Frequency Dependent Limiting
An important feature of high-end limiters is that they engage the gain-reduction in a frequency-dependent way. Consider the following:
This is a 100Hz test tone at -6dB peak that has two "pulses" riding atop the base wave. The first pulse is a single-sample, high-frequency spike that peaks at 0dB; the second is a wider pulse of 100Hz tone that also peaks at 0dB. If this sample is processed through a limiter with a threshold setting of -6dB (or +6dB on some limiters), it will attempt to bring all material up to 0dB. In order to give both pulses “breathing room” and to avoid clipping, the area surrounding the pulse must be lowered in level. This can be done quickly on the leading edge (attack), but should be done more slowly on the trailing edge (release).
Here is the result of processing the sample through an arguably inferior Limiter.
The majority of the sample is now peaking at 0dB. The holes, or dents, in the waveform are the result of gain-reduction intended to make room for the two pulses. Note that the high-frequency spike and the bass pulse have created the same pattern of gain-reduction or denting. Cheaper limiters often exhibit this behavior. In the majority of cases where the source material has a mix of high and low frequencies, the bass is unnecessarily reduced by the gain-reduction, because to the algorithm, "a peak is a peak". On the other hand, if the release time is set too short, bass material will distort as though it were run through a vacuum tube. The solution is a type of program-dependent release that uses frequency-dependent gain-reduction.
Now take a look at the Concrete Limiter output:
In the diagram above it is evident that the high-frequency spike hasn't imposed much on the 100Hz base tone; however the release time on the low-frequency pulse is appropriately longer. This preserves the integrity of the bass signal. In the Concrete Limiter, the amount of auto-release, for frequencies between the two extremes, is continuously variable making it a much more sophisticated design overall.