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Activate Your EQ

The AE400 Active Equalizer is a significant advancement in the field of active equalization. Completely overlapping bands, positive and negative active gains, separate fixed equalizer gains, independent key filtering, side chain input option, active equalization threshold, ratio, attack and release controls per band, band control linking, and McDSP’s world famous equalization algorithms all in one plug-in. Hot dang. But what good is all that control if you don’t know how to use it? Queue the next Colin’s Corner article please.

Why Active EQ

Before going further, one may ask why you need active equalization in the first place.

Consider the typical EQ scenario – the track in question is adjusted with EQ as needed, but over the course of the performance, the audio changes timbre. Many acoustic instruments, vocalists, voice-over specialist, etc. have a different tone at different dynamic levels. The equalization settings for the quiet part of the performance may not be adequate (or flat out wrong) during the louder parts.   
Control automation is one solution – varying the equalization gain as needed. But such an approach can create more problems if the automation is not accurate enough, and the session becomes complicated. A better way is having an equalizer that can vary its gain based on incoming signal levels. Louder segments of a track can have one equalization setting, and quieter segments have another.

Active Parametric EQ

Most folks understand parametric equalization. The three main controls – Gain, Freq, and Q, are used to determine the amount of cut/boost at a selected frequency, in a width-controllable bell shaped curve. Very useful for a variety of applications in music, post-production, live sound, and so on.

Active parametric equalization, as implemented in the AE400, has the same three main controls – Gain, Freq, and Q, and operates similarly to a fixed equalizer, with the useful exception being the gain varies from 0 dB (unity, no effect), to the selected gain, as the signal rises above the selected Threshold, at a rate determined by the Attack (time needed to reach selected gain) and Release (time needed to return to 0 dB). Additional adjustment in overall sensitivity is controlled by the Ratio control. The AE400 also offers an inverted threshold mode, wherein the active gain is 0 dB (unity, no effect) until the signal level drops below the threshold.

Using the AE400

AE400_singlebandOperating the AE400 like a standard fixed parametric EQ is a good strategy to get familiar with it, and set it up quickly. Using the Output, Freq, and Q control text fields in any of the four bands, or the ‘dot on the plot’, the fixed EQ response can be adjusted easily. Note dragging the mouse over the Output, Freq, and Q text fields will also update them – a much faster method than typing! This new feature is included in every McDSP AAX plug-in.
Once the fixed EQ response is created, then the active EQ response Gain control can be dialed in via the vertical slider next to the IO Curve plot in each band. When doing so, a transparent curve representing the maximum equalizer response appears behind the fixed equalizer response.

The maximum equalizer response is the sum of the fixed equalizer response and maximum active equalizer response.
Start playback, and slowly decrease the Threshold control. The Peak meter text readout shows the peak values of the key signal for each band. When the Threshold control is below the values shown in the Peak meter text readout, the solid line representing the current (active) equalizer response will start to move towards the maximum active equalizer response. Adjustments to active equalization sensitivity (Ratio) and response time (Attack/Release) can also be observed in the movement of the current (active) equalizer response curve.
 The AE400 offers an inverted threshold mode that reverses the behavior of the active equalization. When the ‘INV’ button located underneath the Threshold control slider is pressed, the current (active) equalizer response will start to move towards the maximum equalizer response when the signal level is BELOW the threshold level. 
The key signal for each band is selectable – the original audio input, or a side chain input. The key signal for any band, as processed by that band’s key signal filter, can be auditioned by pressing the speaker icon button at the bottom right of each band.

AE400 Applications

Once you understand how to operate the AE400 Active EQ, you’ll start to ponder the possibilities of how it can be used in your work. Here are a few ideas to get you started.

Vocal Control

Vocal performances are some of the most dynamic and challenging material to mix. In particular, the frequency response of the singer/speaker changes over their dynamic range. A good EQ setting may work for the low volume parts, but need an update for the louder parts of a performance. 
Using the AE400 makes this process much easier. First EQ the vocal with the fixed EQ of the AE400. Then observe and note how the performance response changes on the louder portions. The key signal peaks, shown just below the IO curves for each active EQ band, will tip you off on where the most signal energy is going. Then make an according adjustment with some negative active EQ gain at a threshold just under these peak key levels to reduce the signal energy in those EQ bands.
A singer/speaker typically has a more harsh sound (more signal energy in the 1k – 3 kHz range) when they get louder, so attenuating these levels with active EQ works well. The active EQ gain, frequency, and Q (1/bandwidth) can all be adjusted, and the AE400 bands are fully overlapping.

Vocal Insertion

The vocal performance – the center of attention in any track – is in direct competition with all the other elements of a mix. A good way to insure the vocal always has space in a mix is by using the AE400 and its side chain (external) capabilities.
Route the main mix through the AE400, and the vocal into the side chain input. Sweep the key filters around in the AE400 (done from the ‘Key’ plot display button) to find what frequencies the vocal signal levels are at – use the key peak text read outs to assist in this effort, along with the key monitoring button (the speaker icon button).
Upon noting a few key frequencies (that is a pun folks), select a modest Q (say 1.0 for 1 octave), and a few dB of gain reduction (say -3 dB). Now as the main mix is heard, the vocal will actively EQ it with its own frequency response to carve out some space for the vocal to be added into the mix (post AE400 processing).

Drum Buss Equalization

The process of equalizing a drum set to make it suitable for the entire mix is a good exercise in challenging audio engineering. The problem is compounded by the performer’s varying performance levels (whether for the benefit of the entire musical opus or not!). Indeed, one may consider multi-band compression for such an occasion. But active EQ may be an even better. As mentioned in other application tips, first EQ the drums with the fixed EQ controls. Upon completing this task you will likely note where some ‘compression’ could be useful – on the kick, a touch on the snare perhaps. Using the active EQ gains, dial in some negative gain, maybe about 25% to 50% of the positive gain you added in the boosting of the kick drum (ex: if the fixed EQ gain is 8 dB, an active EQ gain of -2 or -4 dB would be good. Lower the threshold until the active EQ is being triggered as shown by the bright white line across the AE400’s EQ plot response graphic. Adjust attack and release times as needed. Use a high ratio for a steep transition to the active EQ maximum gain selected, a low ration for a more gradual transition. Repeat for other bands as needed!

Attack and Release Time Strategies

One of the most common mistakes in multi-band processing is the use of attack and/or release times that are ‘too fast’ relative to the audio in the lower frequency bands of the multi-band processor. If the attack and/or release times alter gain changes produced by the gate, expander, or compressor more quickly than changes in the audio itself, then buzzing or distortion artifacts are created.
When setting attack and release controls in the AE400 active equalizer bands, consider the band frequencies displayed above each band active response plot. A good rough estimate of the fastest attack time needed is given by this simple equation:
Attack Time = 1.0/(band frequency)
So for the attack time in band 1, if the band 1 frequency is 100 Hz, then a 10 msec attack time would be adequate actively equalizing whatever signal levels entered the active equalizer band.

Another good method for auditioning attack and release times is to make use of the Solo button for the band being adjusted. Once the Solo button is engaged, only the output of that band is heard. Distortion caused by attack and release times will become much more audible, and can then be corrected as needed.

Get Back To Work

Once the basics of active equalization are understood, the usefulness of this kind of audio processing technology will find its way into many of your sessions. Hopefully the AE400 finds its way into some of yours.

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