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Pro Tools Corner - Mixing on AIR: Part 1

Jun 29, 2009
While the new interface is stunning to say the least, it is hard to ignore the mother load of new plug-ins that ship with Pro Tools 8. Dubbed the "Creative Collection," the fresh suite of plug-ins from Digidesign's Advanced Instrument Research division shines with creativity from every angle. While many have fixed their ears eagerly on the new virtual instruments, the 20 new AIR effects are sure to have the mixer in us screaming for attention. As a toast to all of these free new effects, I thought it would be appropriate to showcase a handful here at the corner as an on going series I am dubbing "Mixing on AIR." The goal being to both introduce you to some of the plug-ins you get with Pro Tools 8, and share with you some tips and tricks for integrating them into your projects. This week I thought I would kick of the series by showing you how to add some presence and sparkle to lifeless signals using the AIR enhancer.
Enhancer Basics
    When it comes to audio processing, the term "enhancer" is anything but concrete. Not quite an EQ, they are often used to add sparkle, presence and perceived loudness to lifeless signals. While EQ shapes tone by selectively increasing or decreasing the levels of specific frequency ranges, enhancers work with tone in an entirely different way. To many engineers, enhancers have always had this sort of "magic button" quality to them as a mix tool, often over-used but rarely understood.

    Although many enhancers feature company specific proprietary designs, most work on the principle of harmonic synthesis or phase manipulation. In harmonic synthesis based enhancement, the signal is subjected to some form of distortion. This distortion in turn excites high-frequency harmonics that may not have been present in the original signal. The result, which actually sounds like distortion of the signal at lower frequencies, is then filtered and restricted to the higher end of the spectrum where the human ear is fooled into thinking the additional hi-frequency content is a natural part of the original signal (Aphex uses these techniques in their Aural Exciter). In some cases, the enhancement is dynamic, and works only on the transient material of the audio. Harmonic synthesis enhancement can be very useful when there isn't enough hi-frequency content for an EQ to be effective.

    In phase manipulation based enhancement, the signal is broken into multiple frequency bands, which are then delayed slightly from each other, usually never more than a few milliseconds or less. Delaying the lower harmonics from the higher ones can increase presence in the signal and improve clarity. Many times this is coupled with high frequency compression or expansion and low frequency boost to compensate for any loss in low end (the BBE Sonic Maximizer design is based on these concepts).
Using the AIR Enhancer

    The AIR Enhancer plug-in can be found under the "harmonics" sub-menu and is a multi-channel plug-in when inserted on stereo tracks. Generally one would use an enhancer inline as a direct insert on a signal, rather than in a send-return format, but you could definitely use an enhancer to process the return of a reverb or delay effect if desired. The AIR Enhancer is fairly basic, and while the documentation from Digidesign is sparse, it is clear that it works in the harmonic synthesis domain generating new harmonics by applying distortion and filtering the signal.


    At the left hand side of the plug-in you will see two knobs labeled "High Gain" and "Low Gain." Think of these like EQ shelving filters, allowing you to boost the high or low frequencies of a signal as set by the "Tune" section. While I am not positive, to my ears the AIR enhancer does not do any sub-harmonic generation like Digi's Recti-Fi plug-in. That said, if the signal doesn't have much in the bass department this plug-in isn't going to magically create it, so think of the "Low Gain" control as more of a low shelf boost on an EQ and a way of compensating for the loss in low-end that often occurs when harmonic enhancement increases the high-frequency content of a signal.


    The "Tune" section sets the desired center frequencies of the gain controls, much like the frequency parameter of an EQ. The "High" frequency parameter also serves double duty as the high pass filter frequency of the harmonic distortion added via the "Depth" parameter. Remember how I said enhancers create high-frequency harmonics by subjecting the signal to distortion? Well, this distortion can be very obvious and audible at lower frequencies, so it often needs to be restricted to frequencies well above 2Khz to be effective and natural. When used in conjunction with the "depth" parameter, the "high" control restricts the additional harmonic content to frequencies above the desired setting. As an experiment, try increasing the depth parameter to 12dB and sweeping the high frequency control to hear the how the distortion is filtered.


    As stated, the depth parameter in the "Harmonic Generation" section determines the amount of additional high-frequency harmonics added back into the signal, since this parameter also increases the distortion inherent in harmonic generation you want to pay attention to the high frequency control and depth amount to minimize the damage, remember the goal is to enhance not destroy. I generally find depth settings above 3-4 dB to be a bit much when enhancing natural sounds, while the sky is the limit on synthetic stuff. You can also play with the "phase" button, which changes the polarity of the generated harmonics. Sometimes this can yield a cool effect on synth or electronic sounds.


    Because you are increasing the level of the signal with enhancement, use the output knob to reduce the chance of clipping and help return the signal to its preprocessed level in the mix.
Tips and Tricks
    When a signal lacks hi-frequency energy, no amount of EQ boost is going to help increase its high frequency presence or "airyness." Used effectively, an enhancer can help re-introduce high-frequency content to dead tracks, especially things like cymbals, string instruments, and vocals. As always, a little goes a long way. Too much can be ear fatiguing and shrill, often increasing problems with sibilance (especially in vocals). When I feel like a signal needs a bit of life added, I generally adjust the enhancement until I can hear it working and then back off a bit. Remember, a mix is all about creating a focal point for the listener, so while it may be tempting to enhance every track along with the entire mix, doing so may obscure the contrast of the key elements you want to stand out (such as the lead vocal).

    In my opinion, I find that the presets included with AIR enhancer tend to be a little over the top. I generally achieve the best result by starting with a factory preset and then reducing the gain controls and depth controls by at least half, maybe more in some cases. Be sure to adjust your "Output" control with extreme settings to avoid clipping. While the AIR Enhancer can be effective when used in small doses, I'm not going to lie and tell you it is the best or only one on the market. There are many enhancer and enhancer style plug-ins available and each has a unique approach to signal treatment, I encourage you to download some demos and see what works for you.
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