Created on 2013-02-01 00:37:00
THD+N is an abbreviation for Total Harmonic Distortion plus Noise. Total Harmonic Distortion is the sum of all the harmonic distortion products in a measurement, as opposed to selective harmonic distortion, which measures discrete harmonic products separately. THD+N adds noise to this figure.
THD+N is the most common distortion measurement for audio signals. In the APx500 meter (single value) THD+N and Stepped Sweep results, the measurement is performed in the conventional manner: a single sine wave stimulus tone is applied to the DUT input(s). In analysis, the bandwidth is usually controlled using filters; the stimulus tone is removed, and the signals that remain are reported as one measurement. These remainders are the total harmonic distortion (all harmonics within the bandwidth), noise from the DUT, and any other signals such as hum, spurious signals and other distortion products.
THD+N is an excellent and trusted quick check on several aspects of a system’s overall performance, a quick, easy and robust mark of quality. However, because of its lack of discrimination, THD+N is not always the most useful measurement for troubleshooting.
For THD+N Level, the measured distortion plus noise (the signal with the stimulus tone removed) is expressed as an rms level.
For THD+N Ratio, the rms level of the measured distortion plus noise (the signal with the stimulus tone removed) is divided by the rms level of the total signal. The result displayed on a bar meter or as a point on an XY sweep graph. THD+N ratio is most often stated in as a percentage or as a decibel value, where 0 dB represents the total signal.
THD+N is the reciprocal of SINAD.
THD is an abbreviation for Total Harmonic Distortion. As above, THD part refers to the sum of all the harmonic distortion products in a measurement. Noise is not included in a THD measurement.
THD without the noise was not easily done until the advent of FFT measurement techniques. The APx500 implementation measures only the bins that contain harmonic distortion products, producing a result that is not influenced by DUT noise or spurious interfering signals. In a very low-noise system, the THD level result will be the same as the THD+N level result.
The FFT method that provides THD measurements also brings the capability to selectively measure discrete harmonic distortion products.
In the various THD and Distortion Product ratio measurements in APx, the rms level of the measured distortion is divided by the rms level of the fundamental, unlike the THD+N ratio results discussed above, where the ratio is to the total signal. The difference is negligible for all reasonable distortion levels.