Creating an impressive stereo panorama is one essential facet to mixing mainstream music. While you might be in the luxurious position to record in great-sounding reverberant spaces with high-end mics, many of us need to apply in-the-box trickery to help create an impactful soundscape — especially if we’re using non-acoustic sound sources.
Before we get started, let’s begin by saying that the use of such stereo-widening techniques should always be done with one ear on the mono mix; over-emphasizing your stereo field might compromise the mono sum, which is important to the solidity of playback on mass-market single-speaker playback devices, such as public address systems, portable radios, and mobile phones. This is why processing your effects channels, rather than your main voice and instrument tracks, is beneficial. So, what are some good techniques?
If your effect return is already generating a stereo output (as most people's reverb, delay, and modulation effects tend to), then the simplest way of emphasizing the width is to use a Mid-Sides plugin that lets you adjust the relative levels of the stereo signal's Mid component (i.e. its mono sum) and Sides component (i.e. what's left over if you remove the Mid Component). Voxengo's excellent freewareMSED is one such plugin. By turning up the Sides level, the width of the effect channel's stereo image increases.
Beyond the obvious appeal of wider stereo, this trick can also help when adding artificial reverb to already ambient-sounding mono recordings. By widening the reverb channel, you shift its emphasis away from the centre of the stereo image, avoiding too much overlap between the added effect and the pre-existing mono recording. Whenever you use Mid-Sides processing in this way, though, the side-effect you need to be aware of is that the more Sides level you have in the stereo mix, the more the effect will seem to disappear in mono.
Another easy way you can widen a stereo effects signal is to EQ its two sides with equal and opposite curves. In other words, by cutting on the left channel at whatever frequency you boost on the right channel, and vice versa. A logistical problem here, though, is that it works best when you use lots of EQ bands, and that's fiddly to set up using normal EQ plugins. Fortunately, you can achieve identical results much more elegantly, as follows… First send from the effects channel to a simple 8ms stereo delay, and then invert the polarity of the delay effect's left channel only. (If you don't have anything in your DAW that will let you do this, try Flux's freeware Stereo Tool plugin.)
Now, if you fade this delay effect into the mix it will cause comb-filtering — a long chain of equally-spaced frequency troughs — of your main effects return, but will do it at different frequencies for the left and right channels, thereby widening the stereo image. The beauty of it is that you can adjust the severity of the comb-filtering, and hence the strength of the widening effect, simply by tweaking the delay effect's level in the mix. One side-effect of this approach is that it'll change the perceived tone of the effect channel somewhat, both in mono and stereo. Although if the former worries you, you can always head that off by using a Mid-Sides plugin to mute the delay effect's Mid component. You'll also find that your effect channel's low frequencies may start to drift to one side if you use a lot of this comb-filtering delay in the mix, but you can easily combat that if required by high-pass filtering the delay itself.
The general concept of differentiating the left and right channels to widen a stereo effects signal can be applied in other ways too. For example, you might use a pitch-shifter to move the left channel a few cents sharp and the right channel the same number of cents flat. The bundled pitch-shifter plugin within your DAW should be perfectly capable of doing such small shifts with very few unwanted processing artifacts, even if you're processing drums and percussion (which pitch-shifters often find tough to process without generating digital 'gargles'). So there's no excuse not to try it! The potential downside with this widening method, though, is that it adds a hint of chorus-like shimmer to the mono mix because of the way the left- and right-channel pitch-shifts interact when summed.
All the widening schemes we've looked at so far are essentially static, but there are further possibilities available by using modulated processes instead. The most straightforward technique is adding a simple stereo chorus effect to the chain — try Blue Cat's freeware Chorus plugin if your DAW doesn't have one of its own. At the heart of this kind of chorus effect is a delay, whose time setting is modulated cyclically, but in opposite ways for the left and right channels. This results in a subtle vibrato effect, the speed and strength of which are set by the chorus plugin's Rate and Depth controls, and where the pitch varies upwards on the left-hand side whenever it varies downwards on the right-hand side. For stereo-widening purposes, it's best to set the Rate fairly slow (less than 1Hz) and avoid increasing the Depth control to the point where the pitch-variation becomes distracting. Although in principle this effect suffers from the same kinds of shimmering side-effects in mono as the simple stereo pitch-shifting method mentioned earlier, the time-varying nature of these side-effects often serves to make them a little less subjectively undesirable in practice.
Finally, simple auto-panning can also add stereo interest to any effect channel, although the danger is that, in its most basic form, it can feel like a bit of a 'Hey, watch me sweep things side to side!' gimmick. To make it much more generally applicable, use it as a multi-band effect to pan different frequency ranges in different ways. If your DAW doesn't have a multi-band auto-panner, you can work around that by duplicating your effects channel and then using EQ to isolate different frequency regions of each duplicate for independent panning.