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The challenging problems stated here are not based upon clipping functions of singular frequencies. Modern clipping methods, with
distortion management, reduce clipping side effects over a preset range and only up to a specified level. It appears modern recording
techniques either overload the present distortion mechanisms, or they cannot process this content aggressively without generating this
frying/sizzling-like distortion. Since this problem exhibits itself with full range linear recordings, data reduced content (mp3 audio files) is
even more distorted. This question becomes, where’s the rub?
When additional audio is added to a fundamental, lots of fun activity occurs! Sum and difference frequencies are created along with
another component known as intermodulation, or IMD for short. Simply stated, this is where one signal will ride alongside, on top of, or
modulate another. Sometimes this is done for specific effect. Music synthesizers use various intermodulating functions to create
desired sounds.
In an audio processor, the dynamic action of compressors and limiters are examples of modulators, as they generate a level controlling
signal to change the gain of the audio. The level controlling signal and audio is routed to a multiplier function, and the audio is multiplied
by the controlling signal. Through this action, the level is dynamically adjusted. This is an example of intermodulation, as the audio is
modulated by the control function. When the control signal starts to operate too fast, it generates a controlling rate with an additional
frequency of its own. This operating frequency will possess additional harmonics and those get factored (multiplied) into the audio
during the multiplication stage. The resultant contains the level adjusted audio along with harmonics from the controlling signal that
were intermodulated into the final product. This is what happens when the control signal operates in an overly aggressive manner: the
sonic quality becomes fuzzy, dull, and lifeless. We refer to this as dynamic intermodulation distortion.
With the above example in mind, let’s consider what happens within a clipper, when multiple audio signals are present and clipping is
applied. A clipper, in reality, is a zero-attack/zero-release time limiter operating with a ratio of infinity-to-one. When multiple frequencies
are present and clipping is active, the lower fundamental frequency will push the higher fundamental frequency into, and out of, the
clipper at the rate of the lower frequency. This is known as clipper induced IMD. An easy example of this would be music with deep
defined bass and a solo guitar or vocal. When clipping is active, the guitar or vocal will warble at the rate of the bass frequency due to
the action of the bass signal pushing the guitar/vocal signal in and out of the clipper. Some audio processors employ bass processing
techniques to reduce - and in some cases - remove this annoyance. On account of this, IMD components are amplified in level and
spectra. Even modern distortion cancelling clippers (or whatever other marketing name is given to them) generate IMD.
Up until now, it’s been an accepted notion that clipper induced IMD was a by-product of deep bass and enhanced
midrange/presence/treble content. When studying the example of the Kelly Clarkson track, it became evident the problem was related
to clipper induced IMD, except the example does not possess any bass spectrum of any significance.
WHAT HAPPENS WHEN ADDITIONAL SPECTRA IS ADDED?
Notice in Figure-3, a segment taken from the Kelly Clarkson
track, the dominance of signal centered at 500 Hz, and the
range between 10 kHz – 15 kHz. Wonder what happens if some
IMD tests were run on present clipping systems?