Why did you write this post?I hear many problems in my own recordings, and have decided I should educate myself a bit on the subject. This is mainly written as a summary of things I read and remembered.
Mixing? Mastering?First things first: what's the difference between mixing and mastering? In mixing one tries to bring together different tracks into one recording. In doing so, one can apply a whole range of effects to each track separately before combining them to a complete song. I intend to discuss some of those effects in this post. In mastering one takes a mixed song and applies effects to the whole mix at once. This would be done e.g. to make all songs on an album share a similar hear and feel. A photoshop (or gimp ;) ) analogy would be that in mixing you combine clipart into a picture, and in mastering you apply effects to the picture as a whole (cropping, change colors to sepia, ...). My current investigation is mostly about mixing audio.
Mixing objectivesIn mixing audio one strives to create interest, mood, balance and definition. These four dimensions can be heavily influenced by cleverly applying effects to each of the tracks.
- Interest: is all about keeping the listener's attention by adding enough variation in the mix. Example: make the chorus sound subtly different than the verse, or make verses with dark lyrics sound darker than verses with lighter lyrics. Variation keeps the interest higher. You can also decide where to put the instruments in an (imaginary) 3d audio scene. If you pay attention to recordings, you can start to hear how certain instruments seem to sound as if they are placed on different places on a sound stage.
- Mood: how do you make the same music sound darker or lighter? More mellow or more aggressive ?
- Balance: make sure each instrument gets the space it needs and make sure that the instruments don't sound like a bunch of aliens that happen to play simultaneously. The different instruments should sound as if they belong together, and none of the instruments should overpower all of the other instruments.
- Definition: make sure enough details can be heard in each of the instruments, voices, and at the same time get rid of the unwanted details like breathing.
Mixing techniquesIn order to achieve those objectives, we can apply effects to the individual tracks that must be mixed together. Some effects will operate on the time domain (i.e. it will influence how volume changes over time, or how long certain sounds remain hearable), whereas other effects will mostly influence the frequency domain (change how a specific sound sounds, e.g. make vocals sound clearer or darker).
PhaseIn itself, the phase of a single track is pretty meaningless. Sound is made of waves, and phase says something about at which moment in time the wave reaches its peaks (low and high), and passes through zero. Phase starts to matter when you combine two or more tracks. A phase difference between two tracks is a small delay between the two tracks. Funny things can happen when you mix sounds with different phase. When you mix two tracks, you basically sum them. If you take one track, make a copy of it and apply phase inversion to the copy, then mix both tracks together you end up with no sound at all: the phase inversion causes both tracks to cancel each other out. (Indeed when the original reaches its peak, the phase-inverted copy reaches its valley and vice versa. At each moment in time the waves are each other's complement.) Of course this is an extreme example that you would never encounter in practice. But if you record the same track simultaneously with different distances to different microphones a phase difference could be present, and if it is not compensated before mixing, it might lead to unwanted (partial) cancellations of sound. Applying certain effects will also affect the phase of the track. Mixing together tracks with phase differences will result in something that sounds a bit different (usually worse) than what you expected, typically a more metallic, hollow sound. Sometimes this effect is applied on purpose: then it's called flanging. There's another related effect called phasing. Both effects suppress certain frequency components (a phenomenon called comb-filtering takes place). In flanging the frequencies that are suppressed are harmonically related, in phasing they are not. Phasing is usually a little bit more subtle than flanging. Sometimes you can play with phase to achieve interesting effects. One such effect is known as the "Haas" effect. Basically you take a track, and hard-pan it to the left speaker. Then you take a copy of that track and hard-pan it to the right speaker + let it play starting a few milliseconds after the first track. As a result you get a very spacious open sound. Try it out in your favourite audio tool. Another trick is the out-of-speakers trick where you keep the tracks time-aligned, but you invert the phase of one of the tracks. This results in sound that seems to come from all around you. Works best with low-frequency (i.e. low notes) sounds.
FadingFading is adapting the volume of your track, in order to give each instrument equal chances of being heard. To add interest to a recording, many programs allow automating volume levels, so that variations can occur throughout the song. Beware though: humans are only human, and psychoacoustics dictate that "louder" gives the impression of sounding "better". The result is often that volumes tend to be increased, to the point where they don't make sense anymore, or don't leave enough room for other instruments to be added. If volume is set too high, also clipping can occur, which results in considerable distortion (typically clicking or crackling sounds) in the end result.
PanningWith panning you can give different instruments a different spot on the virtual sound stage you are creating in your mix. Its effect is to make the sound come more from the left or from the right. When speaking about panning, one often refers to the sound as coming from a different "hour". Hard left would be 7:00, hard right would be 17:00 and right in front of you would be 12:00.
Compressors and other dynamic range processorsThe word compression has two different meanings. On one side it is used to denote the process of representing digital recordings with fewer bytes (like .mp3 is a compressed version of .wav). This is not the meaning that is used in audio mixing. In audio mixing, compression means something different: it means reducing the dynamic range, i.e. reducing the difference between the loudest and most silent parts of a track. Like that the recorded track can blend better with other tracks. Compression is often used on vocals: without it, the more silent parts of the singing risk to drown in the sounds of the other tracks. In this context: apparently in the recording industry there's an ongoing loudness war: by (ab)using compression, recordings are made to sound as loud as possible. The downside is of course that a lot of dynamic range is lost that way and the music becomes less interesting as a result. Different applications of compressors include:
- Compressor: make loud sounds more quiet (while keeping a sense of louder and more silent); keep silent sounds at the same volume
- Limiter: ensures that the volume never exceeds a given maximum. The volume of any sound that is louder than some threshold is brought back to the threshold. Volume of sound that is more silent than the threshold is kept as-is.
- Expander: making quiet sounds quieter; keep louder sounds at their original level
- Upward compressor: make quiet sounds louder, keep loud sounds at their original volume
- Upward expander: make loud sounds even louder, keep silent sounds at at their original volume
- Gate: make all signals with a volume below some threshold a lot more quiet (with a fixed amount known as the range; often: completely remove)
- Ducker: make all signals with a volume higher than some threshold a lot more quiet (with a fixed amount known as the range