Mastering versus mix-bus processing

It’s a murky world, this mastering.

Mastering is a process by which a mixdown is prepared for distribution. Traditionally, this has been performed by a dedicated mastering engineer with specific skills and equipment. The esoteric skills and expensive equipment gave the mastering engineer a sort of mythical status. No-one outside the mastering studio really knew what the engineer was doing, other than ‘making it sound better’, or ‘sprinkling magic dust on the record’.

Today, music technology is affordable enough that almost anyone can start creating recorded music with not much more than a computer. These recordings are often self-published online. In many cases, the audio that it heard by the listener is exactly what came from the original computer that recorded it.

Even though no dedicated mastering engineer is being used, there is still a ‘mastering process’. Sometimes this mastering process consists of little more than rendering the mixdown and encoding to MP3. Sometimes it might be an elaborate process of mix bus plugins, comparisons with other songs, advanced multi-band processing and more.

The confusion lies with the deliniation of processes. Mastering is a process performed by people. It may also include modifying the audio, using means that are sometimes used in mixing. More confusingly, these means are sometimes used in mixing, but in a processing chain that is similar to mastering (such as using plugins on the mix bus).

So, to clarify:

Mixing is a process of combining the individual elements/instruments and balancing them so they all work together. The end result is a mixdown.

Mastering is a process of taking the mixdown and preparing it for distribution. The end result is music that translates well to all the expected playback scenarios.

The mix bus is a way to apply some mastering-type processing while mixing. Not all processing on the mix bus is actually mastering though – it depends on the intent. For example:

  • Mixing into a bus compressor is not mastering when it’s done to help the different sonic elements fit together.
  • Using an EQ and limiter on the mix bus is mastering if it’s used to balance the overall tone and ‘loudness’ so that the music sounds best in a mixed playlist.
  • Using a limiter on the mix bus is not mastering if it’s used to make the sound pump in time with the kick drum.

While mixing and mastering are two different processes, the use of the mix bus makes it possible to overlap them so that mixing and mastering are both done in the same environment.

Additionally, not all mastering processes can be applied on the mix bus. Trimming and fading is usually done in an audio editor. Encoding to mp3 or burning to CD are best handled by dedicated software and hardware. Preparing a collection of songs for an album usually can’t be done in the same environment that was used to mix them.


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