When choosing amplifiers, monitors and similar equipment, an inexperienced person is often guided by such indicators as power and frequency response. More savvy people are interested in the value of the coefficient of harmonic presentations. And only the most knowledgeable mention intermodulation distortion. Although their detrimental effect is the largest among all those listed. In addition, they are very difficult to measure and define.
Introduction
Initially, let's start with a definition. When a signal formed from two frequencies is applied to the input of an amplifier that does not have a very linear response, this leads to the generation of harmonics (overtones). Moreover, not only these two indicators take part in this, but also their mathematical sum and difference. This last one is called intermodulation distortion.
Smallexample
Let's say we have a signal. It consists of two frequencies - 1000 and 1100 Hz. This means that signals with a frequency of 2100 Hz (1000 + 1100) and 100 Hz (1100-1000) will also be generated at the amplifier output. And these are only derivatives of the first order harmonics!
One more example. Two frequencies are taken that differ by a fifth. Somehow 1000 Hz and 1500 Hz. In this case, the second order harmonics will be 2000 Hz and 3000 Hz, and the third - 3000 Hz and 4500 Hz. Relative to 1000 Hz, the values at 2000 Hz, 3000 Hz and 4500 Hz are octave, duodecim and none. With 1500 Hz, things are a little different. In relation to it, the harmonic of frequencies at 2000 Hz, 3000 Hz and 4500 Hz is the fourth, octave and duodecim.
It should be noted that the produced overtones of both considered frequencies correspond to the fundamental tones. However, this is not surprising given that all musical instruments produce natural harmonics when used.
What are the features of intermodulation distortion?
Their specificity lies in the fact that signals are generated, the frequencies of which are the sum and difference of overtones. It should be noted that the combinations produced do not always correlate with the values of the main indicators. Moreover, with a complex spectral distribution of the results, this not only does not lead to an enrichment of the harmonic structure (as is possible with low-order overtones), but also begins toresemble the usual addition of noise.
This is especially true when creating or reproducing a complex musical signal. Measurement of intermodulation distortion implies an attempt to determine the degree of non-linearity of the system. For example, in loudspeakers, similar effects arise due to different values of elasticity of the movable diffuser system. This also applies to the behavior of magnetic fields under different excitation conditions. Incidentally, the loudspeaker is a good example of a system that exhibits unbalanced behavior at different volume levels.
Actually, this leads to the appearance of nonlinear phenomena at the acoustic output from it. If the loudspeaker were a system with symmetrical behavior, then there would be no possible prerequisites for intermodulation distortion to occur. From this, by the way, it turns out that if there is a harmonic at the output of the system, then there must always be a certain non-linearity.
What intermediate conclusion can be drawn from this?
Summarizing the above, it should be noted that harmonic distortion does not demonstrate the occurrence of processes leading to non-musical systems. Moreover, a direct comparison of various devices by this parameter can lead to significant misconceptions about the quality of the generated signals.
One very telling example is intermodulation distortion in amplifiers. There, many believe that tube ones have a better sound than transistor ones. Although the latter generate an order of magnitude less distortion.
Aboutmeasurement and distortion
It is already clear that intermodulation distortion is a problem - real and hidden. If the task is to reduce it, then for this you have to strain and work, having previously studied it. Good results were achieved by the Russian electro-acoustician Alexander Voishvillo. His works are recommended for study by anyone who wants to expand their own knowledge in this area. First of all, it should be noted that distortions appear depending on the generated frequency.
In this case, exceeding the threshold level is fixed. This is observed in those cases when intermodulation distortions of the third order, as well as the second, are fixed. At any given frequency, the level of harmonics can be found by subtracting the distortion from the level of the response, which is observed in the axial direction.
What are the methods for measuring intermodulation distortion?
Theories of connection and probability, as well as mathematical statistics, are used as a basis. They are supplemented by spectral analysis, methods for approximating nonlinear characteristics, and computer simulation of multipath diagrams. If we talk about more specific solutions, then these are:
- Computer-based method for analyzing and calculating the spectrum of the output signal with the approximation of transfer characteristics using Bessel functions. It is characterized by high accuracy, which ranges from 0.1 to 0.2dB.
- Group of numerical-analytical methods for modeling multipath diagrams. Due to their novelty, they have not become widespread, but their viability has been confirmed by experimental studies.
- Using an array of parameters and models of parasitic and main lobes of polar and spectral radiation patterns. This is widely used with satellite communication systems that provide area service.
These are not all methods for measuring intermodulation distortion. The radio path can be characterized by the presence of specific features that must be taken into account both when conducting work and when solving the problem of reducing the influence.
Practical protection solutions
There is no single universal answer to this challenge. Therefore, refer to:
- Hardware-software corrector of transfer characteristics. It allows you to increase the efficiency by 10-15%, while reducing energy consumption by 15-20%. In addition, the system bandwidth is increased by 5%.
- Algorithms and programs of theoretical calculation, allowing to control the Raman spectrum and spurious radiation. They make it possible to achieve an increase in the efficiency of transmission paths by the same 10-15%, reducing energy consumption by 15-20%.
- Using a computer-based method for analyzing the combination spectrum using approximation by Bessel functions. This solution allows you to calculate theoretical indicators, control and reduceparasitic emissions in functioning systems.
And a number of others. Something specific is selected depending on what goals are being pursued, as well as focusing on current problems.
A little about practical work
How to listen to intermodulation distortion in order to react to it? Why measure them at all? It should be noted that this is not such an easy task as it might seem at first glance. The magnitude of intermodulation distortion values depends on the frequency range of the signal, its absolute level, complexity, the ratio between the peak and average value, on the waveform, the interaction between the mentioned factors and a number of other reasons. Therefore, it is difficult to measure values. After all, there are processes where some frequencies affect the generation of others. And the number of variations, purely theoretically, can approach infinity.
An important role in the assessment is played by the coefficient of intermodulation distortion. It is an indicator of the ongoing harmonic distortion of the amplifier. The intermodulation distortion factor is used to show how much of the main signal is made up of additional generations. It is believed that the value of this indicator cannot exceed 1%. The smaller it is, the greater the fidelity of the sound is characterized by the source. High-end amplifiers boast ratios that are hundredths of a percent or even less.
Not only single sources
The occurrence of distortion is not limited to onepoint of their formation. Certain problems arise when trying to catch signals. This is how intermodulation distortion appears in receivers. This is especially true for various radio equipment. After all, it is very relevant for it to reduce the level of the useful signal, as well as the deterioration of its ratio with noise. It should be noted that powerful interference can even interfere with work on adjacent signals. In this case, they speak of the presence of crosstalk.
This phenomenon occurs when the signal and radio interference do not match the frequencies of the main and similar channels. What is the nature of this phenomenon? Crosstalk manifests itself as a particular result of the interaction of the spectral components of the modulated interference and the useful signal on the nonlinearities of the receiver. Distinction deteriorates, and in case of significant problems, normal reception becomes impossible.
Remember important moments
Intermodulation distortion tends to turn into modulated noise. To understand the essence of the phenomenon, it is enough to imagine situations when someone wants to listen to a good music system at home, and outside the window there is a person who is fully wielding a chainsaw for its intended purpose. The noise level will depend on the spectral density and loudness of the music.
Although it should be noted that there is no direct relationship in this case. In the presence of intermodulation distortion, insight and clarity of sound will be lost. At low signal levels, detail is lost, and also lostcharacteristic lightness. This is especially problematic for brass bands and choirs. If a person is used to listening to them live, then when trying to hear the same songs through the loudspeaker, you can be very disappointed.
This is because when everything is mixed and played through two speakers, the distortion becomes very obvious. Whereas if you place objects at different points in space, then the number of problems will be an order of magnitude smaller.
Interesting research
I would like to mention the research results that can be obtained by the multifrequency method. There is an essence that several signals are passed through the system at the same time, which have a different tone. In this case, the frequencies are selected based on the fact that to ensure the maximum separation of the intermodulation components. This allows you to more accurately understand the problem area.
The multi-frequency method made it possible to find out that in many cases the total amount of recorded intermodulation distortion exceeds the total value of the nonlinear distortion factor by four times. From this a simple conclusion is drawn. Namely, what is often considered to be harmonic distortion, in fact, to a greater extent consists of phenomena of an intermodulation nature. In this case, it is very easy to explain why the value of the coefficient does not correlate well with the real sound, which is perceived by ear.
Conclusion
That's basically all you need to know about intermodulation distortion to the average person. It should be noted that this topic is very broad and covers many areas, even space! But the large amount of knowledge that you can get acquainted with will be of interest only to specialized specialists who are engaged in serious research and research.
