What is the difference between a THD and THD+Noise measurement?  When is each measurement used?

What is the difference between a THD and THD+Noise measurement?  When is each measurement used?

In older analog distortion meters, distortion is measured by using a notch filter to eliminate the fundamental signal component and measuring the RMS of the remaining signal that passes through the device-under-test (DUT). The measured signal contains both harmonic components and noise. Thus, the analog measurement method detected THD+noise.

Using digital signal processing, the measuring instrument can compute the harmonics of the fundamental and ignore non-harmonic components such as noise. Thus, DSP-based instrument can compute THD without any noise components included in the measurement.

THD gives the most direct indication of the distortion of the DUT. THD+noise gives a total indication of the distortion and the noise generated in the DUT. Noise, such as from the power line, can make the DUT's distortion appear much higher than it actually is. THD+noise also includes non-integer-related harmonics that occur from the stimulus of mechanical systems such as speaker systems. The 2015 makes both measurements.