From Tutorials

The Beginner’s Guide to Mid/Side Processing

Mid/side processing is a powerful technique that can be extremely useful in the right circumstance. But it can be a little difficult to get your head around and understand exactly what it’s for. Here we look at what mid/side processing is, how to achieve it and what to use it for.

What is mid/side?

Mid/side – sometimes called sum/difference – is a technique whereby a stereo recording is changed from the usual left and right to mid and side. The ‘mid’ or ‘sum’ is the mono portion of the recording (everything the left and right signals have in common) and the side is the stereo portion (everything else).

Using mid/side, we can more easily zone in on different parts of the stereo recording for EQ or compression say, or even manipuate the stereo image itself.

Manipulating M/S with free plugins

The easiest way of constructing an M/S matrix is to use a free plug-in like Voxengo’s MSED. This let’s you chain the relative gain of the mid and side signals to effect stereo width (another free plugin for this is included in BlueCat’s free gain package).

MSED will also act as an M/S encoder and decoder. Set it to encode and the left portion of the signal will become the M and the right, the S. You can then use any plugins that allow you to manipulate the L and R channels separately (but that don’t include an M/S matrix themselves) to manipulate the M/S signal, before sticking in another instance of MSED set to decode to convert it back to stereo.

How to construct a mid/side matrix without plugins

As easy as using MSED is, for additional flexibility (and understanding of what’s happening) it’s worth constructing your own M/S matrix at least once or twice.

The principle is simple – the M signal is everything common to the L and R signals, and the S is everything that is not. So, to construct your own M/S matrix:

  1. Split your stereo recording into 2 mono tracks – 1&2. Pan centre.
  2. Make duplicates of each one – 3&4. Pan centre.
  3. Send each pair to a separate group – buss 1&buss 2
  4. Invert the phase on track 4.
  5. Create 2 new buses – M&S
  6. Send buss 1 to M and buss 2 to S
  7. Pan M hard left and S hard right
  8. Invert the phase on buss S

This way you can now use any plugin that works in mono to manipulate the M&S signals separately.

Common uses of M/S processing

M/S processing is useful whenever you need to really zone in on a particular part of a stereo recording and the stems aren’t available. For example, if there’s an edgy sounding guitar at the edge of the field you can use an EQ cut on the S channel only to smooth it out.

How successful this sort of thing is depends on where instruments are panned, sometimes you just have to leave it be. Some common uses of M/S processing though are:

  • Stereo width
  • Increasing/decreasing stereo width by adjusting the relative gain (more S = wider). This has fewer artifacts than using ‘stereo widener’ plug-ins that often make use of comb filtering. Another approach is to use a high boost (8-16kHz) on the S channel to increase top end width.
  • Mono-ing bass
  • Tighten up bass by using a high pass filter at around 100Hz on the S channel.
  • De-essing
  • Use a de-esser on only the M channel (for lead vocals) or S (for backing vocals) in a complete mix.

As you can see, the sorts of problems that M/S processing can address are usually better dealt with earlier in the chain – but when that’s not an option, mid/side can be tremendously powerful.

The Beginner’s Guide to Buss Compression – Part 2 – Master Buss Compression

Last time we looked at the basics of buss compression – what it is, where to use it, and what compressor to use. This time we’ll be looking at master buss compression: a subject that flummoxes many budding engineers but that is essential for cohesive mixes.

What compressor should I use?

Compressors built for the master buss are subject to much adulation, mythology and devotion. Everybody has their own favourite so the answer really is to download as many demos as you can and try them out.

The same rules apply as for mix buss compressors (and really they’re generally the same models/plug-ins). There are the fast and ‘grabby’ models in the tradition of the SSL G series and the more gentle ‘vintage’ style.

Probably the most popular software master buss compressor at the moment is the Waves SSL G-Master Buss. But a cheaper alternative (just as credible, and preferred by many, including myself) is Cytomic’s The Glue.

Famous hardware compressors used for the master buss include API 2500, SSL G Series and Smart Research C2 in the fast and grabby vein, Manley Vari-Mu and Thermionic Culture Phoenix in the slow and gentle tradition of the infamous Fairchild 670.

Try out various software emulations and you’ll quickly get a feel for what style of compressor you prefer.

Why compress the master buss?

Master buss compression is most often cited as necessary for ‘glue’ i.e. a sense of cohesion. It’s often talked about as ‘what makes a record sound like a record’. Good use of master buss compression will give a mix definition, punch and depth.

When you first experiment, you’re listening mainly for all the parts gelling together, sounding like one piece of music rather than separate parts. As you get more confident you’ll learn to use slight pumping to introduce movement and punch – but be careful not to overdo it. As with all use of compression, too much can result in everything sounding flat and unexciting.

How do I set it up?

Needless to say, the two different styles react very differently and so require very different settings. This again is subject to much individual style. It’ll take a few mixes for you to zone in on your own personal favourites, but when you get them you can save it as a preset so it’s all set up when you start mixing. Always mix into your compressor rather than slap it on afterwards.

When mixing, I tend to set up The Glue with the following settings:

  • Threshold -14dB
  • Attack 30ms
  • Release 200ms
  • Ratio 2:1
  • Sidechain HP filter at 100Hz

I then mix so that the needle is just about pushed by the kick. If I want a gentler compression I use Stillwell Audio’s Bombardier using the following:

  • Threshold -20dB
  • Attack 40ms
  • Release 200ms
  • Ratio 2:1
  • Feedback/forward depending on the song
  • Again, there should be no more than 2-3dB of gain reduction – just enough to achieve depth and glue.

    The advantage of having a set threshold to work to is that you end up with mixes that are roughly the same level, without having to turn it down to prevent clipping. It gives you a good starting point for your kick and vocal to act as reference levels for everything else.

The Beginner’s Guide to Buss Compression – Part 1

Buss compression is one of the most effective techniques in mixing, but also one of the easiest to get wrong. This is a guide for those new to the discipline, wondering where to get started.

What is buss compression?

Simply put, buss compression is slapping a compressor over a stereo buss. Most commonly this is used on drums and on the master buss, but it can be used on anything.

The idea is to compress everything at once, making it sound more cohesive. This is why it’s particularly popular on drums where each part can sound oddly separate from the others.

What compressor should I use?

Any stereo compressor can be used although some work better than others. Buss compressors are the subject of much adulation as some impart a certain ‘magic’ that others don’t.

Try out everything you’ve got and see what works best for you. My favourite buss compressor for mix work is the Stillwell Audio Bombardier, but I’ll also often use the Fabfilter Pro-C for something a bit more transparent.

If you’re looking at analogue emulations, buss compressors usually fall into two camps: gentle and slow in the vein of classic tube compressors like the Fairchild 670 and the Manley Vari-Mu, or fast and punchy in the SSL tradition.

Basic technique

If you want to put buss compression over a group, send everything in that group to its own buss. So if you’re doing drums, route the kick, snare, hi-hat etc. to a stereo buss and stick your compressor over that.

Basic buss compression is all about using a gentle squeeze for a sense of cohesion:

  • Use fairly slow attack and release times – 30-50ms attack, 200ms release
  • Low to medium ratio – 1.5:1 to 2:1
  • Set the threshold for no more than 2-3dB of gain reduction.

This works with anything you want to add a sense of ‘togetherness’ to – drums, double tracked guitars, backing vocals, strings. Good buss compression is that mythical ‘glue’ you hear so much about.

For a drum buss feel free to go anywhere up to 6-10dB of gain reduction to achieve ‘pumping’ – when the kick triggers compression in the higher frequencies to add more movement and a real smack to your kit.

Mix into or add after?

You may need to compress individual elements of the group as well as the stereo buss. This is fine, but the best advice here is to apply the buss compression first and then work backwards. Doing it this way makes you far less likely to over-compress.

Next time – master buss compression