Thursday, 21 June 2018

Tech Review: Guide To Professional Audio Engineering Practices: Proper Gain Structure Demystified.

A primary focus on input channel gain structure for both digital and analogue consoles...

While I’ve written about gain structure before, I continue to run across people who don’t fully understand it. And that means one thing; I need to keep writing about it.

I actually understand why so many have a limited understanding of proper gain structure. It’s not glamorous like plug-ins or digital consoles, and there is really nothing new to discuss. However, if you’ve never gotten a good handle on how to properly set gains on your console, there is no time like the present to learn.

If your gain structure is whack, no amount of EQ, plug-ins or compression will fix it. For this post, I will focus primarily on input channel gain structure (overall system gain structure is another post, but I’ll mention it briefly).

To Hit The Preamps Hard Or Not? It has often been debated whether it is better to hit the pre-amps (gain) hard then turn down at the main output, or run the mains up around unity and dial back input gain to get the SPL you want out of the system.

As a general rule (there is an exception, which I’ll detail in a minute), I would argue the former is the correct (or at least better) method, and here’s why. Most pre-amps sound best when you hit them pretty hard—at least up to the point of clipping, which is too hard. By running your pre-amps hard—and by hard I mean within 6-12 dB below full-scale on a digital board, or within 6-12 dB of clipping on an analogue board—you’re maximizing your signal to noise ratio.

Typically, preamps just sound better when you keep the levels up. Keep in mind, that’s a general rule, your mileage may vary. Now, it’s quite possible that if you dial your input gains up so that all your pre-amps are running high, your overall system level will be too high. That’s when you would lower your main level to compensate. This method will keep the signal to noise ratio high throughout the mixing chain, and will attenuate the signal at the last possible moment.

Goals Of Proper Input Gain Structure Before we get to setting up the gain structure, let me lay out my goals in for the process.

First, I want to maximize S/N ratio. Keeping the input level high means I won’t be boosting it later, which adds noise.

Second, I like to mix with my faders around unity. Mixing with faders at unity is another key ingredient to good mixing.

The fader resolution is highest right around unity, so you can easily make small adjustments. If you try to mix with your faders at -20, a slight change in fader position might yield a 3-5 dB change rather than the 1-2 you actually desire.

Finally, I want to be sending a very solid signal out of my mixer to the processors to again maximize signal to noise.

Using All The Bits? At one point, I believed that if we were mixing in the digital world, we wanted to try to use up as many bits as possible to keep S/N high. And that may have been true at one point, but now that even inexpensive consoles are using 24, 36, 40 and higher floating point resolution internally, we effectively have unlimited dynamic range and high S/N ratios.

Modern digital consoles are very quiet and have great dynamic range. So the point of running the input gain high is really to extract the best tone out of the pre’s.

So now that we have some understanding of the goals of the process, we need to consider how to properly set up the gain on the console. This may not be the only way to accomplish it, but I’ve had great success with this method for over 20 years.

Next, I’ll explain how I approach the process.

Gain Setting In The Digital World

For each input channel, I would have the musician play their instruments at their loudest level.

I then dial up the input gain until I’m within about 8-12 dB of full scale (minus 8-12 dB on the meters). I like to leave a little room for the musician to play louder when the lights go up (they always do).

Many digital boards also have a trim (or attenuation) control in addition to the input gain. I use my trim to dial the level back to where it should be in the mix with my faders at unity. Because I’ve gained my entire system properly, my main fader is sitting at unity as well, and all is right with the world.

I’m also using VCAs to manage groups of faders (drums, guitars, keys, background vocals, etc.) and those live at unity as well, at least to start. All of this ensures that my signal to noise ratio is optimized at the A/D stage (just after the mic pre), and my starting point for my mix is faders at unity.

Now, if you don’t have a digital trim control on your board, you have a decision to make. You won’t likely be able to run the mic pre’s hard without having too much signal at some point, so you’ll need to dial the level back somewhere. Of course, you can always turn the fader down, but then you lose fader resolution.

A better alternative would be to use a VCA to keep your fader at zero. That can get tricky, however. Take a drum kit for example: If you optimize the gain on the kick, snare and hi-hat, chances are, the hats will be way too loud in the mix. But more than likely, you’re using a single VCA for the entire drum kit.

So now what? Well, you could break the drums up into zones and use one VCA for each; kick & snare, toms, hat & overheads might work. That way you can pull back the faders at the VCA level (a VCA is really an electronic remote control of the faders), and maintain fader resolution. You could do a similar trick with groups if you have them.

If you run short of VCAs, I would suggest breaking my rule and set the input gain up so that the fader remains around unity for a proper mix. Audio is a lot about compromise, and in this case I’ll give up absolute input S/N to gain faders at unity. I have found that to be the wiser trade, especially for things like cymbals.

Gain Setting In An Analog World

Really, the process is much the same, though you are much less likely to have a trim control after the gain control. In that case, the same rules apply as a digital board without a trim knob.

You still want to have good input level coming into the channel (at least as much as you can), then turn it down as needed later in the mixing stage. You also want to keep your faders running around unity. Make the trades where you have to.

In either the digital or analog world, what you don’t want to do is under-drive your mic pre’s and have to add a lot of gain down the road. Sure you can push a fader up for a guitar solo, but you don’t want to regularly run your input faders at +8, your groups at +10 and your main at +5 because your input gain is set too low.

The Exception To The Rule Now, all of this assumes you’re running on a professional grade mixer that has a mix structure with enough headroom. If you find yourself mixing on a music store brand, chances are you’ll run out of headroom in your mix bus very quickly.

If you set input gains on one of these mixers the way you should, when all those hot signals hit the mix bus, things tend to go south quickly. The buses saturate and you lose all sense of dynamics.

In that case, you need to really keep an eye on your overall output level and run input gains down accordingly. This isn’t a dig on cheap mixers—you can only expect so much for what you pay for them—it’s just reality.

That’s a quick guide to setting up your gain for an input channel. As I mentioned earlier, if you go through this whole process only to find that your overall SPL in the house system is either way too loud or way too soft, you have some work to do at the system processor or amp level. But that’s another post…

Thanks Guys and always remember to keep the creativity perfected.

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