Intermodulation
From Wikipedia, the free encyclopedia
Intermodulation or intermod is the result of two signals of different frequencies being mixed together, forming additional signals at frequencies that are not in general at harmonic frequencies (integer multiples) of either. The largest intermodulation products appear at f1 + f2 or f1 − f2 (second-order intermodulation), and less so at 2f1 − f2 or 2f2 − f1 (third order intermodulation).
The cause for intermodulation is the existence of non-linear characteristics of the according equipment. The theoretical outcome of these nonlinearities can be calculated by conducting a Volterra series of the characteristic, while the usual approximation of those nonlinearities is obtained by conducting a Taylor series. According to the summands in those series, the above numbering of orders is counted.
Intermodulation is rarely desirable in radio, as it essentially creates spurious emissions, which can create minor to severe interference to other operations on the resulting frequency. Usually the physical element, in which intermodulation occurs, is an ‘active’ circuit, such as an amplifier, and more specifically the semiconductors and electronic valves with unintentional non-linearities in their transfer characteristics. However ‘passive’ elements, such as connectors or conductors made of two galvanically unmatched metals, can too have non-linearities in their characteristics. The intermodulation caused by such passive elements is termed as passive intermodulation but its nature and character are not different from those of the ordinary intermodulation – the distinction is due to historical reasons only. (See the Rusty bolt effect for more details).
Intermodulation may be desirable in audio if the intent is to create specific sound effects; for instance, intermodulation is the basis of the power chord technique in rock music.
Contents |
[edit] Intermodulation noise
In a transmission path or device, Intermodulation noise is noise, generated during modulation and demodulation, that results from nonlinear characteristics in the path or device. Intermodulation noise occurs when the frequency sum or difference of a particular signal, S1, interferes with the component frequency sum or difference of another signal, S2.
Someone listening to a car radio while driving close by an AM or FM radio transmission tower may hear two types of 'interference' / distortion:
- 'break-through', where the transmission from the near station overwhelms the car radio; and
- intermodulation, where another station entirely is heard.
[edit] Intermodulation distortion
Intermodulation distortion is nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals.
Harmonic components themselves are not usually considered to characterize intermodulation distortion. When the harmonics are included as part of the distortion, a statement to that effect should be made. This is usually considered Total harmonic distortion.
IMD in its most basic and most testable form shows up as presence of frequencies not in the input signal. If the sum of two pure tones is the input to the system, IMD shows up as the presence of new tones in the output whose frequencies are the sum and difference of the input tone frequencies.
[edit] Use in music production
In modern record production, it is a commonplace technique to exploit the intermodulation distortion characteristics produced by vacuum tube electronics and audio tape. For example; once a recording engineer has mixed the various tracks that make up a song into the stereo format, he may send the mix to a vacuum tube based stereo compressor and overload the vacuum tube electrical components. The resulting output will sound fuller and smoother due to the creation of second and third order harmonics.
This technique applies mostly to vacuum tube based equipment though some use electro-optical based compressors to similar effect. Solid-state or integrated-circuit based equipment is rarely used for this effect as its harmonic distortion character is not favorable.
A recording engineer may also record the mix to an audio tape format called reel to reel. In this technique, the engineer will increase the level at which the mix is recorded to audio tape far past the level recommended by the tape's manufacturer. This will result in a slight compressing of the dynamic (volume) range and the production of several second and third order harmonics.
[edit] Problems in Live Audio
RF technicians and audio engineers often experience problems with intermodulation distortion when setting up wireless equipment for live performances and events. Oftentimes, wireless equipment for performer’s in-ear monitors or wireless microphones operate on similar frequencies to digital televisions signals, creating harmonic frequencies that interfere with other equipment. With security, technical crew, performance and other wireless signals in use at larger live sporting or concert events, it has become common for hundreds of individual frequencies operating in the same area. Audio engineers have to rely on complex software to calculate all of the possible overlapping and distorted frequencies when setting up such a large live event.
[edit] References
- This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.
