Mixing and Mastering
You've already been mixing - adding effects to tracks, adjusting faders with relation to one another, and so forth. Mastering is the final touch that helps make all tracks on a CD sound uniformly clean and balanced. Mastering includes frequency analysis to boost some parts of the spectrum that may be getting swamped, and notch out frequency spikes. It's an art unto itself. In fact, top musicians with years of studio experience tend to leave mastering to professionals. It can't hurt to at least understand what mastering is and how it differs from mixing, though. For the purposes of most people reading this book, an excellent mix may not require much mastering work, if any.
Earlier, I suggested normalizing a track that had a small waveform. Now I will take that back because normalizing tends to leave little to no "headroom" - the track is now very loud, so loud that if you apply compression, it will clip. Clipping is when the level meter goes into the red, and can result in horrid distortion on playback. Compression narrows the difference between loud and soft sounds in a track, hence allowing more room to increase the gain before the track clips (because the peak volume is lowered). This leveling has the effect of adding crystal clarity and presence to just about everything. Once you apply it, you may find yourself wanting to trim back those faders below zero Db! But that's a good thing, because it shows the signal-to-noise ratio is improved.
You will also see limiters in the list of audio processors - these do essentially the same thing as compressors, only with a high ratio and faster attack time. Sometimes applying a low- or high-pass filter can quickly eliminate frequencies you just know are irrelevant and possibly muddying a track - think piccolo or bass drum. While the timbre of some sounds such as the human voice can rely on harmonics over a broad frequency range, for other sounds a filter might be the right tool, especially if there is a glaring artifact showing up. These processors can be applied to individual tracks and/or to the whole song. In the Mixer, click on that blank region above the fader of any track or the Master track and select plugins/plugin manager. There you will see dozens of pre-installed effects and processors, many of which are frankly redundant. Notice the ability to favorite - checking that off will add the current effect to a short list under 'favorite' plugins the next time you add effects. I recommend starting with Calf Analyzer to get a view of levels across the spectrum. You can then deploy an equalizer or other tools to tweak those levels, but some caution is warranted. If you just go by your ears, you may be deceived by whatever monitors you are using, especially headphones where one is tempted to overcompensate the low end. A conventional piece of advice is to play back over a few different speaker systems. Also, trust the 'scope! The best way to understand mastering is to see it done live. Here is a great video on the topic from YouTube channel Mastering Monday, that includes a list of gorgeous free processors to try out: https://www.youtube.com/watch?v=FzNweEPg-2U
These did not all run on my system, but I liked Bluecat Audio's FreqAnalyst, and Tokyo Dawn Productions' Slick EQ, which includes saturation, a way to add a bit of warmth to digital recordings. Having tried out a few of these tools, my favorites list ended up with mostly standard Calf plugins. They come with an intuitive GUI and get the job done.
Mastering is an art, and getting the mix just right on a complex recording can be vexing. Really doing the subject justice is beyond the scope of this book. Please check the library, online resources, and the electronic music section of your local bookstore for more information on this very deep topic.