—The Gain Stage—
Let’s start with the gain stage. Any time an audio signal is passed from one device to another, there is a possibility of a change in the overall volume of the signal. This can happen because of the physical differences between different components, or because of limitations of a device’s ability to reproduce the signal in all of its integrity. But, for whatever reason, it becomes necessary to include a control set for volume changes within each device so that the proper volume of signal can be reached. A classic example is the gain knob on a guitar amplifier. Another example would be the throw faders on a mixer’s tracks. In the cases where a signal is passed to a device and then altered by the use of a control, this is called a gain stage. There are a few important concepts to be discussed here!
First: is the gain. What does the word actually mean? To change the volume of the signal, an amplification of the original signal must be introduced. This means that an addition or subtraction of the original signal takes place. If we were to measure the input signal and compare it to the output signal after the new amplification stage has been applied, there is a simple way to know if the volume has gone up or down. This is called the gain. If the original signal has been brought up in volume by 3db then it is referred to as a +3db gain on the signal. If, however, the signal goes the other way, then we measure in negative db. (The letters, ‘db’ mean decibel which is an audio measurement created by Alexander Graham Bell.)
Second: is the word nominal. If you look at a mixer’s track and read the numbers that are placed along the side of the track, you might notice that zero is placed in the upper middle section of the fader’s control and the numbers go up or down from there. This reflects the measurement system of gain stage that we just discussed. If the fader stays at zero, then no change has been made to the input signal and we are at a nominal gain. Furthermore, a movement up or down can be measured by the numbers placed here.
Third: is signal loss. You may also see an infinity symbol as the lowest number on the fader’s scale. This is a reminder of the signal loss factor which is brought on by a loss in gain. If a signal is lowered too much, it can no longer be passed to another track or device, because the amplification needed to bring it back up would not be sufficient and the signal would be lost.
Fourth: is distortion. If we were to pass a signal which has been severely altered, then we face the problems which can arise from doing so. Let’s say that a signal is too soft in the first place. This commonly happens when recording with microphones. Now we want to use the gain stage to change the volume. The mix engineer may use the track’s fader or, perhaps a separate control such as the track’s trim knob, to raise the gain by a significant amount. There is a phenomenon of signal degradation which takes place when the audio of a track runs too hot. This is called distortion. While there are plenty of places where distortion may be a desirable effect, it is most usually avoided, especially in the example given. When the input signal is above the nominal level, which can be read by the signal load indicators included on every mixer, then the signal usually gets clipped. A wave is unable to pass through without the tops of the crests being cut off and the audible result is noise.
NOTE: DISTORTION IN THE DIGITAL DOMAIN AND DISTORTION IN AN ANALOG SYSTEM BOTH INCLUDE CLIPPING BUT THE RESULTS ARE VERY DIFFERENT. THE SOUND OF DIGITAL CLIPPING IS HARSHER AND GENERALLY UNDESIRED.