The Classic Symmetrical is the most popular bell shape used in many mixers and outboard equalizers. It is almost “constant Q.” That is, as resonant frequency changes, the bandwidth changes, widening as the frequency goes from low to high to maintain a constant ratio of center frequency divided by bandwidth. Constant Q is roughly the way our hearing perceives an EQ’s effect.
The three Proportional PEQs emulate bell characteristics which change their bandwidth proportional to their boost or cut. Many think they behave more musically as you don’t have to correct the Q after every amplitude change. There are also three other constant–Q equalizers available, with characteristics similar to some classic American brands.
thEQblue also offers two proprietary constant–Q varieties. The first, Constant-Q Ideal, is a perfect version of the Classic Symmetrical equalizer, having exactly the same bandwidth at any amplitude within its range of Q. This is measured 3 dB below maximum amplitude at and above +6 dB of boost. Such PEQs are impossible to implement in the analog domain, hence the “ideal” name. Another specialty variety is the Constant-Q New, which follows a new Q definition. It preserves exactly the same bandwidth at the half of the maximum amplitude for all amplitude values. Unlike the classic Q definition, this includes the range below 6 dB of boost. Due to its amplitude/bandwidth dependency in terms of a classic Q definition, this new kind of constant-Q PEQ could also be thought of as a member of the proportional group.
Finally we emulated two vintage parallel equalizers. With parallel construction, the main input feeds every filter in parallel, with each filter’s output being summed together for the final composite output. With a series implementation, filters are cascaded; each fed from the output of the previous filter. Unlike a series parametric equalizer (almost all paramets are serial), parallel–connected filters combine or cascade differently. They also behave differently in terms of phase. Their special interactive behavior between bands is offset by a pleasing sonic character. Our Parallel LC emulates old skool parallel passive PEQ circuitry built with inductors and capacitors. The Parallel Feed–Forward/Feedback topology emulates the feed–forward/feed–back designs still popular in low noise analog graphic EQs. While the feed–forward path is trivial to implement, the feedback path is impossible to implement in traditional digital signal processing, because of the need for so called “delay–free” feedback loops. In thEQblue, we apply a unique and very elaborate technique for true emulation of delay-free feedback. The result is a characteristic sound and perfectly complementary filters for boost and cut.
The Filter Library
In thEQblue, not only are various bell–shaped filters available but also different kinds of shelving filters. Nine of the series equalizers use a special shelving design characterized by a cut–off frequency defined in the middle of the transition region. We found these filter definitions more intuitive than the classical “–3 dB below maximum.” The old skool definition is only used in the Classic Asymmetrical architecture in order to conform to its analog antecedent. All 2nd order shelving filters have a Q adjustment to emulate vintage characteristics, with their inherent and specific bumps at higher slopes. Also, the parallel equalizers are equipped with respective shelving filters shapes that are typical of old parallel PEQs. They also have interacting bands as did their analog predecessors.
Every complex PEQ is equipped with cut filters. With thEQblue, each of the twelve equalizer topologies can provide 1st and 2nd order cuts. The 2nd order filters have a Q adjustment to create a resonant filter response, and can be combined to provide very steep “brickwall” slopes by cascading several Sections.
Best of Both Worlds
thEQblue is a creative equalizer tool combining the best of both the analog and digital domains. To avoid bell filter asymmetry at high frequencies for baseband sample rates, typical of many digital equalizers, we have applied reference–quality upsampling, which automatically kicks in for 44.1 or 48 kHz. By using proprietary filter algorithms, we have achieved a huge dynamic range, as well as extremely low noise and distortion. This yields unparalleled sonic purity, impossible for any analog circuitry.