Blue Cat's range also includes a healthy selection of EQs and filters, and their Widening Parametr'EQ is another mastering tool with a twist.
It's a stereo equaliser that operates in M/S mode, allowing you to apply different EQ curves to the Mid and Sides components of the signal before they are matrixed back into conventional stereo. There are five fully parametric bands of equalisation, plus high and low bands that can be switched between shelving and filter responses. A graphical display shows the curves for Mid in blue and Sides in red, but it's not possible to draw in curves with the mouse. Instead, you have to use the rotary controls and gain sliders in the lower half of the interface, which in the default skin are too small and fiddly for my taste.
One feature that is, as far as I know, unique is the ability to "choose the position of the virtual centre for the Mid/Side matrix". The manual says this allows you to "really control where you want the equalisation to take place in the stereo field", but doesn't give firm examples of why you might want to move this virtual centre around. I didn't find any practical uses for this feature, but I was surprised at how effective the M/S equalisation can be for opening out a mix. A little high shelving boost and low-frequency cut on the Sides channel only can really make things more expansive, without introducing obvious artifacts or leaving a hollow centre, as some stereo width processors do. Sound-wise, the equalisation is transparent without being thin or unnecessarily clinical, and the width of the parametric bands can be set as low as 0.01 octaves, for surgical tasks such as hum removal. From Soundonsound
