An inexperienced person when choosing amplifiers, monitors and similar equipment is often guided by such indicators as power and amplitude-frequency characteristic. More savvy people are interested in the value of the harmonic exponent. And only the most knowledgeable mention intermodulation distortion. Although their detrimental effect is the most ambitious of all those listed. In addition, they are very difficult to measure and determine.
Introductory information
First, let's start with the definition. When a signal formed from two frequencies is fed to the input of an amplifier, which has a not very linear characteristic, 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.
A small example
Suppose we have a signal. It consists of two frequencies - 1000 and 1100 Hz. This means that the output of the amplifier will also generate signals with a frequency of 2100 Hz (1000 + 1100) and 100 Hz (1100-1000). And these are only derivatives of the first order harmonics!
One more example. Two frequencies are taken, which 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 nona. With 1500 Hz, the situation is a little different. In relation to it, the harmonic of frequencies in 2000 Hz, 3000 Hz and 4500 Hz is a quart, an octave and a duodecim.
It should be noted that the produced overtones of both frequencies considered are related to the fundamental tones. However, this is not surprising, given that all musical instruments when used create natural harmonics.
What features have intermodulation distortion?
Their specificity lies in the fact that they generate signals whose frequencies are the sum and difference of the 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 enrich the harmonic structure (as is possible with overtones of lower orders), but also begins to resemble the usual addition of noise.
This is especially true when creating or playing a complex music signal. The measurement of intermodulation distortion implies an attempt to determine the degree of nonlinearity of the system. For example, in loudspeakers, such effects occur due to different elasticities of the movable diffuser system. Also applies to the behavior of magnetic fields under different excitation conditions. By the way, a loudspeaker is a good example of a system in which asymmetrical behavior is observed at various 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 follows that if there is harmonic at the output of the system, then there must always be a certain nonlinearity.
What intermediate conclusion can be drawn from this?
Summing up the above, it should be noted that harmonic distortions do 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 regarding the quality of the generated signals.
One very revealing example is intermodulation distortion in amplifiers. There, many believe that tube ones have better sound than transistor ones. Although the latter generate an order of magnitude less distortion.
About measurement and distortion
It is already clear that intermodulation distortion is a problem - real and at the same time hidden. If the task is to reduce it, then for this you have to work hard and work, having previously studied. Good results were achieved by Russian electro-acoustics Alexander Voishvillo. His works are recommended for study to 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, the threshold level is exceeded. This is observed in cases where intermodulation distortions of the third order, as well as the second, are recorded. At any given frequency, the harmonic level can be found by subtracting the distortion from the response level, which is observed in the axial direction.
What methods for measuring intermodulation distortion exist?
The theory of communication and probabilities, as well as mathematical statistics, are used as a basis. They are supplemented by spectral analysis, approximation methods for nonlinear characteristics and computer simulation of multipath diagrams. If we talk about more specific solutions, then this:
- A computer-based method for analyzing and calculating the spectrum of the output signal with approximation of the transfer characteristics using Bessel functions. It is characterized by high accuracy, which ranges from 0.1 ... 0.2 dB.
- Group of numerical and analytical methods for modeling multipath diagrams. They did not become widespread because of the novelty, but their legal capacity was confirmed by experimental studies.
- Using an array of parameters and patterns of spurious and main lobes of polar and spectral radiation patterns. This is widely used when working with satellite communication systems for which zone service is provided.
These are not all intermodulation distortion measurement techniques. 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 influence.
Practical Security Solutions
There is no one universal answer to this call. 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, system throughput is increased by 5%.
- Algorithms and theoretical calculation programs that allow you 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%, lowering power consumption by 15-20%.
- Using a computer-based Raman spectrum analysis method using approximation by Bessel functions. This solution allows you to calculate theoretical indicators, control and reduce spurious emissions in functioning systems.
As well as 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 to react to them? Why measure them at all? It should be noted that this is not such an easy thing as it might seem at first glance. The magnitude of the values of intermodulation distortion depends on the frequency range of the signal, its absolute level, complexity, the relationship between the peak and average value, on the waveform, the interaction between the mentioned factors and a number of other reasons. Therefore, measuring values is difficult. After all, there are processes where some frequencies influence the generation of others. And the number of variations, purely theoretically, can approach infinity.
An important role in the assessment is played by the intermodulation distortion coefficient. It is an indicator of current non-linear distortion of the amplifier. The intermodulation distortion coefficient is used to display how much of the additional signal is from the main signal. It is believed that the value of this indicator cannot exceed 1%. The smaller it is, the more faithful the sound characterizes the source. High-end amplifiers boast a factor that is equal to hundredths of a percent or even less.
Not only single sources
The occurrence of distortions is not limited to one point of their formations. Certain problems arise when trying to catch signals. This is how intermodulation distortions appear in receivers. This is especially true for various radio equipment. Indeed, for her, it is very important 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 operation on adjacent signals. In this case, they talk about the presence of crosstalk.
This phenomenon occurs when the signal and the radio noise do not coincide with the frequencies of the main and similar channels. What is the nature of this phenomenon? Cross distortion appears as a particular result of the interaction of the spectral components of the modulated noise and the useful signal at the nonlinearities of the receiver. Distinguishability in this case worsens, and with significant problems normal reception becomes impossible.
Remember important points
Intermodulation distortion tends to turn into modulated noise. To understand the essence of the phenomenon, it is enough to imagine a situation when someone wants to listen to a good music system at home, and there is a person outside the window who is fully working with a chainsaw for its intended purpose. The noise level will depend on the spectral density and volume of the music.
Although it should be noted that in this case there is no direct relationship. In the presence of intermodulation distortion, insight and purity of sound will be lost. At low signal levels, detail disappears and the characteristic lightness is lost. This is especially problematic for brass bands and choirs. If a person is used to listening to them live, then when you try to hear the same compositions through a loudspeaker, you can be very disappointed.
This is due to the fact that when everything is mixed and reproduced through two speakers, distortions become very obvious. Whereas if you place objects at different points in space, the number of problems will be an order of magnitude smaller.
Interesting research
I would like to mention the results of studies that can be obtained by the multi-frequency method. The bottom line is that several signals that have a different tone are passed through the system simultaneously. In this case, the frequencies are selected based on the fact that to ensure 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 number of recorded intermodulation distortions exceeds the total value of the non-linear distortion coefficient by four times. A simple conclusion is drawn from this. Namely, what is often considered harmonic distortion, in fact, to a greater extent consists of phenomena of intermodulation nature. In this case, it is very easy to explain why the coefficient value does not correlate well with the real sound that is heard.
Conclusion
That's basically all that is necessary to know about intermodulation distortion for an ordinary 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.