly pop music, is not unlike pictorial art, in that artists' approaches can range from the emulation of real life to a completely unnatural style that sometimes appears to break all the rules. Picasso did much the same with his portraits. Whereas the painter relies on his palette of colours and his painting techniques, the producer uses the mixing console and effects to shape and direct what has already been recorded onto tape. This phase of the production is of paramount importance as the producer brings together several independent musical elements that may originally have been separated by space as well as time and blends them into a cohesive whole. Quite how the sounds are mixed and treated depends on the result that you want to achieve but the processes can usually be divided into two groups: those that are necessary as a result of the physical limitations of the recording equipment or the performers, and those that are applied to create a specific creative effect. I'll call these two areas corrective and creative, though they can overlap as we shall see later.

Corrective Measures

By corrective measures, I mean the type of processing that has to be applied to minimise noise or crosstalk, control dynamic range and to correct for tonal inadequacies created somewhere along the line.

The first line of defence here is the basic noise gate but there are more sophisticated expanders and dynamic noise filters on the market, so that there's no shortage of weaponry in the fight against noise. Where possible, gating is best applied on a track by track basis rather than on a complete mix or submix because a gate is only of use if there are gaps in the music where the gate can be allowed to close. When the signal is present, so is the noise so there's no point in having the gate open at the start of a mix only to stay open to the end as you gain nothing except for a quieter start. Gating a single track however, such as a vocal track for example, effectively mutes the tape noise between words and cuts out any crosstalk that might be present on the track as an added bonus. However, you have to set the gate's controls carefully to avoid losing any wanted portion of the sound. The threshold and release controls are most vital in this respect and you'll always have more success gating a dry sound and then adding delay effects to the gated signal than trying to gate a signal containing a significant proportion of reverb or echo. Apart from the obvious problem of making sure the reverb tail doesn't get chopped off, adding an effect such as echo or reverb to the gated sound may well hide any premature cut-offs caused by imperfect gating techniques or by awkward signal dynamics. As a rule, expanders are a little more forgiving when it comes to the setting up, but are still best used on a track by track basis. However, dynamic noise filtering can be applied to a complete mix with minimal side effects providing that you are careful not to apply more than the bare minimum of processing needed to improve the noise performance to an acceptable level. (For more on this, see 'Making the Most of a Bad Job' in H&SR September 86.) Used on individual tracks, these devices work exceedingly well as they're almost undetectable in operation but it's rarely cost-effective for the small studio owner to own more than a single 2-channel unit.

Naturally prevention is better than cure; it's always worth minimising noise when recording by keeping an eye on levels and planning track bounces carefully. As we've covered the use of gating quite thoroughly over the past year, I shan't go into further detail at this point.

Dynamic Control

Now the noise problems have been dealt with, what next? Dynamic control is a complicated way of saying 'controlling levels'. Obviously you have control over levels using the sliders on the mixing desk but what I'm referring to here is the use of compressor/limiters to tame those sounds which vary too much in level to be usable as they stand and which do so in such an irregular manner that correction using the console faders becomes difficult, if not impossible.

A professional studio might well use a bank of compressors to control several tracks within a mix, but again the small studio is limited by more stringent monetary restrictions. From experience, I've found that the vocalist who doesn't need some degree of compression is very rare, though you can get away without compressing most other parts of a mix if they are carefully recorded and well played. It's possible to get around the problem of having only one compressor by applying some compression to the vocal track as it is recorded. This has the advantage of giving you a higher than average signal level on tape and consequently a better noise performance. However, you can also compress the signal coming off tape just as long as you bear in mind that every dB of compression is a dB of degradation in the signal to noise ratio of that particular track. Again though, this isn't too much of a problem if you keep the amount of compression as low as possible commensurate with doing the job in hand. Many units now feature built-in expander gates which are a great help in cutting out any noise brought up by the compressor action.

Lastly, a few words about compressors' user-variable parameters and their implications. You will probably be familiar with the term compression ratio, but in this instance it doesn't refer to your new Cortina, it's simply a way of expressing the change of output level in dBs for a given change in input level. A compression ratio of 2:1 then means that a 2dB change in input only causes a 1dB change in the output level. The higher the ratio, the more severe the compression and the less effect any change in input level has. A really high ratio may be called limiting, as the maximum output level becomes virtually independent of the input level, and you have in effect a fixed maximum output signal level. For normal applications such as levelling vocals or bass guitar, low ratios between 1½:1 and 4:1 are generally adequate and the lower the ratio, the less obtrusive will be the processing.

Apart from the ratio control, there are also the attack, release and threshold control settings to consider but these are simple enough to choose, as long as you understand what they are actually doing. The easiest way to explain the action of these controls is to look at what used to happen before compressors were invented, ie. gain riding. This was the process whereby the engineer had to manually change the gain during a mix using only the mixer's faders. As soon as the engineer hears that a sound is getting too loud he turns the gain down. Of course he will be too late to correct the first part of the sound due to the fact that he can't respond instantly, so some of the loud sound gets through before it is brought under control. This delay in responding is analogous to the attack parameter on our compressor. After all, it too takes time to respond to a rise in level but is obviously much faster than any engineer. With a fast attack time set up, there's very little overshoot because the compressor reacts very quickly, but there are occasions when it is actually desirable to have a slower attack time to lend emphasis to the leading edge of a sound. In this case the attack control is set up by ear. Typical examples of this kind of application would be adding impact to bass drum sounds or bass guitars.

The release time is the time it takes for the compressor to stop applying gain reduction once the input signal level has fallen below the threshold. If the release is set too fast, then the signal seems to pump, but if it is set too long, the compressor will not have time to recover and will apply gain reduction to following low-level sounds that don't need it. If you're in doubt as to what setting to choose, set a fast attack time and a release time of half a second or so and start from there.

On a gate, the threshold is the level above which no change to the signal takes place; for the compressor it's just the opposite. A signal below the threshold passes through unchanged but once it exceeds the threshold, the compressor starts to apply gain reduction. When compressing vocals, I tend to set the threshold so that gain reduction is only just starting to occur on average level signals, and most compressors have a readout of some sort, showing the gain reduction to help you. With this method low level signals remain unchanged, but the louder sections are controlled, which in turn allows you to use a higher average level in the mix so that quieter sections don't get lost.

I don't want to dwell on the use of compressors too much as, like gates, their use has been covered in depth in previous issues, but I make no excuses for restating the basics as this subject constitutes a significantly well-defined area of doubt and uncertainty if our readers' letters are anything to go by.

Tonal Correction

Tonal correction is mostly self-explanatory and you'll be familiar with using the EQ controls on your own mixer for this, but what should you do if these aren't enough to cope? Well, you can patch in an external equaliser which gives you the choice of graphic or parametric equalisation and either will let you home in more accurately on that section of the audio band that needs treatment. The parametric offers more precise control but isn't as easy to use as a graphic. The choice is yours. If you need to cut a frequency because of an unpleasant colouration in the sound, the best way to find it when using a parametric or sweep equaliser is often to set the equaliser to maximum boost and then home in on the offending area using the frequency control before applying cut. This is far easier than setting the equaliser to cut and then vainly hunting for the area that needs cutting. The graphic is easier to use because the slider positions give a good indication of what is going on but, if you don't have a graphic with a lot of bands, the frequency you want to affect might fall in between two bands where you can't tweak it without affecting frequencies to either side. However, the graphic does give you the chance to simultaneously equalise different parts of the audio spectrum.

There are occasions where EQ alone can't help because it can only affect the sound that already exists. So what do you do when you are faced with a muddy sound that refuses to be brightened? Regular readers will know the answer to this one straightaway. That's right, the trusty psychoacoustic enhancer. This wonderous device generates additional upper harmonics to complement whatever input signal is fed into it and this technique actually synthesises a new top end rather than trying to boost one that doesn't exist. The most common models on the market are the Aphex Aural Exciter and the Scintillator, though there are budget units available from Access it and Tantek.

Whether you use enhancement or EQ, adding top invariably means emphasising any noise present in the recording so you may need to use gates or a dynamic noise filter to keep things clean if the recording is particularly noisy in the first place. Equalisation and enhancement should hold no surprises for regular readers but the next subject on the list does require a little explanation, judging by the questions we've received on the subject.

Effect Connection

It's all very well having loads of outboard toys but where do they fit into the scheme of things when it comes to connecting them up? Exactly what plugs into where?

As far as connecting up goes, there are two basic types of signal processor; those that treat and alter the whole signal and those that create an effected version of the input intended to be mixed back with the dry signal. In the first category we have effects like gates, compressors, equalisers, enhancers and dynamic noise filters. Whatever is fed into them is processed and the output then replaces the original signal.

The second category comprises effects like echo units, reverbs and so on which generate an effect that needs to be added to the dry sound before it sounds acceptable. Some of these units contain a balance control so that some dry sound may be mixed into the output but, when used in conjunction with a mixer, it is normal to set these so that only the effected sound appears at the output. This way, the right amount of effected sound can be added to the dry sound using the mixer controls.

Any piece of equipment from the first category should be connected via the mixer's insert points and should not be connected to the auxiliary or effects send circuit. If you have no insert points, then you can unplug a tape return from the mixer and feed it into the processor; the output of the processor would then feed the mixer input. If you want to effect the entire stereo output, then you must connect a stereo or 2-channel processor to the master insert points or between the mixer and your master recorder if this latter option is not provided on your desk. When using a 2-channel unit to process a stereo signal, use the stereo link switch (if one is fitted) so that both channels track properly. This is particularly important when using compressors, gates or dynamic noise filters as the control circuitry needs to be fed from a mix of the two input channels if serious image shifts are not to result.

If you do have to patch in between the mixer and the mastering machine, remember to monitor the 2-track output rather than the mixer's left/right outputs, otherwise you won't know what effect your processing is having. Also, if you're using anything other than EQ, this last arrangement can make fading difficult. For example, if you are using a level sensitive device such as a compressor on the whole mix, this will try to turn up the gain as you are turning it down. You will win in the end but the fade won't sound right. If you are forced into this situation, you can avoid the problem by doing the fade using the level controls on your mastering machine rather then the faders on your desk.

Auxiliaries

The second category of processors is generally connected via the auxiliary (or echo) send controls on your desk because in this way, several channels can share the same effects unit without being restricted to the same amount of effect. However, if you have already used all your effects send facilities, you can patch in say a single DDL to a specific channel via the insert points and use the DDLs own mix control to set the dry/effect balance. This limits you to using that effect on only one channel (or subgroup) but then that may be all you need.

Echo send controls are known as post fade auxiliaries because they're connected internally at some point after the channel fader in the signal path, so when the channel gain is reduced, the send to the effect unit is also reduced. Therefore turning a channel down will also vary the level of the added effect in exact proportion. And if you turn the channel right off, the effect will be turned off too.

The other type of auxiliary control that you'll almost certainly have on your desk is the foldback or pre-fade auxiliary. This is independent of the channel fader and it's important to bear this in mind if it's used to drive an effects unit. If you have a reverb connected here for example, you can set a perfect balance between the dry and the effected signal but if you then fade the dry signal using the channel fader, the effect level will remain the same as it was before. So it's apparent this method of connection should only be used if you're not going to change the levels much within a mix or if you actually want to fade the dry sound leaving only the effect present as a special production trick.

You also have to take care with effects when subgrouping. Take a situation where you have several channels, each with effects added via the auxiliaries, all mixed down to one or two subgroups. You'll find that unless your effects returns are routed to the same subgroup or subgroups, you will get the same problem; turning down the subgroup level using the subgroup faders will not affect the level of any added effects. Conversely, you may want to use a specific effects unit on some channels that are subgrouped and some that aren't, so you can't simply route the effects returns to those subgroup faders. So, you have a problem, and one to which there is no easy answer. At the end of the day, life's a compromise. However, if you can anticipate these difficulties before they arise, you can often plan around them.

Now, let's look at effects returns, or auxiliary returns as they are sometimes known. These are often considered to be something separate from the rest of the mixer, but in reality they are just simplified input channels; you can plug line inputs into them and use them as channels or you can plug effects into channels and use them as returns. The only real difference is that the auxiliary returns only work at line level and their facilities are far simpler than those found on a main input channel. If however you do use an input channel as an effects return, make sure that the auxiliary sends on that channel are turned right down or you'll create a feedback loop and the whole thing is liable to shriek at you.

Well, so far we haven't touched on the applications of creative effects in the context of mixing. That will have to wait for part two of this series, which will be featured next month.