Good gain structure is vital to mixing and mastering audio for electronic dance music. Without a proper understanding of the theory behind gain structure, digital productions tend to sound dull and lack depth when compared to their acoustic counterparts. Gain structure relies on a thorough understanding of signal levels and how level meters are calibrated. While different equipment and plugins may offer their own proprietary meters, it is best to rely on the same meter type across all of your channels, as different meters will often provide different readings. To understand why this is considered a best practice, we will take a moment to explain the often misunderstood unit of measurement known as the decibel (dB).
Level meters in most digital audio workstations allow us to monitor both incoming and outgoing signals in decibels (dB). The term decibel refers to a tenth of a "Bel", a unit named after Alexander Graham Bell. A common source of confusion is that the dB does not refer to a fixed signal level, but rather a ratio between two signal levels. Since dB is a ratio, 0 dB typically refers to whatever the operating voltage on our equipment is. Since voltage and gain increase together, we can rely on voltage levels for reference purposes. Some years ago, an audio signal of 75db above the noise floor would indicate a voltage level of 1 milliwatt, which would read as 0 dBm on a volume unit (VU) meter. As hardware evolved new equipment gradually required greater voltages, the milliwatt was no longer a sufficient reference level.
Using the calculation (P = V2/R (001 W = V2/600 W (V2 = 0.001 W * 600 W (V = sq (0.001 W * 600 W))) engineers determined that .775 volts RMS applied to a load of 600 ohms, would be the new standard reference level for audio equipment. This new reference level became what we now refer to as a dBu. While the dBM referred to a fixed voltage level of 1 milliwatt, the dBU referred to a ratio. Since the majority of studio equipment relied on characteristically noisy tube amplification, the voltage reference was boosted to +4 dBu, which allowed for the best signal-to-noise-ratio. Because of this, a signal of +4 dbu would be required in order for a signal to register as O on a VU meter. This reference level is what is considered the Professional Standard (balanced) today. Most professional audio equipment feature balanced inputs and outputs. Some equipment is specified at working at -10 dBv, but we won't address that reference level here, as it is reserved mostly for consumer audio.
Although these reference levels described above do not exist in the digital domain, the same conversion ratios are determined by bit depth and sample rates, which make the same theory of gain structure extremely relevant. Most audio interfaces operate at 24-bit which provides a dynamic range of 144 dB. Even more headroom is available in sequencers which operate in 32-bit floating point, as word length is increased.
Since digital audio levels are measured differently than analog levels, we use 0 dBFS in the digital domain, with the FS referring to "full scale", which is the loudest sound level that is to be digitized. If we assume that 0 dBFS is the highest audio level we can achieve before clipping occurs, it would correspond to an analog level of 24 dBu (or -20dBFS). Keep in mind that this standard may change when digital audio values are converted back to analog, as digital audio equipment often provides level selections to offset the analog output levels of 0 VU to -18 dBFS or -14 dBFS. Thus, by lowering the dBFS ratio we increase the level output from the digital-to-analog converter.
The best method of maintaining proper gain structure in the digital domain is to treat your mix as you would in the analog domain. That said, leave at least 3dB but no more than 12db of headroom. This ensures that the level will not have to be boosted excessively at any point while maintaining a good level throughout the signal path. Monitor the input level of all plugins using the same meter and make sure none of your channels are running an overloaded signal. If a signal is overloaded attenuate it via the plugin itself if possible, before adjusting it on the mixer. In extreme cases a gain plugin can usually be found in most DAWs which will help you bring the signal where it needs to be. Be sure to keep the main output channel from clipping, and adjust the channel faders, not the main output fader to bring levels down. As a last resort you can bring down the master fader, with no need to worry about degradation of signal quality. We recommend that you adjust faders on the individual channels first if they are running hot as they will likely make their way to the stereo bus overloaded, in which case no amount of attenuation on the master fader will resolve the overload.
Proper gain structure is a key component to producing, mixing, and mastering audio for electronic dance music and other genres as well. Without a proper understanding of gain structure, you will be at a disadvantage when attempting to go up against engineers who are versed in mixing on hardware. Even if you work entirely in the digital domain, proper gain staging is just good practice and will reinforce the understanding you have of your mix session. Moreover, some processors will not work properly when fed an overloaded signal. A side-chained compressor, for example, will be directly affected by an overloaded signal as its threshold control is not capable of going higher than 0 dB (in most cases). Also keep in mind that many plugins cannot handle overloaded signals and will impart distortion artifacts onto the audio. Some analog modeling plugins (such as the Waves CLA Compressors) have added noise which will decrease the signal-to-noise ratio. Without properly structured gain these plugins will raise the noise floor and cause audible problems in a mix.