M/S mastering with Multipressor

Combining the Multipressor with M/S processing can add big benefits to the mastering process in Logic.

Like all multiband compressors, the Multipressor is a powerful and surgically precise tool that’s an asset to any mastering activity. By slicing the frequency spectrum into a series of manageable frequency bands, it enables you to apply compression in a more controlled and instrument-specific way, whether you’re tightening a bass line, for example, or fine-tuning the compression across the overheads to make it a little less pumpy. However, by bringing in the dimension of mid/side processing, we can make the Multipressor an even more powerful sonic tool, controlling issues that relate to dynamics, timbre and stereo dimensionality.

Technically speaking, of course, the Multipressor doesn’t come with M/S processing as standard, but with the addition of a simple freeware M/S encoding matrix and Logic’s own Direction Mixer plug-in you can start to use Multipressor in an M/S capacity. For the purposes of this tutorial we’re going to use Brainworx’s bx_solo (www.brainworx-music.de), which does the job perfectly, although there are other M/S encoder/decoder solutions available. Of course, if you like the bx_solo – and what M/S offers in general – it’s well worth taking a closer look at Brainworx’s other M/S-flavoured signal processors.

Enter the matrix

To begin, we’ll build a simple M/S matrix to transform our existing master from L/R format – where the stereo soundstage is represented by two channels covering opposite sides of the soundstage – to M/S. The advantage of M/S (and the reason that it differs from L/R stereo) is that it divides the soundstage into mid and side components – the mid channel forming the centre of the soundstage and the side channel containing the extremities of the stereo image. Although this approach might sound strange at first, it soon makes sense once you start playing with the matrix.

The initialised state of the project is our track, ready to be mastered and split across two adjacent channel strips. Now, instantiate bx_solo across each of the channels and set them to solo the mid and side channels respectively. Pan these channels hard left and right and we now have a complete two-channel M/S encoding of our original L/R mix. The M/S version sounds slightly one-sided, but don’t worry. (1)

Having separated the L/R version into M/S format to apply the Multipressor, we also need to ensure that our M/S signal is returned to stereo so we can audition it in L/R format. To do this, route the two M/S channels to a spare buss by changing the channel’s current output assignment. Over on the newly created aux fader, instantiate the Direction Mixer plug-in and set its input format to M/S. The Direction Mixer will decode the M/S signal, returning it back into L/R format. (2)

Middle men

At this stage, the M/S matrix is complete, but it’s worth adding a few additional components across the main output fader so you can monitor what’s going on. The first plug-in is the MultiMeter, which should be set to Goniometer mode. The Goniometer is a neat way of visualising the stereo image – try reducing the fader levels on the mid and side components to see what happens. Also, you’ll want to instantiate another bx_solo, this time across the main output fader, so that you can solo the mid and side channels without having to switch in and out the Direction Mixer plug-in. (3)

Now that our setup is complete we can turn our attention to the Multipressor, in this case, being used to control and refine the mid channel. The mid channel is an important part of the M/S equation, as any instrument sitting in the centre will be prominent in this channel – instruments such as bass, for example, vocals, or anything panned centrally. To hear this, engage the M Solo button on the final bx_solo and insatiate the Multipressor across the mid channel in our M/S matrix. (4)

Keep control

Despite the abundance of controls, the Multipressor is relatively straightforward to understand. Across the top you’ll find a graphical representation of the four frequency bands with their three respective crossover points. In this part of the exercise we’re attempting to direct some ‘tightening’ compression towards the low end of the mix, using the mid channel as this is where the bass currently resides. Try soloing the lowest frequency band, therefore, and then adjusting the crossover to around 170Hz so that you accurately separate the bass from the rest of the mix. (5)

Keeping the bass band solo’ed, try adjusting the Compr Thrsh and Ratio controls to engage the gain reduction. The ratio is about right (although you could go as high as 4:1 if you want harder compression), so you’ll need to lower the threshold to yield about 3–6dB of gain reduction. You can see the gain reduction movements in the top half of the display, with a blue bar graph ducking down in response to movements around the threshold. (6)

Adjusting the attack and release controls as well as applying a degree of gain make-up are useful ways of refining the compression. Try softening the attack to around 20ms, for example, to hear more of the percussive energy in the low end, as well as softening the release to 100ms so that it breathes in a more sympathetic way with the track. Gain make-up will also restore the relative level of the band, but be careful not to overdo this as you might start to skew the spectral balance of the track. In this case, about 2.5dB should provide a suitable restoration of level. (7)

On the winning side

One of the real benefits of having access to the side channel is a means of controlling the stereo width of the bass end. If you want a really tight bass, accepted wisdom is that the left and right channels should be working in tandem rather than conflicting with each other. Removing bass from the side channel, therefore, doesn’t remove bass from the mix, but instead puts a distinct focus towards the centre of the mix. To explore how we can achieve this, start by instantiating another Multipressor across the side channel and solo the side channel using the instance of bx_solo across the main stereo outputs. (8)

Given that the Multipressor uses filtering to divide the sonic spectrum, we can effectively use the Gain Make-up control on the lowest band to remove (or at least severely attenuate) bass from the side channel. Set the Gain Make-up control to -20dB, therefore, and move the Crossover control to tune-out any unwanted bass. In this example, a setting of around 200Hz provides enough low-end reduction. Try bypassing the side solo and switching the Multipressor in and out to hear the subtle tightening effect it delivers. (9)

The M/S advantage

As you start to understand the differences between the mid and side channels you begin to see the potential of the Multipressor in M/S mode. If you can imagine a traditional frequency-based multiband compressor as slicing the mix horizontally (working up through the frequency bands, in other words), M/S adds an additional dimension in respect to vertical slicing – where the mix is further divided into the side and mid components. In effect, therefore, you can direct compression across eight distinct areas of the mix, which is an impressive degree of control considering that you began with just a single stereo file!

Fully understanding the potential of M/S takes time, but as one last example, let’s explore the effect of high-end enhancement in the side channel. Open the Multipressor on the side channel and turn your attention to the highest frequency band. What we want to achieve here is some high-end enhancement – reducing the dynamic range, but avoiding any excessive pumping. Dial down the ratio to a relatively soft setting (around 1.533:1) and then pull the threshold down to around -40dB so that the Multipressor starts to massage the signal levels. (10)

To retain the transient detail, try increasing the attack time to around 20ms, as well as lengthening the release to around 100ms so that the compressor responds in a more musical way. Assuming that you’ve carried this out with the side band solo’ed, try going back the stereo version of the track and experiment with differing amounts of gain make-up. Around 2–3dB would arguably retain the original signal levels, but it’s also interesting to push the output levels slightly hotter. Notice how the effect isn’t entirely timbral in that it also changes the stereo dimensionality of the mix, almost making the sides of the mix more vibrant. (11)

Band master

As you can see from what we’ve explored here, the combination of M/S processing and the Multipressor is a powerful one, enabling you to deal with three primary issues of mastering – timbre, dynamics and stereo dimensionality – in a single process. While many novice engineers see a limiter as the secret weapon of mastering engineers, it’s arguably more holistic techniques like M/S multiband compression that offer the most effective means of processing and enhancing your music in its present state.

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