Mix bus processing is an often-overlooked but very important part of the mixing process. The mix bus is where all of your sounds are combined, and it's also where you can apply a wide variety of different effects to enhance your final mix.
You can use many different mix bus processors for this purpose, but some common ones are the EQ, compressor, limiter and exciter. This blog post will show you how to set up each of these effects in your mixes!
First, set up an auxiliary track and route the output of your individual tracks to this aux (or use the sends).
The best place to start is with an equalizer. Equalization allows you to cut or boost certain frequencies that may be causing issues in your mix-down. For example, if there's too much bass then a high pass filter on the low end would help reduce it while not affecting other elements like vocals or guitars — which don't need any additional filtering at this point. Try sweeping through the ranges until you find where the problems lie, and go from there! It's easy enough to add more than you remove, so it's good to be conservative with your cuts.
Next up is a compressor! This allows you to reduce the dynamic range of certain sounds and make them more uniform throughout the mix-down — which makes everything sound much smoother and easier on the ears. You want to set this based on what works for each individual track, but generally speaking an attack time of around 50 ms will work well as long as there isn't too much bleed from other tracks (in which case go faster). A release of somewhere between 500 and 1000 ms will also do nicely here. The key factor is that every instrument benefits from having its dynamic range reduced in some way or another; if something doesn't then no amount of compression will give you a good result.
The limiter is the next processor that can be used on the mix bus, and it's generally used to keep levels under control. A good starting point is to set the threshold so that the loudest peaks in your mix are hitting-0.25dB or so; this will ensure that nothing goes over 0dBFS (the maximum level before clipping) and helps to avoid any nasty surprises further down the line. Attack and release times should be kept as fast as possible here, with an attack of around 20 ms and a release of between 50 and 100 ms being ideal for most situations.
Finally, we have the exciter! This allows you to add some extra "sparkle" to your mix-down and can be used to enhance certain elements like vocals or guitars. A gentle amount of saturation will give the most pleasant results here, so try not to overdo it! Saturation is generally applied with an input gain somewhere between-20dB and 0 dB (depending on how much you want), though this varies depending on what exciter plugin you're using.
There are many other bus processors that could also work well in a mixing context; tape simulators for example which emulate the sound of analog hardware, but these five effects cover almost everything that I consider must-have when it comes to processing audio through the mix bus. Use them wisely!