Stem Mastering
While conventional mastering involves fine-tuning and processing of the final two-track mixdown, stem mastering focuses on the processing of the component stems (or mix groups if you prefer) of the mixdown/song. For example, the different instrument groups of a mix such as drums, bass, guitar, keyboards and vocals. Stem mastering gives the mastering engineer much more freedom of choice in the processing stage of the mastering and the ability to fine tune your work in greater detail.
The Benefits of Stem Mastering
Consider the following scenario: you have a mix you are generally happy with, where the instruments are well balanced but the lead vocal is a little dull in the high frequency range compared to the instrumentation of the track. With conventional stereo mastering increasing the high frequencies in the track will give the intended enhancement to the vocal. But by doing that all the other elements of the mix will be affected since the process is applied to the whole mix. The problem with conventional mastering in this instance is that the instruments within the mix that are already bright enough are likely to end up sounding too bright (too much energy in the high frequency range) which may be unpleasant to the ear of the listener. For instance the cymbals from the drum kit might become too dominant or the electric guitars might become too fierce. It is in situations like this where stem mastering can really make a difference. Other instances can include situations where the level of an instrument, such as drums or lead vocal might sound ok in the mixdown but due to a requested “loud” master with a low dynamic range, they become submerged deeper into the rest of the instrumentation. With stem groups it is possible to restore the original balance of the mix by adjusting the volume of the component parts.
How to choose the Stems
So instead of going into the mastering stage with just a final mixdown of your song(s), you can provide the instrument group channels for each mix, so that the mastering engineer can shape the tonal and spectral balance of the mix in finer detail. Stem mastering can often be simple. For example you can include as little as two stems, one for the vocals and the second being the instrumental. This way you can maintain the same settings on the instrumentation while the vocals can be processed on their own.
If you want the mastering engineer to have more of a creative input or greater flexibility during the mastering stage you can divide your mix into a greater number of stems. This is particularly useful if you are unsure about certain mix elements or the overall balance of the mix. With stem mastering you can have anything from two stem groups, as discussed earlier, up to eight or more groups. For example the main components of a mix with a standard rock band might be as follows: 1) drums 2) bass 3) guitars and 4) guitar overdubs/solos 5) lead vocals 6) backing vocals. This way the engineer is able to go into more detail, fine tuning the relationship between the bass and the drums, while the guitars and vocals are treated separately.
I would advise for you to get in contact with your chosen mastering engineer to discuss the project in detail and decide upon the number of stem groups that are likely to be required. The mastering engineer will need to hear the stereo mixdown in order to advise you.
Preparing the Stems
Preparing the mix stems is relatively easy. All you need to do is to mix your track as you would usually and when completed create a separate bounce/recording of the groups of instruments you are going to send over for mastering. Also provide the full mix version with the stems as a reference. As is always advisable make sure that you are not exceeding 0 dB (clipping) on the final mixdown or on any of the individual stem groups. Individual processing you have used for the groups should be left on to retain the character you want on the individual elements of your mix, for instance if your drums are going through a compressor or limiter.
As with conventional mastering the same principle applies with stem mastering. We prefer to work with mixes without any additional processing on the master/stereo buss. However, if you have mixed into a compressor, limiter or EQ, rather than inserting it after the mix levels are completed you might find that when you take it off the mix loses its shape. If this is the case, then please send two versions of the stems, one with the master buss processing and one without, so that we can choose the version which will work best for the mastering.
Careful not to overdo it
One thing to bear in mind is that stem mastering should not replace the mixing process. We advise not to provide more than eight group stems, because the aim is to give the mastering engineer greater flexibility during the mastering stage, not to re-mix the track, which is more time consuming and consequently more costly.
Conclusion
The advantages of stem mastering are self-explanatory. I hope we have covered some of the potential benefits in enough detail here to make the process clear. In the end it comes down to how confident you are with your final mixdown. If you are already happy with your final mix then you can choose conventional stereo mastering, but if you are unsure about certain elements of or the balance of the mix, or you want to give more creative freedom and more room for adjustments to the mastering engineer you can achieve that by providing stems.
Written by Spyros Stasis and Greg Chandler
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