When saturating a full mix it first helps to know what saturation is doing to your mix, and how the harmonics used will improve its sound. You can use saturation or analog emulation plugins, or you can combine compression and distortion to create your own effect.
Saturation as a term gets used a lot and is often conflated with distortion, but, it’s a unique phenomenon in audio engineering. It occurs when a signal is run through an electrical or magnetic component, but, that component can no longer handle the amount of electricity being run through it.
This results in a non-linear input to output ratio - in other words, compression. The compression if often gradual, or has a soft-knee, and becomes more aggressive stronger the signal.
Additionally, the process creates harmonics, which will look and sound different depending on the electrical component.
The gradual compression caused by saturation will help control the dynamic range of the signal, while the harmonics will increase the perceived loudness and shape the frequency response. Harmonics evoke a psychoacoustic response in which the fundamentals to which they relate become easier to hear.
This effect is so strong, that even if I remove the fundamental, our brains will still generate it so long as the harmonics are there.
So in short, saturation will make whatever you put it on easier to hear.
Analog emulation plugins are probably the easiest way to achieve saturation - they mimic both the dynamics processing and the harmonic generation associated with analog equipment. You probably are well aware of this but, just like with different electrical components, different emulations will result in various harmonic formations.
If you’re ever curious about this, run a sine wave through an emulator and use an EQ to measure the harmonics.
Since saturation is more or less distortion and compression, we can recreate it using separate distortion and compression plugins - this way you can really control how the 2 elements are combined. For example, I could use optical compression, with a strong second-order harmonic to make a really full sound.
Or I could combine really fast compression with an exciter for a super bright and aggressive sound.
When it comes to your mixbus, it’s best to be cautious with saturation - it can no doubt greatly improve the sound, but it can easily be overdone. Fortunately, a lot of plugins offer a wet/dry for the effect meaning you can blend in just the right amount of saturation.
If available with the plugin you’re using, I’d recommend a subtle or gentle setting as well.
Just a moment ago I mentioned using the wet/dry or mix dial when introducing saturation via your mixbus - one important thing to note is that you should use linear phase when performing this type of parallel processing. This way you avoid phase cancellation between your dry and saturated signals.
Since the saturation will add harmonics, it’ll shift the frequency response. When the shifted frequency response of your wet signal is combined with your dry, it can cause issues.
Linear phase will ensure that this cancellation won’t occur.
While we’re covering technical aspects of saturation, let’s talk about how oversampling will make your saturation sound more like analog equipment. If you’re heavily saturating, saturating your high frequencies, or both, you’ll need to use oversampling to avoid aliasing distortion - a distinctly digital form of distortion.
If harmonics caused by the saturation go above the maximum frequency that your sampling rate supports, they’ll get reflected down the spectrum, and will cause comb filtering as well as just generally sound unpleasant.
Oversampling both increases the max frequency that can be occupied, and utilizes filters to attenuate any reflections that may occur. As a result, your emulations sound more like the aliasing free analog equipment on which they’re based.
Admittedly this last chapter is much more on the technical side than the practical, but by using some tricks you can measure your total harmonic distortion. Some plugins have these meters built-in, as does some hardware, but if you’re using a plugin that doesn’t here’s how you measure it.
In short, we’re doing a null test in which we have our saturated and original or clean signal. I’ll invert the phase of one, and then play both simultaneously.
The signal that’s left will be only what’s different between the two - in this case, the saturation. Now we can measure the level, peak, RMS, LUFS, or whatever form of measurement you want, of our saturated signal and compare it to the clean signal.
Furthermore, we get to hear the delta or soloed version of our saturation.