If multiplying a number by a signal scales the amplitude, adding a number to a signal performs an offset. From a waveform plot, addition shifts the value of every sample up (or down) by the same amount.
The term, “DC Offset,” is commonly used by audio engineers to describe this operation. From the days of analog circuits, there were two types of current: direct current (DC) and alternating current (AC). When direct current was applied to alternating current, the result was a signal shifted away from zero.
Generally speaking, when we play audio signals over loudspeakers, it is better for the signal to be centered around zero. Therefore, most of the time a DC Offset should be avoided. However, there are a few situations when it will be useful within the context of other processing. For educational purposes, it also helps illustrate another way the amplitude of a signal could be changed.
Next, let’s look at how audio engineers regularly think about changing the amplitude of a signal – on a decibel scale.