Last month we looked at the corrective role of signal processors and the use of insert points and auxiliary sends. Now we'll move on and examine the mix itself, beginning with ways to create the illusion of stereo.

AS I INTIMATED last month, there are no hard and fast rules when it comes to mixing. If there were, everyone would produce exactly the same result from any given multitrack tape.

But there are guidelines to get you off to a good start, and once you've put them into practice, you'll be able to decide for yourself which rules to break and which to stick with.

I'm assuming you're starting out with a four-, eight- or 16-track master multitrack tape and that few, if any, effects have been recorded onto tape. I'll assume the instrument line-up consists of drums or drum machine, guitar, bass, synths and vocals, as even in these pioneering times, this still seems to be the most popular combination. If part of your instrumental line-up is driven via a MIDI sequencer synced to tape, then the same techniques apply. My last assumption (isn't generalisation wonderful?) is that you intend your finished product to be in stereo, rather than mono or multi-channel surround-sound.

Now, a misconception exists about mixing in stereo. To many people, it simply means positioning the sounds in a mix using a pan-pot. Though this is a valid mixing technique, there are many other approaches that can work just as well without being quite so obvious.

But first a quick look at the mechanics of stereo in real life. Although most sound sources are essentially mono in that they emanate from a single point source, they may be nearer to one side of the listener than the other, giving an equivalent of the pan-pot effect. But this alone doesn't create the sense of depth and reality that we are looking for. (Our ears and brain actually react to off-axis sounds in a much more complex way than the previous couple of lines might imply, but to investigate further would mean an unacceptably lengthy, though interesting, digression.)

The sense of depth that we get from hearing a sound in a normal environment is not merely a function of its position, but is largely due to the addition of reflected sound. You may instantly think of reverberation, which is certainly a step in the right direction, but in many everyday acoustic environments, the reverberations may be too heavily damped for us to perceive.

Take, for example, your living room. Though it probably wouldn't sound very live if you clapped your hands in there, if you were able to do a direct comparison between that sound and the same handclap in a completely dead (or anechoic) room, the absence of those vital few early reflections would make a startling difference.

In recording, we tend to keep everything as dry and separate as possible. Close-miking techniques eliminate most of the room's natural reflections, and the room itself will probably be as dead as we can make it anyway. What we've done is to rob the sound of its spatial identity and record it in mono onto one track of tape. In exceptional circumstances, you might have found enough spare tracks to use two mics and record in stereo, but the chances are that the room ambience will still be lacking, as is typically the case when two mics are positioned close to a piano or drum kit.

An alternative approach is to record these instruments in stereo in a live room, but if your studio is at home, you may well find this impractical. And if you're using a drum machine instead of real drums (or indeed any DI'd signal), the sounds are likely to be very dry and lacking in natural ambience.

Having set this grim scenario, we'll now look at things we can do in a mix to recreate the missing sense of space, other than simple left/right panning.

False Ambience

A REVERB UNIT can be used to create a stereo ambience to complement a mono sound, but a simple spring unit may have too long a decay time to add ambience without adding a noticeable amount of reverb at the same time.

"The unsubtle use of flangers is rather passé nowadays, but used with imagination, they do have their more creative uses."

A better approach is to use a digital unit. These generally accept a mono input and provide two outputs to simulate the different reflection patterns reaching the ears of the listener, and most will allow you to set up a short reverb time of between 0.4 and 0.8 seconds, which will give an impression of "roominess" without much in the way of obvious reverb.

Some machines even allow you to set up an early reflection pattern without the following body of reverb, which can be useful on vocals to add presence in the spatial sense of the word, without cluttering the sound with reverb. If you want a certain amount of obvious reverb as well, then fine - just select a longer reverb time or use a spring.

Now, you may well face the dilemma of having one reverb unit and requiring it to do several jobs simultaneously. You can plan for this at the recording stage by adding a little reverb in mono to the desired tracks, and then adding an overall short stereo reverb to them all when mixing. This will reintroduce a sense of space that simply adding reverb in mono won't achieve.

Of course, you may be faced with a tape that's already recorded, or you may not have access to a digital reverb in the first place. So what can you do, apart from shake your fist at the skies? Fortunately, quite a lot.

One source of ambience that tends to get overlooked is the natural ambience of rooms themselves. Wouldn't it be nice if you could steal the ambience from, say, your bathroom and add it to your drum machine?

Well, this is how to do just that. After evicting your sister from the shower and running some extension cables up the stairs, the setup should look something like this: one of your hi-fi speakers now resides in the bathroom facing a hard, preferably tiled, wall and this is fed from your hi-fi amp which, in turn, has its aux input wired to one of the aux outputs on your desk. Turning up the drum track aux send on the desk may now elicit howls of protest from the rest of the household, as the drums come thundering forth from the bathroom.

The next step is to pick up this new "live" sound, and a little experimentation will pay dividends here. The object is to position two mics (preferably the same type) in the bathroom so that they pick up as much reflected sound as possible, and as little direct sound from the speaker as possible. A couple of PZM microphones taped to the walls are ideal for this job, due to their excellent clarity when working at a distance.

The next bit is simple. Just make sure nobody flushes the lavatory for the next half-hour, and proceed with your mix while pretending you have the latest ultra-sophisticated digital room simulator on the other end of the line, rather than a bathroom.

There are dedicated stereo simulators which you can use to widen mono sounds, and these use delays to simulate the sound bouncing off adjacent walls. Usually you only get one reflection per side, but you often have the option of sweeping the delay in a slow chorus-like manner to add movement to the sound.

"An early reflection pattern can be useful on vocals, where you want presence in the spatial sense without cluttering the sound with obvious reverb."

I've found such devices to be most effective on synthesised sounds, or the types of sound that usually benefit from a little chorus, but they don't really create ambience in the true sense of the word, and they can be expensive. The stereo simulation programs of the Alesis MIDIfex utilise a greater number of reflections and are more convincing, though the unit offers no modulation facility.

If you're limited in resources, here are a few things you can try for yourself. The following treatments work well on synth sounds, guitars and vocals, but may not give the desired effect on drums.

DIY Ambience

FIRST WE HAVE the old trick of panning a sound hard to one side, and then placing a delayed version at the other side, the delay being in the order of tens of milliseconds.

The reason this is so effective is that it simulates, to some extent, what happens when we hear a sound in real life. First we hear the direct sound, and then a short time later we hear a reflected version bouncing off a wall or floor. True, subsequent reflections complicate this state of affairs in nature, but the single delay can still be surprisingly effective.

One thing to be aware of, though, is that this kind of trick is not always mono-compatible. As students of the art will realise, adding a signal to a delayed version of itself sets up what is known as a comb filter, which is the basis of flanging and phasing. Depending on the delay you've chosen, some sounds will add to each other and some will cancel each other out. And although this is not apparent when the two sounds are panned to either side, it will show up as tonal colouration when the sounds are summed to mono.

If you have a delay unit that offers both in-phase and out-of-phase outputs simultaneously, try positioning the dry sound in the centre of the mix, with the two outputs from the delay unit panned to either side. This will give an impressive illusion of depth, and when you sum to mono the two delayed outputs will cancel, leaving you with the original mono sound. Neat.

And that's not all. If your DDL has a built-in modulation facility, as most of them do, you can use a little slow, shallow pitch-shift to add a sense of movement as well as stereo separation.

Now, if your DDL is tied up with more important work, how about using that old analogue chorus, or even the flanger pedal you have tucked away somewhere? You can perform the same trick here by feeding a dry signal to one side and a chorused version to the other, keeping the levels roughly equal. The result is a wide, three-dimensional sound that works particularly well on synths, basses and clean guitar. If you're using a flanger, turn the feedback control down to minimum and you should get a similar effect.

The unsubtle use of flangers is rather passé nowadays, but with a little imagination, they do have their more creative uses. For instance, a bassline generated by a synth, or even a bass guitar, can sometimes start to sound muffled in the context of a mix, and may tend to get lost or sound boomy. Feeding this through a flanger (set to a low speed with only shallow modulation) can give the sound a harder edge without it sounding too obviously 'flanged', and may even succeed where EQ fails. By adjusting your flanger's range control, which basically varies the delay time, you can tune the effect to some extent; this may allow you to emphasise some frequencies or cut others as needed.

"The natural ambience of rooms themselves tends to get overlooked; you can steal the ambience from your bathroom and add it to your drum machine."

There's no magic formula here: just sweep through the range until the result sounds good to you. You can then either use the new flanged sound on its own, or add a little of the dry sound to make the effect even less obvious.

As to other uses for flangers, consider this: split a signal into dry and flanged versions as before, but then feed the output of the flanger through a noise gate. Trigger the gate via its key input from a rhythmic sound such as the drum track, then set the release time for half-a-second or so and see what happens. Every time a drum beat occurs, a flanged version of the dry sound will be added to the mix, which will then fade away according to the time set by the release control of the gate.

You might even try to miss out the dry sound entirely and have, say, a burst of flanged string synth popping up only where triggered... Use both channels of the gate with the same setup but with different attack and release settings, and you could get a flanged sound that moves from left to right as it's triggered.

Has this set you thinking? I hope so, because there are endless tricks based on this arrangement which all sound as original as your own application of the technique.

Pitch Changing

UNTIL RECENTLY, I wouldn't have dwelt too long on the uses of harmonisers or pitch-shifters, simply because, to be honest about it, the good ones were too expensive for widespread home use, and the cheaper ones were not that good.

This last year has changed all that, though, and we now have the pitch-shifting section of the Yamaha SPX90 and the little Boss RPS10 pitch-shifter; both fall easily into the semi-pro price range, yet still turn out excellent performances.

The creative uses of large pitch-shifts are rather limited, as the "chipmunk" effect becomes all too obvious. But using just a fraction of a semitone of shift can fatten a sound enormously, and doesn't cause "churning" the way a chorus unit does.

The preferred technique is, once again, placing a dry sound at one side of the soundstage and a shifted version at the other. But if you have a dual-channel device like the SPX90, you can keep the dry sound in the middle and then place an upward-shifted version at one side and a downward-shifted version at the other; this really fattens things up.

This treatment is extremely effective, yet is subtle enough to be used on virtually any sound source, voice or instrument, without giving the game away.

Next month, I'll be explaining how the above techniques are actually put into practice during the mixdown.