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Quick understanding of THD and THD+N
Although most acoustic engineers hear THD and THD+N every day, they have never understood the similarities and differences between THD and THD+N. Even some engineers who understand these two concepts find it difficult to express them briefly.
Xiao M has put a lot of thought into this article recently, and finally finished it. He hopes to help everyone better understand these two concepts from three aspects: definition, calculation formula, and listening experience. I hope it can be helpful to everyone. I also hope that friends who like it will help forward it and give it a thumbs up!
1.1 Definition of THD
THD (Total Harmonic Distortion) refers to the percentage of the energy of the total harmonics of a signal to the energy of the fundamental wave. The harmonic frequency is the frequency point where the frequency is an integer multiple of the fundamental wave, as shown in the power spectrum below:
1.2 Causes of THD
Let's take the power amplifier as an example. When it is working, due to the inevitable oscillation or other resonance of the circuit, the second harmonic (V2), third harmonic (V3), fourth harmonic (V4), etc. are generated. The output signal is not just a component that is exactly the same as the input (V1), but a signal that includes harmonic components (V2, V3, V4...). The ratio of these excess harmonic components to the actual input signal is the total harmonic distortion.
1.3 Calculation of THD
THD is expressed by the voltage amplitude ratio, and the harmonic energy amplitude TD is:
Internationally, there are two THD calculation standards, the IEC and IEEE standards. In fact, the only difference is the denominator. IEC considers both fundamental wave energy and harmonic wave energy as denominators, while IEEE only considers fundamental wave energy as the denominator.
IEC standard, the ratio of each harmonic amplitude to the fundamental wave plus harmonic amplitude
IEEE standard, the ratio of each harmonic amplitude to the fundamental amplitude
Therefore, when the harmonic energy is very small, the IEC and IEEE results are similar, but when the harmonic energy is very large, the IEC will be significantly smaller than the IEEE calculated value.
In addition, the order of distortion is also very important. In many cases, distortion greater than the 6th order can be almost ignored. Therefore, some software calculates THD based on the 2nd to 5th order by default.
Next time, if you want to communicate with colleagues or customers about the THD indicator, in order to be more rigorous, you can clearly explain what standard you are using and which THD orders. AudioExpert supports the selection of standards and THD orders. Users can select orders and combine them according to the product structure characteristics.
2.1 Definition of THD+N
THD+N is total harmonic distortion + noise (THD+Noise)
2.2 Causes of THD+N
From two aspects, THD and Noise
* THD (described above)
* Noise, from equipment noise or interference noise
Equipment noise - caused by the operation of equipment components, circuits, etc.
Interference noise - caused by interference from other signals in the environment or equipment
The THD+N measurement includes all the unwanted sounds mentioned above. If any of these two items are too large, it will be reflected in the THD+N.
2.3 Calculation of THD+N
Let's take the power amplifier as an example. First, send a single-frequency sinusoidal signal to the power amplifier, and get a voltage amplitude V1 at the output end. At this time, there will be some unwanted sound components in the spectrum (such as background noise and harmonics in the figure below).
If we can add an ultra-narrowband notch filter to the output signal to filter out the baseband component, so that the baseband signal is almost completely filtered out and can be ignored, we will get a voltage amplitude V2 at the output end (as shown below).
At this time, the energy ratio of V2 and V1 we get is the measured THD+N parameter
3. THD and THD+N auditory experience
After reading the above introduction and formula, if you still feel confused, that's right, it means you are a normal person. Normal people are not sensitive to formulas, but very sensitive to sounds. So, next, let's try the auditory experience brought to us by different THD and THD+N signals. (It is recommended to wear headphones and turn down the volume first, and then turn it up according to personal preference)
Video download link: http://www.megasig.com/Sequence 01.mov
The first segment is a perfect 500 Hz signal with relatively small THD and THD+N. The power spectrum and real-time AudioExpert distortion test data are shown in the figure below.
The second section is also a 500Hz signal, but it has a larger THD signal, that is, a large harmonic signal. The power spectrum and test data are shown in the following figure:
The third section is a 500Hz signal with a relatively large noise floor. The power spectrum and test data are shown in the figure below. The noise floor energy is greater than -100dB, and we can hear obvious noise.
summary
THD and THD+N are essentially distortion, which refers to the deviation of the signal from the original signal during transmission. If our products are distorted, we will hear sounds that should not exist. If you listen carefully to pop music in the 1980s, you will find that the background noise is relatively high, which may be limited by the THD+N index of the recording equipment at that time.
AudioExpert has complete and accurate test methods for THD and THD+N. Among them, the electroacoustic signal test of THD+N has higher requirements for instruments. MegaSign has instrument configuration with THD+N index lower than -105dB, which can meet the electroacoustic test requirements of almost all consumer electronic products.
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