Mastering gain structure

Posted: 27th February 2012 by Mezzanine Floor Studios in Live sound, Mixing Techniques

Gain structure is at once one of the easiest concepts for people to grasp and one of the hardest for folks to master.

At it’s core, it is simple. Proper gain structure is about two inter-related things:

  1. Avoiding distortion, crackling, bad sound from overloading a component in the mixer/daw
  2. Giving the engineer “headroom” on each fader/control, so they can make minor volume boosts and full range volume cuts with each fader

There are two things that make practical applications of gain structure complicated-

  1. Routing (both submixes and auxiliary channels) – where the signal that comes into each channel goes
  2. Signal processing – what each signal is processed with

On a mixing board proper gain structure includes many components: the actual signal coming in, the trim level on the mixer channel, the EQ and any FX that are inserted, the individual channel faders, sub-mix faders, aux returns, the master fader, and the amplifiers that run the PA system. The components are very similar in most modern DAW software as well.

To put each component in context and make sense of proper gain structure, let’s start with an analogy. Think of a mixer like a series of streams that form fromĀ  snow melt in the Rocky Mountains. Each stream would eventually flow into a river that would eventually flow into the Mississippi river, which finally empties in to the Gulf of Mexico. In the analogy, each channel on a mixer (and the signal that it controls) would be like one of the springs. The Mississippi would be like the master output on the mixer, which feeds sound out to the audience. In-between the main river that feeds into the ocean there would be confluences- places where multiple streams come together to form one stream, where multiple rivers come together to form the main river that flows into the ocean. There forks in some of the rivers, where the water would end up following two different paths to eventually meet again and flow into the Mississippi. The same snow would also lead to snow melting on the West side of the Rockies, which would go through a similar process and eventually empty into the Pacific Ocean ( in our case, think of the Auxiliary sends that are often used for monitor mixes. The Pacific and the Gulf both have a common source of fresh water feeding into them, just like the monitors and the main outputs on a mixer are fed by the channels.)

The trim knob on a mixer is a bit like sunshine causing the snow to melt faster, which makes each of the initial streams flow with more water. In many cases this is good, allowing for crops to grow, water for drinking, cleaning, etc. However, if too much snow melts too quickly the streams fed by snow melt will flood and overflow their banks. This flooding will cause silt and dirt to enter into the stream, making the water dirty and muddy. This is similar to what happens to a channel if you turn the trim up too high- you get distortion or “dirt” polluting the sound that was clean and pleasing just moments before. While there are a number of ways in nature that such dirt or pollution might be filtered out before the water reaches the ocean, mixing boards are not so forgiving. Distortion that enters a channel on a mixer will reach the output of the mixer until that channel is muted or the cause of the distortion is identified and removed.

With a mixing board each channel also has a fader. It may be useful to think of this like a dam in our analogy, except that the fader can actually increase the volume of sound above and beyond the signal that it received. A dam can’t do that. Needless to say, though, the relationship between the trim and the fader fits pretty well into the analogy. The goal is to have the right level of sound feeding to the audience, similar to having the right amount of water flow out of a dam. If there is too much sound reaching the audience, the engineer could choose to turn down the fader, which makes a lot of sense. If the engineer later turns up the trim, however, they could cause “flooding” on that channel.

When multiple streams come together into one, the amount of water they collectively flow into the Mississippi could be controlled by another dam on this smaller river. This is analogous to a sub-mix fader (although, again, a fader can output more signal than it is given, unlike a dam.) Think closely about the implications of having a dam on this river. It is fed by a lot of individual streams, and if enough of them flood individually they could cause flooding on this river as well.

Lastly, think about what it would be like to have a dam on the Mississippi river. If this existed it would be very similar to the Master fader on a mixer.

With this picture clearly in our minds, let’s start to examine the implications.

The best practice for setting levels for a single channel with a trim and a fader is as follows:

  1. Turn down the trim on each channel all the way
  2. Set the fader(s) at unity
  3. Bring up the trim up until the volume coming out of each channel is at a balanced level, with each individual channel providing a strong level with plenty of headroom

This ensures that the signal is not too loud at the fader (and causing distortion) because the trim is too loud.

A few key principles:

  1. The trim should never be significantly louder than the fader
  2. Channel faders should not be louder than submix faders
  3. Submix faders should not be louder than the master fader
  4. If you need to turn anything down a lot, use the earliest control in the flow. Trim, fader, submix fader, master fader.
  5. If you need to turn anything up a lot, think twice. If the fader is below unity, turn it up. If the fader is at unity and you have a lot of room on the trim, turn up the trim. If you don’t have headroom on the trim or fader, turn other channels down to balance the mix instead of turning up what you want to hear
  6. Whenever possible, be sure to utilize buss or sub-mix faders on your mixer, as this makes it easy to make the vocals louder than the band by turning down your drum submix and band submix, rather than turning up the vocals if you don’t have enough headroom.

At the heart of it, that’s proper gain structure. Once you’ve mastered an understanding of routing and gain structure together, you’re well on your way to being a solid front-of-house engineer.

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