As explained in the first part of this article, the process of audio test and measurement is fundamentally about six performance benchmarks, often referred to as 'the Big Six': Level, Frequency Response, Total Harmonic Distortion plus Noise (THD+N), Phase, Crosstalk, and Signal-to-Noise Ratio (SNR). In the first installment of this piece, we looked at the different kinds of testing required for different audio devices, explored the right signal path for connecting an analyzer to carry out test and measurement on a particular 'device under test' (or DUT — in these articles, a home theater receiver was used), and discussed the first couple of tests in detail. In this, the second and final part of this article, we'll look at the final four tests in similar detail.
As mentioned in the previous article, THD+N stands for Total Harmonic Distortion plus Noise. Harmonic distortion is the unwanted addition of new tones to the audio signal. These tones are harmonically related to the original signal: when the signal is one sine wave of frequency f1, harmonic tones are f2, f3, and so on, at integral multiples of the frequency of the original tone. Total harmonic distortion is the sum of all of the harmonics measured in the DUT’s bandwidth. Why THD+N? Why not just measure THD (the distortion) and N (the noise) individually? Well, in the early days of audio measurement, it was difficult to measure the THD by itself, without the noise, but it was relatively simple to measure the THD and the N together. So the accepted techniques handed down from years past specify THD+N, because that’s what was practical. In addition, THD+N is a convenient and telling single-number mark of performance, widely understood and accepted.
Bandwidth & THD+N
The measured THD+N of a device will vary with the measurement bandwidth. You will almost always want to restrict the measurement bandwidth using high-pass and low-pass filters, if your analyzer offers them, and you must include the bandwidth used when you state the result. THD+N is typically measured and reported in a 20 Hz-20 kHz bandwidth, which is what we'll use and refer to here.
Level & THD+N
The measured THD+N of a device will also vary with level and the frequency of the applied signal. Audio THD+N is typically measured and reported at a mid-range frequency (1 kHz or so) at the either the device’s nominal operating level or at its maximum output level (MOL).