The Lost Art of Gain Staging
Clean sound. That’s one of the marks of a professional recording, or professional sound reinforcement. “Clean” means free of noise and distortion. Getting that sound is a result of correct gain staging.
Gain staging (or gain structure) is the process of setting the gain optimally in each stage of a recording system or reinforcement system. We’ll explain how to do that so you can create some beautiful audio.
First, let’s define noise and distortion.
Every audio component produces a little noise—a rushing sound like wind in trees. Noise in a recording is undesirable unless it’s part of the music.
You can make noise less audible by keeping the signal level in a device relatively high. If the level is low, you have to turn up the listening volume in order to hear the signal well. Turning up the volume of the signal also turns up the volume of the noise, so you hear noise along with the signal. But if the signal level is high, you don’t have to turn up the listening level as much. Then the noise remains in the background.
If you turn up the signal level too high in a device or software, the signal distorts and you hear a gritty, grainy sound or clicks. This type of distortion is sometimes called “clipping” because the peaks of the signal are clipped off so they are flattened. To hear distortion, simply record a signal at a very high recording level (with the meters going well into the red area) and play it back.
Optimum Signal Level
You want the signal level high enough to cover up the noise, but low enough to avoid distortion. Every audio component works best at a certain optimum signal level, and this is usually indicated on a level meter built into the device or software.
Figure 1 shows the range of signal levels in an audio device. At the bottom is the noise floor of the device—the level of noise it produces with no signal. At the top is the distortion level—the point at which the signal distorts and sounds grungy. In between is a range in which the signal sounds clean. The idea is to maintain the signal around normal operation level on the average. Generally you want the signal level to be as high as possible without clipping or distorting.
With a digital recorder (such as computer recording software), 0 dBFS (0 decibels full scale) on the meter is the maximum undistorted signal. In that case, an average level of -20 dBFS and a peak level of -6 dBFS is a typical normal operating level. In a digital audio file that is used to master a CD, a peak level of -3 dBFS or so is a good goal.Signal-to-Noise Ratio The level difference in decibels between the signal level and the noise floor is called the “signal-to-noise ratio” or S/N (see Figure 1). The higher the S/N, the cleaner the sound. An S/N of 60 dB is fair, 70 dB is good, and 80 dB or greater is excellent. To illustrate S/N, imagine a person yelling a message over the sound of a train. The message being yelled is the signal; the noise is the train. The louder the message, or the quieter the train, the greater the S/N. And the greater the S/N, the clearer the message. Headroom
The level difference in dB between the nominal signal level and the distortion level is called “headroom” (see Figure 1). The greater the headroom, the greater the signal level the device can pass without running into distortion. If an audio device has a lot of headroom, it can pass high-level peaks without clipping them. “Peakroom” is the dB difference between signal peaks and the distortion level.
You want to set your mixer controls (software or hardware) so that the signal has some headroom, is well above the noise floor, and is below distortion. Here’s the general procedure:
Try to get the most gain at the beginning of the signal chain. Set the mic preamp gain as high as possible without causing clipping, then back off about 6 to 10 dB to allow some headroom.
Set the master fader and group faders to design center, the shaded part of fader travel about 3/4 up
Adjust the channel faders so the peak level in the master bus output reaches about -3dB.
Set the power amps or active monitors to get the loudness you want.
Let’s go over each step in detail. First, set the master faders to design center.
Set Mic Preamp Levels
We’ll start at the beginning of the signal path. Have each instrument play the loudest part of the music, one at a time or all at once. For each input signal in the mixer or audio interface, set the TRIM (mic preamp gain) control so the level is as high as possible without causing distortion. While setting levels for a digital multitracker, peak each track around −6 dBFS (−6 decibels full scale) maximum in peak meter mode, not RMS mode. That allows some headroom for surprises. Also, musicians generally play louder during a performance than during a level check. If you exceed 0 dBFS you’ll hear digital clipping, which can make a loud click.
Most audio interfaces have gain knobs to set recording levels. A few interfaces use onscreen volume controls for that purpose.
Check Pad Switches in Mics and Direct Boxes
With the mic preamp gains set, do you still hear distortion from a mic or DI box? If so, switch in its pad (if any). Be sure to NOT use pads in mics or active direct boxes unless you hear distortion. Pads reduce signal-to-noise ratio, so they should be switched in only if a musical instrument is so loud that it causes distortion in its microphone or DI box.
If you are mixing several instruments to one or two groups (as in a drum-kit submix), follow this procedure:
Monitor the group(s).
Set its submaster fader (group fader) to design center.
Set the submix balances, panning, and recording level with the input faders.
Fine-tune each submix level with the submaster fader.
As you’re mixing, try to keep the master faders at or near design center. If the master faders are set low, you will set the channel faders up high to get a good mix level. That results in high-level signals which can overload the mix bus. After your fader balances are roughed in, adjust all the channel faders by the same amount up or down so your master output level peaks around −3 dB maximum. A typical average setting for channel faders is about -6 to -12 dB. The more faders in use, the more their signals sum together. So the more channels you are using, the lower the fader settings will need to be to prevent overloading the mix bus.
Don’t exceed a 0 dBFS recording level because that will create clipping distortion. If a mix-bus meter clips, it’s okay to lower the master fader a few dB until the clipping stops.