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Stem Mastering - Part 2: Selecting and Exporting Stems

Stem Mastering - Part 2: Choosing and Exporting Stems - Alessandro Fois' Blog


Article by: Alessandro Fois

Biografia di Alessandro Fois

Alessandro Fois, born in Cagliari, is a musician, composer, pianist, arranger and sound engineer. Since 2014, he has also been a writer, blogger and webmaster. After living in Oristano and Cagliari, he moved to Ivrea, where he runs Lycnos, an audio, video and web services studio, and the Glamour Recording Studio. He studied piano, music theory and composition at the Cagliari Conservatory, specialising in sound recording with Fonoprint and Sony Italia. He perfected composition and arrangement with Mogol at C.E.T.. He has explored various musical genres, including classical, jazz, blues, pop and progressive, composing for Italian artists and participating in live concerts. He released the album 'Dialogue', while his new project is 'Kaleidoskope'. He is co-author of the Sardinian-language musical 'Boghes de Domo', also directing and performing. He has written soundtracks for RAI, advertising music and for prestigious events such as those of Ferrari. For over 25 years, Fois has managed the

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NOTE: This is the 2nd part of the article: Stem Mastering - Definition and Operating Principles


Which stems

Gang of Four

One possibility, suggested by sound engineer Bob Katz and applicable to most pop-rock, funky, dance and similar genres, is the simple division into 4 stems', which he nicknamed The Gang of 4 (Gang of Four):

  • Stem 1 - Whole Mix
  • Stem 2 - Tools 
  • Stem 3 - Instruments + Choirs
  • Stem 4 - Lead Voice or Solo Instrument of the song 

Considering the possibility of using stems additively or subtractively, it will be easy to imagine the simple operations that can be achieved.

Example

Suppose, for example, that in the mix the voice sounds too loud, not enough body and too compressed.

With stem no. 4 used with reverse polarity, we can subtract a lot of volume from the voice even to the point of making it disappear.

With another track with the same stem 4, but used in straight polarity, we can feed the vocal track back into the audio programme at the desired volume, taking care to give the track more body and a hint of expansion to counteract excessive compression.

Magnificent Seven

 You will forgive the 'conceptual game' with the 'Gang of 4" in defining the choice that follows as "The Magnificent 7".

I personally believe, in fact, that in some cases it would be better to operate with a few more stems:

  • Stem 1 - Whole Mix
  • Stem 2 - All instruments
  • Stem 3 - Instruments + Choirs
  • Stem 4 - Voice or Soloist of the song
  • Stem 5 - Bass
  • Stem 6 - Bass drum
  • Stem 7 - The instrument (or group of instruments, or session of instruments) that best defines the rhythmic harmonic bearing of the song (often an acoustic or electric guitar or piano, but sometimes it can also be a synth, string quartet or other).

In this way, in addition to the stems suggested by Bob Katz, other elements will be available, including particularly critical ones that are expressed at low frequencies, such as the bass and kick drum.

By using additive and subtractive processes, we will be able to resolve many of the imbalances in the mix very well.

N.B.: In special cases, it is not excluded that the mastering operator may request a single source or a group of sources not listed above. This may occur if the source or group in question manifests a tonal or volume disproportion with the context.

Advantages of Stem Mastering

It allows ample scope for intervention by the mastering engineer in order to best resolve any mix engineer-induced distortions. In many cases where a single track mastering was planned, a Stem mastering had to be carried out in order to check and solve some flaws in the mix.

Risks of Stem Mastering

The risk is to slightly distort the overall 'flavour' of the original mix, especially if many tracks are involved. For this reason, it is advisable that it is carried out in the presence of the production manager.

How to export stems correctly

A foresighted mix engineer should take care, at the time of bouncing, to also export an appropriate number of stems. By doing so, he would protect himself against the risk of having to make revisions to the mix or even simply having to export the stems requested by the mastering engineer later.

It is remarkable that not all sound engineers know how to export stems correctly, in the absence of previous experience of this kind.

In summary, the correct export of a stem produces a file whose audio programme is perfectly identical to that of the same source immersed in the mix including reverberation fields, with no variations in volume, tone or dynamics.

N.B.: It is also possible to proceed with a reverberation-free stem, which will allow full control over the source but not over the reverberation fields. In this case, for example, a subtractive action will not eliminate source reverberation. The use of a stereo stem within the M/S matrix will finally give us the possibility of diminishing to a good extent the incidence of reverberation of the stem itself, simply by lowering the volume of the S component of the matrix.

The export procedure is very strict but simple, but it is easy to lose control of some details, making the stem almost unusable in many applications.

Basic rules

  • The tracks, whatever they are, must be exported once the mix is perfectly defined. 
  • Synchronised stems are obtained by starting and ending the export at the same point on the timeline, similarly to the complete mix. 
  • You simply have to mute any tracks that you do not wish to export.
  • This mode of operation forces any reverb or other process that has been applied to the entire mix to be reflected in the stems. In certain cases, however, the export of a stem of the reverb alone may be required.
  • In order to maintain the balance of the original mix, it is absolutely necessary that all tracks routed to the same bus are exported as a single stereo element (this recommendation is particularly important if a dynamics processor is operating in the group and an effects return is present in it.

In additive correction, but even more so in subtractive correction, the mix between the complete track and a single stem will only work satisfactorily provided that the stem corresponds 'perfectly' to the same source inserted into the mix.

In comparison to the corresponding track immersed in the mix, the stem works very well:

  • must contain the same stereo reverb (so the stem will also be stereo)
  • shall not interact via the control of a sidechain compressor
  • must have undergone the same equalisation, compression and other processes, if any, by means of the same plug-ins
  • must have the same volume
  • must be positioned in the same position in the stereo image, by means of the pan-pot
  • must also be identical with regard to the variations imposed by the automatisms programmed by the mix sound engineer, both for the volume function and for all others
  • must not be part of a group or bus in which dynamic control processes of several sources have taken place, as such processes would create a dynamic interaction with the other sources contained therein, making the stem different from the corresponding source contained in the mix; for the same reason, any dynamic intervention on the master track will also be prohibited

Reduced effectiveness of stems, especially in the subtractive field

If one or more of the above elements are not fulfilled, there may be a partial effectiveness in the use of additive techniques and a total inability to use subtractive ones.

Bear in mind that many 'coloured' filters used in a stem, especially analogue emulation filters (even a simple analogue emulation equaliser), will often not be able to guarantee a waveform perfectly identical to the previous ones at each export cycle, which may diminish the effectiveness of the process, especially in subtractive use (but not only).

Finally, the use of automations of all kinds on the track, especially volume automations, may also produce stems that do not match the 100% to the characteristics of the same source within the mix, due to differences in reactive latencies that sometimes occur in different exports.

However, even a partial but high effectiveness (e.g. the 90%) can be used successfully in most applications.

To assess effectiveness, simply add the stem to the mix, after reversing its phase polarity to 0dB Unit Gain; if the sound of the source stem disappears completely, then effectiveness will be total; if, on the other hand, a trace of the source remains, its volume will give us a measure of effectiveness.


NOTE: This is the 2nd part of the article: Stem Mastering - Definition and Operating Principles


For more on Digital Audio Mastering

https://www.alessandrofois.com/prodotto/manuale-di-audio-mastering-digitale/

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