25 - 06 - 2018
Understanding Gain structure and applying it to our signal chain
Gain structure is giving the smoothest ride of your audio signal from one point to another. You want to a achieve a nice runway from the beginning until the end of the journey, not a rollercoaster ride where the signal is boosted at one end, and heavily reduced at another point because there is too much level coming in (for instance, having all faders of your mixing desk at the highest point of their course and your master output very low on his).
Scales and measurements
We will be addressing gain structure with digital audio units so everything will be measured in dBFS (decibels related to Full Scale ). To have a coherent gain structure you need a reference point (a value on the dBFS scale) and measuring tools (Bargraphs, Vu-meters)
Because of the nature of a guitar sound, we will not be speaking of the signal which goes from your guitar, through the pedals if you have any, to the amp. There are two reasons:
- there are no practical and reliable measurements happening while the signal goes from the guitar to the amp. There are no Vu meters or scale that show how high or how low the signal is.
- guitar sounds are usually made with a variable amount of crunches, fuzzes, distortions which in the audio world all belong to distorted signal, achieved by clipping different parts of circuits (pedals, input of the amp, etc...)
We will focus on the signal going from the amp, through a loadbox that also acts an Amp DI, to a DAW via a soundcard.
Once you have dialed the desired guitar sound on your amp, you now want to make sure that this signal will be pass through each step without being damaged (either by clipping or by added circuit noise). If we look at the front panel of a Live or a Studio, there is an input level and an output level. The amp sends your desired sound through the speaker cable to the input of the rack. Depending on the master output level of your amp, you will set the input level of the rack.
The aim is to have a proper level entering the rack with enough headroom so as to avoid clipping. With a dBFS scale, proper level is around the -20dBFS area with peaks around -12dBFS. Do all the treatment that needs to be done (mic and cab simulation, signal treatment (eq/comp/reverb) if necessary) then focus on having the right level coming out of your rack. Again, it is important to avoid clipping, so aim for the -20dBfs region with peaks around -12dBFS.
The resulting signal of the rack will now go to your soundcard where the gain can be set again. Measuring with the dedicated software or the input channels of your DAW. Again, set the inputs so you have enough headroom. Once you have matched all the levels, you can start recording.
Why care for gain structure
You could achieve the same waveform as an end result in your DAW with very different settings of gain throughout the chain. But without appropriate gain structure, you could be clipping your signal at some point. This clipping will then be carried to your recording in the DAW. Or you could have a signal too low at the beginning that is heavily boosted at the first unit. All kinds of noise will then be added to your original signal (hums, hiss, etc...). And this degraded signal will be recorded as such in your DAW. (try recording through the microphone input of a random laptop and experience the amount of noise in the recording) You can of course in the DAW set your gain so that you have headroom for both the previous situations. But your recording will be of poor quality because you did not make sure your gains were properly set at each point of the signal path. You need to make the levels appropriate to keep noise away from your signal (each preamp, circuit generates noise. It is the quality of the components and the build that makes more or less significant. This is called signal to noise ratio), and have enough headroom for your channel.