Tuesday, May 02, 2006

Intro to Gain Structure

Today's Topic: The Basics

Gain structure is one of those slightly mysterious, absolutely critical parts of good sound reinforcement.

Perfect gain structure will lead to mostly distortionless and noiseless audio.

Improper gain structure ensures hissy, crunchy sound. Let's all say it together: Yuck!

Most of you sound people are probably familiar with setting the gain or trim knob on the mixing board. But gain structure is more than just getting that one knob in the right place. All the input and output levels of all the components of your sound system determine the system's gain structure, and therefore sound quality.

So, let's try to understand gain structure as a whole. Today, we'll examine how dynamic range and gain structure interact. In the next few weeks we'll find out how to make sure you have good gain structure between system components and on your mixing board itself.

First, I need to introduce the important term "dynamic range." Dynamic range is the difference between the lowest voltage and the highest voltage a piece of electronics can reproduce accurately. For sound equipment, then, dynamic range is the difference between the softest sound and the loudest sound that equipment can pass without distortion or noise. All pieces of audio equipment have a noise floor, which is the lowest level where the signal can be heard over the noise. They also have a clipping point, above which the signal will sound distorted.

So, the effective dynamic range of a piece of sound equipment is the difference between its clipping level and its noise floor.

Gain Structure is the alignment of the dynamic ranges of the different components in your system. Here are some drawings to demonstrate...

... Bad Gain Structure :|: The dynamic ranges do not line up.

... Good Gain Structure :|: The dynamic ranges do line up.

For the purpose of these pictures, the sound signal travels through each component from left to right. If it helps you, replace components 1-6 with "microphone, mixer, compressor, equalizer, signal processor, amplifier, speaker" or whatever your setup looks like. Also, the rectangles represent the dynamic range of each piece of equipment. The bottom of the rectangle is the level of the noise floor for that piece of equipment. The top of the rectangle is the clipping level.

Your system has good gain structure if the maximum level of the dynamic range of each piece of equipment "aligns" with the next.

The bad gain structure shown in the first picture causes several potential problems.
  • First, a loud but non-distorted signal coming out of component 2 will distort component 3. That distortion will remain in the sound throughout all the rest of the system and will be heard by the congregation.
  • Second, a moderate sound coming out of component 3 will be below the noise floor of component 4. That noise will remain in the sound throughout the rest of the system and will be heard by the congregation.

Here are two important things to notice:
  1. The maximum overall dynamic range of your system is equal to the dynamic range of the worst piece of equipment in the system. Even with the best gain structure possible, component 4 still limits the possible dynamic range of the system.
  2. If you gain structure is poor, and your dynamic ranges are not aligned, your effective dynamic range for the whole system is the difference between the lowest clipping level and the highest noise floor. In the bad case above, the effective dynamic range is the overlap of component 3 with component 4, which is significantly worse than the whole dynamic range of component 4.

What is the audible result of that poor gain structure? Your system is noisy generally and will distort at lower levels.

That's why you want to make sure you have correct gain structure throughout your system.

I'll stop there for now. Send me your questions, if this is unclear. I'll try to answer them on Friday or in the next few weeks as we explore gain structure further.

Next week we'll discuss how to adjust your gain structure between components to align everything correctly. The week after that, we'll cover how gain structure works within your mixer.
More Posts on "The Basics"

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