The measurement of SNDR can be made by using a technique called SINAD.
The actual definition of SINAD is quite straightforward. It can be summarised as the ratio of the total signal power level (Signal + Noise + Distortion) to unwanted signal power (Noise + Distortion). Accordingly, the higher the figure for SINAD, the better the quality of the audio signal.
The SINAD figure is expressed in decibels (dB) and can be determined from the simple formula:
where:
SND = combined Signal + Noise + Distortion power level
ND = combined Noise + Distortion power level
It is worth noting that SINAD is a power ratio and not a voltage ratio for this calculation.
In the equation given by DIXIT, one measures the powers. So, one gets the average square of the whole distorted and noisy signal and then get the white noise power and the distortion power by building the average of the square of all the distortion components.