A common approach in stereo-widening plugins is detuning or a short delay applied across the entire spectrum. One setting changes the upper detail, the midrange and the bass. That is convenient as long as the same processing suits the whole sound.
But try widening a harmonically rich bass synth: you may want to spread the top further while keeping the low end focused. A global knob forces you to find a compromise. At that point, what matters is not only how you create width, but also which part of the sound you can apply it to.
Detune and Haas create different kinds of width
Microdetuning creates a small pitch difference between the channels. The sound becomes fuller, with movement that recalls the differences between two performances. It is not a replacement for real double tracking, but a way to bring a pad, guitar or backing vocal to life.
The Haas effect uses a different kind of difference: time. One channel is delayed slightly relative to the other. The notes do not need to change pitch, but the perceived width and position of the sound change. Sometimes this better suits a part that does not need any extra detuning.
With full-band processing, both techniques affect the entire selected signal. Yet low notes, the body of an instrument and its airy top do not necessarily need the same detuning and delay.
Separate band settings instead of a single compromise
Some plugins let you separate these jobs. For example, WideEngine by PhotonDSP [64-bit VST3] offers five frequency bands with independent Detune and Haas, a choice of the side to process, and their overall blend in each band. You can also pan frequency bands between the channels, which is another way to create width.
With that same bass synth, you can leave the low end free of these effects, widen the mids a little and give the upper harmonics more movement. With backing vocals, you can keep the body of the voice calmer and work mainly on the upper detail.
The difference from a full-band approach is not just the number of knobs. You no longer have to reduce an effect that works well at the top because it is too obvious at the bottom. You can use detuning in one region, Haas in another, and neither in a third.
Pitch and time are not the only ways to create width
When detuning changes the intonation too obviously and a delay pulls the sound too far to one side, try other psychoacoustic techniques. Channels can differ in their harmonics, reflections or phase.
Harmonic colour. Different types of saturation on the left and right create differences in overtones, such as even and odd harmonics. For instance, combining a more symmetrical tape colour with an asymmetrical tube colour can add not only width but also density.
Comb Filtering. The idea is to remove different frequencies from one channel while leaving them in the other, and vice versa. The differences between the channels create a spatial colour. With stronger settings, it can become deliberately hollow or metallic, but careful adjustment can produce remarkable results.
Micro reflections. Short early reflections with different timings on the left and right add a sense of space around an instrument. This is an option when you want more dimension but a long reverb tail would overload the arrangement.
Phase decorrelation. Different chains of all-pass filters change the phase relationships between the channels without conventional detuning. This can create a wider texture without introducing a musical interval between the left and right signals.
These four techniques are collected in WideEngine's Advanced Spatial section [64-bit VST3]. It is a separate section after the per-band Detune and Haas. Its filter lets you keep the low end out of the added spatial signal without cutting the bass from the main signal.
You can also work only on the existing stereo difference
With a pad that is already wide, changing the side component while keeping the centre stable may be enough. This is where Side, the difference between the left and right channels, is useful.
In WideEngine [64-bit VST3], Side processing is available for the per-band Detune/Haas and separately for Advanced Spatial. If the channels are identical to begin with, first create differences in All mode: there is no point in processing a Side signal that does not exist.
After adjusting the settings, listen to the mix in mono and at a comparable volume. If the body of the instrument disappears or its attack weakens, reduce the relevant effect. Being able to combine different methods is about choosing the right character, not turning up every control at once.