Click here to purchase the entire book in PDF format. Dynamic Equalization A dynamic equalizer is one which automatically changes its frequency response according to characteristics of the signal passing through it. You won't find many single devices what fit this description, but you can create a system that behaves differently for different input signals if you add a compressor to the rack. This is easily accomplished today with digital multi-band compressors which have multiple compressors fed by what could be considered a crossover network similar to that used in loudspeakers. Dynamic enhancement Take your signal and, using filters, divide it into two bands with a crossover frequency at around 5 kHz. Compress the higher band using a fast attack and release time, and adjust the output level of the compressor so that when the signal is at a peak level, the output of the compressor summed with the lower frequency band results in a flat frequency response. When the signal level drops, the low frequency band will be reduced more than the high frequency band and a form of high-frequency enhancement will result. Dynamic Presence In order to add a sensation of ``presence'' to the signal, use the technique described in Section 6.1.5 but compress the frequency band in the 2 kHz to 5 kHz range instead of all high frequencies. De-Essing There are many instances where a close-mic technique is used to record a narrator and the result is a signal that emphasizes the sibilant material in the signal - in particular the ``s'' sound. Since the problem is due to an excess of high frequency, one option to fix the issue could be to simply roll off high frequency content using a low-pass filter or a high-frequency shelf. However, this will have the effect of dulling all other material in the speech, removing not only the ``s's'' but all brightness in the signal. The goal, therefore, is to reduce the gain of the signal when the letter ``s'' is spoken. This can be accomplished using an equalizer and a compressor with a side chain. In this case, the input signal is routed to the inputs of the equalizer and the compressor in parallel. The equalizer is set to boost high frequencies (thus making the ``s's'' even louder...) and its output is fed to the side chain input of the compressor. The compression parameters are then set so that the signal is not normally compressed, however, when the ``s'' is spoken, the higher output level from the equalizer in the side chain triggers compression on the signal. The output of the compressor has therefore been ``de-essed'' or reduced in sibilance. Although it seems counterintuitive, don't forget that, in order to reduce the level of the high frequencies in the output of the compressor, you have to increase the level of the high frequencies at the output of the equalizer in this case. Pop-reduction A similar problem to de-essing is the ``pop'' that occurs when a singer's plosive sounds (p's and b's) cause a thump at the diaphragm of the microphone. There is a resulting overload in the low frequency component of the signal that can be eliminated using the same technique described in Section 6.1.5 where the low frequencies (250 Hz and below) are boosted in the equalizer instead of the high frequency components.