"Distortion" is an important feature of audio components and is often listed in the specifications of speakers, amplifiers and other audio acoustic equipment. Unfortunately, this term is often misused and many important additional information is often overlooked. In this article we will help you clarify this concept and their measurement methods and correct usage.
First of all, not all distortions are bad and difficult to accept. Some distortions may be intentionally created, such as the magical "soft distortion" of a tube amp. Unlike transistor amplifiers, vacuum tube amplifier distortion is primarily an even harmonic distortion that is closely related to the fundamental frequency. The sound produced in this way is very pleasing to the human ear.
On the other hand, when you need to record voice or music and want to reproduce it as much as you can, distortion is not welcome. What is discussed below is the classification and measurement of such distortions.
Linear and nonlinear distortion
Linear distortion refers to distortion that varies in amplitude or phase but does not increase the new frequency.
The distortion when a new frequency component is excited is nonlinear distortion. Nonlinear distortion is what people usually call "distortion."
Harmonic distortion and non-harmonic distortion
Nonlinear distortion is further divided into harmonic distortion and non-harmonic distortion. All integer multiples of the fundamental frequency in harmonic distortion are taken into account. This is the classic distortion measurement called THD (Total Harmonic Distortion). In some special applications, the number of specific harmonics is considered.
Non-harmonic distortion is the case where the frequency is not a multiple of the fundamental frequency, which is the case of IMD (Intermodulation Distortion).
Harmonic distortion spectrum
THD and IMD
There are many devices that can measure THD and are easy to use. However, one of the main drawbacks of these devices is that the frequency range is very narrow. When measuring high frequency signals, the harmonic distortion will exceed the range. To measure accurate results, you need a wider frequency range test system or measure IMD.
When measuring IMD, two or more pure tones of independent frequency and amplitude are typically used as test signals. Nonlinear distortion will produce the sum and difference of the multiples of these input frequencies. The distortion will be distributed throughout the analysis band.
Measurement and calculation method
Distortion is basically expressed by the signal distortion ratio. The results are generally expressed in percentage (%) or decibel (dB). Signals can be input either through audio (circuitry) or acoustic (sound).
There are two commonly used THD calculation methods. THD IEEE, mainly used in the United States, calculates distortion and fundamental frequency ratio. The THD measurement based on the IEC method is widely used in almost all regions outside the United States, and it calculates the ratio of distortion to the sum of the original signal and distortion. In earlier measurement systems, signal noise was often added to the distortion, producing a THD+N value (N stands for noise). In any case, the analysis bandwidth must be indicated as this has a significant impact on the measurement results.
There are three main methods for IMD measurements. The MOD method uses two signals of selectable frequency and amplitude in accordance with the IEC 60268/3 standard. The intermodulation components of the second to fifth harmonics are measured. DFD (Differential Frequency Distortion) is somewhat similar to the CCIF method, except that the frequencies are very close and have the same amplitude. The DIM (Dynamic Intermodulation Distortion) method, often used in power amplifier testing, introduces additional square wave signals and modulation frequencies.
DFD (Differential Frequency Distortion) Spectrum
Which measuring instrument is selected?
Distortion measurement in a simple measurement system can only measure THD+N. For multi-channel and more complex analysis, modern measurement systems such as the FX100 audio analyzer with wideband and fast signal processing are necessary. With the FX-Control computer software, all relevant types of distortion can be measured quickly, reliably and easily according to standard methods.
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