After Algorithmix unveiled its linear-phase equalizers, the best mastering engineers certificated their unparalleled sound clarity and transparency, declaring them to be the new benchmark worldwide. At that time, our main intention was developing an equalizer for critical mastering and re-mastering tasks that could significantly boost or cut complex audio material without adding its own sound. After passing this difficult exam, our customers started asking us to develop a classic PEQ for recording and mixing, but with the world-famous Algorithmix purity and transparency. We knew that there has been no single particular PEQ style that could satisfy all the different flavors and habits in the audio community. Our intensive research and development efforts finally resulted in the Classic PEQ Blue, a parametric equalizer with 12 different faces. It contains the most complete collection of classic, vintage, modern and experimental equalizers ever included in one PlugIn. We hope that everybody will find his favorite PEQ characteristic depending on preferences and given applications. To avoid any conflict with trademark owners, we did not assign any real names to the PEQ types. Instead, we gave them rather neutral names related to their technical classification. You are invited to discover sound nuances and share with us your associations with real brands and/or products.

Every PEQ type has 10 bands with five freely assignable parametric filter types including bell, low-shelf, high-shelf, low-cut, and high-cut. The most significant filters determining the equalizer name are bells. Set up to a certain Q, different bell filters change their bandwidth in a specific way depending on the boost or cut amount . To allow practical comparison between different equalizer types in the Classic PEQ Blue, the bells in all PEQs have been normalized to 6 dB boost, i.e., a separate bell looks identical for 6 dB boost and the same Q, independent of chosen PEQ type.

The Classic Symmetrical is the most popular bell shape used in many mixers and outboard equalizers. It is almost constant-Q. The three Proportional PEQs emulate bell characteristics which change their bandwidth proportionally to boost or cut. It is said that they behave more musically when used for recording and mixing; one does not need to correct Q after every amplitude correction. Furthermore, there are three other constant-Q equalizers with characteristics mostly used by some American brands. Algorithmix also offers two proprietary constant-Q characteristics. The first one, Constant-Q Ideal, is a perfect version of the Classic Symmetrical equalizer, having exactly the same bandwidth at any amplitude within Q definition range (i.e., measured 3 dB below maximum amplitude at and above + 6 dB boost). Such PEQs are impossible to implement in the analog domain. Similar curiosum is the Constant-Q New following a new interesting Q definition. It preserves exactly the same bandwidth at the half of the maximum amplitude; this for all adjustable amplitude values (including, unlike the classical Q definition, the range below 6 dB boost). Due to its amplitude/bandwidth dependency in terms of classical Q definition, this new kind of constant-Q PEQ could also be added to the proportional group.

Finally we emulated two vintage parallel equalizers. Unlike the serial parametric equalizer (almost all parametric PEQs are currently serial), parallel connected filters combine differently. They also behave differently in terms of phase. Their special problem with some interactive influences between bands is fully accepted due to compensation by a pleasant sound character. The Parallel LC emulates old parallel passive PEQ circuitry built with inductors and capacitors. The Parallel FF-FB emulates the feed-forward/feed-back structures still very popular in low-noise analog graphic equalizers. While the feed-forward path is trivial to implement, the feedback path is impossible to implement in traditional digital signal processing, because of the so-called delay-free feedback loops. In Classic PEQ Blue, we apply a very elaborate technique for true emulation of delay-free feedbacks for the first time to a commercial product. The result is a characteristic sound and perfectly complementary filters for boost and cut.

In Classic PEQ Blue, not only are various bell filters used but also different kinds of shelving filters. Nine of the serial equalizers use a new shelving filter generation characterized by cut-off frequency defined in the middle of the transition part. We found these filter definitions more intuitive than the classical ‘– 3 dB below maximum’. The old definition is only used in the Classic Asymmetrical type to conform to its original predecessor. All 2nd order shelving filters have a Q adjustment to emulate vintage characteristics with their specific bumps at higher slopes. Also, the parallel equalizers are equipped with respective shelving filters shapes being typical for the old parallel PEQs and interacting with other bands as did their analog predecessors.

Every complex PEQ is equipped with cut filters. In Classic PEQ Blue, each of the 12 equalizers can assign 1st and 2nd order cut filters. The 2nd order filters have a Q adjustment to create so-called resonant filters characteristics and very steep brickwall filters after cascading more of them (Butterworth, Bessel, Elliptic).