Specifications

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Performing an IMD test on a clipping system is quite easy. Two audio frequencies are mixed together, then passed through the
system under test and the output is observed on a scope and spectrum analyzer. In this instance, the clipping systems all employed
the required 15 kHz low pass filtering and zero-overshoot control mechanisms found in broadcast processors.
For the test, 100 Hz was inserted at a level, which generated 3 dB of clipping. A high frequency component was mixed in at the same
level and 75 μs pre-emphasis was applied. The tests were run over the range of 5 kHz up through 15 kHz, while 100 Hz was used as
a constant low frequency source. Figures 4 – 8 are the results of the tests.
Notice as the upper frequency is increased, there is significant difference spectra that falls between the two fundamentals. This is
extremely severe at 10 kHz, 12 kHz, and 15 kHz. If you recall the music example, this is very close to the spectral illustration in the
Kelly Clarkson track. Houston, we’ve got a problem! It is clipper induced IMD!
what’s new! /omnia /
UNDER THE MICROSCOPE
Figure-4, Clipper Induced IMD: 100 Hz & 5 kHz Figure-5, Clipper Induced IMD: 100 Hz & 7.5 kHz Figure-6, Clipper Induced IMD: 100 Hz & 10 kHz
Figure-8, Clipper Induced IMD: 100 Hz & 15 kHzFigure-7, Clipper Induced IMD: 100 Hz & 12.5 kHz
CLIPPER SYSTEMS, DISTORTION CANCELLING, AND TOO MANY BANDS!
As stated, all present day clipping systems employ methods to control distortion. Of interest is that each of these use a static method
to mask harmonic distortion when clipping is active. As the Kelly Clarkson example clearly illustrates, harmonic distortion is not the
concern as it once was. Intermodulation, due to added presence and high frequency spectra, has overtaken the problem that once
was dominated by harmonic distortion. Suffice it to say, all clipping methods must employ some form of harmonic distortion control,
or they will not operate sufficiently enough to generate competitive sounding on-air audio. Modern content now requires additional
processing means to reduce induced IMD.
Suppressing IMD is significantly more difficult, as the constantly different frequency components are a non-stop moving target.
Whereas suppressing harmonic distortion can easily be predicted and controlled through a static filtering system.
Proof of this is demonstrated with an evaluation of present day distortion cancelling systems. All of them employ static filtering to
mask distortion components. They vary in range from broadband to 5-6 band, or more. All of these fail with aggressive processing.
The broadband method suppresses harmonics and some IMD at specific frequencies. The multiband methods are designed to insert
gentle low pass filters after multiband clippers in each audio band. This works over a narrow range, but falls apart with aggressive
levels of clipping. Multiband clipper/filtering is done in parallel architecture and each singular band clipper is not able to understand
what the others are doing. Therefore, the resulting filtered harmonics of each band interact in unpredictable ways - some of which
exaggerate IMD. Adding more bands or steeper filters does not improve or fix the problem!