Creative Audio Processing
06 - Creating EQing

 

 

 

 

As we've seen in chapter 05, an EQ is usually known to be able to boost or cut certain frequencies. Theoretically, it's a smooth curve centered on a given frequency, or above a frequency etc. But in fact, there is much, much more to an EQ than just boosting or removing frequencies. Let's consider two examples, two specific kinds of EQs.

 

1. Convolution EQs Convolution EQs are defined by a very short impulse response (an audio file a few samples long), convoluted with the original file. What's a convolution : convolution in the time domain is equivalent to multiplication in the spectral domain : when I convolute two files, I multiply their respective spectrum by each other. What's interesting in convolution EQs : it's easier to control the EQ's microscopic aspect, the EQ's "grain". Also, complex EQs are easier to create. For example, it's possible to sample a speaker, and modify the impulse response so that it suits the need best, but the original "grain" is intact. Those are typically "stylizing" EQs, not at all "correcting" EQs.

 

2. Analog / analog modeled / special EQs Some outboard EQs cost a lot of money. If people can spend €2000+ on a stereo EQ, that's probably for a reason. Is that for increased transparency ? Not likely : simple digital EQs in DAWs, like the Digidesign EQIII, are already quite transparent. Outboard EQs have specific "colors" : the JoeMeek EQ, for instance, has a very specific timbre. Aphex EQs (cheap outboard analog EQs), also have a very distinctive flavor. Avalon EQs are also quite particular.   When it comes to style, there are two factors on which it's possible to judge such EQs : grain, and behavior in non linear response zones. - 2a Grain : this is difficult to write down with words, but playing on a few moments with a JoeMeek EQ gives an quick answer : for similar settings (freq, gain, Q), it doesn't sound the same as some other EQs. The color is very distinctive. It's very likely that this specific timbre comes from the microscopic aspect of the EQ, and gives some kind of "tactile" aspect to the sound, just like different kinds of films give different kinds of grain, and modify the "flavor" of the image that's on it. For instance, Super8 grain is obviously different than DV grain. Background noise added to the sound which is processed can also be part of the machine's grain. This is especially true when it comes to cheap EQs (eg Aphex EQs) - 2b Non linear behavior This is a well known phenomenon : high end analog gear gives very good results when "pushed a bit" - ie electric level is close to actual distortion. In those cases, the machine exhibits a "non linear" behavior,with added harmonics, and to some extent, arbitrary filtering. This behavior is specific to each machine, at least to each brand. Strictly speaking, if this is not filtering anymore, we are still in an EQ issue : added harmonics bring brilliance, that's still frequency-related processing. Non linear behavior, especially in outboard EQs, and less so in analog modeled digital EQs, is a useful tool when it comes to give style and color to a given sound.