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Acourate Crossovers - UB crossovers

Posted: 13 Jul 2020 11:20
by UliBru
Acourate allows to create a new crossover class named UB crossovers.
Here is some explanation about the new XO types.

From the first release Acourate has included Neville Thiele crossovers. Basically they are crossovers combined with notched responses. The digital version can be considered as a combination of different connected curves in the frequency response. The NT-XO is attractive because of its high steepness but still good time domain behaviour.

But I have thought about how to improve the NT filter after detecting that it still shows some "micro-ringing" in time domain. This can easily be shown by e.g. a 1st order linearphase NT lowpass 1 kHz in RMSLog display

Image

The picture shows the ringing of the NT filter (red curve) in comparison to a UB jPol11 filter (green curve). It should be quite clear that the UB filter has much less ringing time.
Remark: this does not mean that the NT filter is bad at all, the ringing time (width) at level -100 dB is about the same. But anyway why not to try for improvements?

The reason for the micro-ringing is given by the curve continuity in the frequency response. When you connect some curves they may not really connect or there may be kinks (= different tangents at the connection point).
For the continuity of curves see e.g.
Image

The UB filters now contain different curves to improve the continuity. The first two curves UB Cos and UB jPol3 are designed by a cosine function and a polynomial of 3rd order. Both curves fullfill the G1 condition (definition see picture above), the tangents are the same in the connection points (start + end of transition area).
The other curves UB jPol5 to UB jPol11 are polynomial functions and they fullfill the G2 up to G5 condition. Clearly G5 means the smoothest curve.

A comparison of the different filters see their frequency responses
Image

So at the end you may want to pick your poison. You have to decide by your ear which crossover you like most. I wish you a happy tweaking ;)