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What Is Intermodulation Distortion (IMD)?

And why is it so hard to control

Intermodulation distortion is an amplifier or preamplifier feature that measures non-harmonic frequencies added to an input signal. It can also be present for speakers and other audio components such as CD or Blu-ray/DVD players and can be one of the most difficult music-related distortions to reduce in sound systems.

How does intermodulation distortion work?

Similar to total harmonic distortion, inter-module distortion is measured and represented as a percentage of the total output signal. And as with total harmonic distortion, lower numbers are better for improving performance.

Intermodulation Distortion can occur when two or more signals are mixed through a non-linear amplifier device. Each of the tones interact with each other to produce altered (or modulated) amplitudes. This results in the formation of frequencies (often called “sidebands”) that were not present in the original signal. These sideband frequencies are considered highly undesirable because of their non-harmonic and non-musical nature, as they arise in the summation and divergence of the original tones.

For example, let’s say the instrument plays a note and produces a fundamental frequency of 440 Hz. For the first instrument, harmonic frequencies (integer multiples of the fundamental value) occur at 880 Hz, 1220 Hz, 1760 Hz, etc. If an amplifier creates a non-harmonic frequency of 300 Hz along with the fundamental frequency of 440 Hz, a third frequency of 740 Hz is reproduced (440 Hz + 300 Hz) and 740 Hz is non-harmonic at 440 Hz. That is why it is called Intermodulation Distortion as it is between harmonic frequencies.

Why is Intermodulation Disruption Important?

Intermodulation distortion is a more meaningful measure as it is incompatible (non-harmonic). And when present, it is easier to perceive by the ear than harmonic distortion, since audio signals often contain harmonics. At lower volumes or with simpler music, intermodulation distortion may go unnoticed. Discrete tones can be heard clearly, but when the sound rises to a point where non-linearity occurs within the amplifier, unwanted frequency shift and reproduction confuses or blurs the original signal.

This effect is also compounded by more complex music genres (such as orchestra) where there is more interaction between frequencies. The result can be the creation of a noise floor that erodes the detail and fidelity of the sound. At best, Intermodulation Distortion leads to dull, muffled, or lackluster music. At worst, everything sounds vulgar or crooked.

However, like total harmonic distortion, inter-module distortion is often too low to be detected. Most modern amplifiers are designed to make intermodulation distortion negligible. Your ears are the best judges of sound quality, so don’t judge components by their Intermodulation Distortion characteristics alone.

Common questions

  • How can I reduce intermodulation in audio equipment?

    Some intermodulation is unavoidable, but use the best equipment to limit intermodulation, avoid antennas and transmitters, and use as few devices as possible for the signal path.

  • What is RF Interference?

    RF interference is a type of intermodulation disturbance between wireless devices such as phones, intercoms, computers, security systems, and smart home devices. Keep devices as far away as possible to reduce RF interference.

  • What is Passive Intermodulation?

    Passive Intermodulation (PIM) is a form of distortion that occurs when RF signals intersect at non-linear electrical junctions (such as cables, connectors, and antennas), resulting in the generation of new signals.


See more

What Is Intermodulation Distortion (IMD)?

And why it’s so difficult to control

Intermodulation Distortion is an amplifier or pre-amplifier specification that quantifies the non-harmonic frequencies added to an input signal. It can also exist for other audio components like speakers and CD or Blu-ray/DVD players, and it can be one of the most difficult music-related distortions to mitigate in audio systems.

How Intermodulation Distortion Works

Similar to Total Harmonic Distortion, Intermodulation Distortion is measured and represented as a percentage of the total output signal. And as with Total Harmonic Distortion, lower numbers are better for improved performance.

Intermodulation Distortion can occur when two or more signals are mixed through a non-linear amplifier device. Each of the tones interacts with one another, producing altered (or modulated) amplitudes. This results in the formation of frequencies (often referred to as “sidebands”) not present in the original signal. Since these sideband frequencies pop up at the sum and difference of the original tones, they are considered non-harmonic and highly undesirable due to their unmusical nature.

To illustrate, say that instrument one plays a note and produces a fundamental frequency of 440 Hz. Harmonic frequencies (integer multiples of the fundamental) for instrument one occur at 880 Hz, 1220 Hz, 1760 Hz, and so on. If an amplifier creates a non-harmonic frequency of 300 Hz along with the fundamental frequency of 440 Hz, a third frequency of 740 Hz will be reproduced (440 Hz + 300 Hz), and 740 Hz is not a harmonic of 440 Hz. Thus, it is termed Intermodulation Distortion because it is between harmonic frequencies.

Why Intermodulation Distortion Is Important

Since Intermodulation Distortion is discordant (not harmonic), it’s a more meaningful measurement. And when present, it’s easier to pick up on by ear than harmonic distortion, since harmonics are generally present in audio signals. At lower volume levels or with more simple music, Intermodulation Distortion may not be noticeable. Separate tones can be clearly heard but once volume increases to a point where non-linearity happens within the amplifier, the alteration and unwanted generation of frequencies muddles or blurs the original signal.

This effect is also compounded with more complex music genres (such as orchestra) where there is greater interaction between the frequencies. The result can be the creation of a noise floor that erodes sonic detail and precision. At best, Intermodulation Distortion leads to dull, veiled, or lifeless-sounding music. At worst, everything sounds harsh or distorted.

However, as with Total Harmonic Distortion, Intermodulation Distortion is usually so low that it is imperceptible. Most modern amplifiers are designed to make Intermodulation Distortion insignificant. Your ears are the better judge of sound quality, so don’t judge components solely by the specification for Intermodulation Distortion.

FAQ

How do I reduce intermodulation in audio gear?
Some intermodulation is inevitable, but to limit intermodulation, use the best equipment, avoid antennas and transmitters, and use the fewest number of devices possible for the signal path.

What is RF interference?
RF interference is a type of intermodulation distortion between wireless devices such as telephones, intercoms, computers, security systems, and smart home devices. To mitigate RF interference, keep devices as far apart as possible.

What is Passive Intermodulation?
Passive Intermodulation (PIM) is a form of distortion that occurs when RF signals get crossed in non-linear electrical junctions (such as cables, connectors, and antennas), resulting in the generation of new signals.

#Intermodulation #Distortion #IMD


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