Exakt filters are the profiles used in the Exakt Engine:
Original Design release
In the original release of Exakt the drive unit models were rather inflexible. The roll-on and roll-off of each drive unit were modeled separately but could only be deployed to correct for both the magnitude and phase of the natural response, or do nothing. At the lowest frequencies there was very little we could do due to the very large processing requirements, so for most speakers we left the natural roll-on alone. Furthermore we didn't have a viable model for the roll-on of ported systems or capability to correct for any fixed internal electronic filtering.
It should be noted that the original release for many supported speakers is not available as a selection using the current version of Konfig. The file format is very different to what is used today and is not readily supported by the Exakt compiler.
Independent highest frequency modelling
For many speakers there is also a version 4 which is not available for selection in Konfig. This was essentially the same as the independent highest frequency modelling filter. Changes were made to all design files to ensure they fell in line with the generic measurements stored in the cloud for the drive units employed by the filter. This was put in place to guard against cloud drop out. It shouldn't ever cause a problem, so version 5 can be considered as a 'belt and braces' replica of version 4.
Independent drive unit modelling
The first and most sizable change to the Exakt filters happened in October 2015 when we introduced Independent drive unit modelling, often described as 'fourth order filters'. This change introduced the current file format used for all Exakt designs and allowed a huge increase in the processing flexibility of the Exakt engine. Each drive unit could now be modeled with a greater flexibility. Improvements included; ported system models (fourth order), support for fixed electronic filters (e.g. subwoofers), independent modelling of magnitude and phase or phase only for low frequencies, better processing by application of new windowing techniques (allowing extension of linear phase to the lowest frequencies).
Extended drive unit modelling
Linn have invested significant effort in improving our acoustic measurement capabilities (bought a very posh laser measurement system, built an anechoic chamber, written a new measurement suite) which gives us a much clearer picture of what the loudspeaker is doing. With Katalyst in the speaker we decided it was time to re-examine the 350. The results of this work are described as Extended drive unit modelling. Previously we treated each unit in general terms, correcting for the roll-on and roll-off behaviour using a model based approach. The extended models treat any consistently measurable acoustic aberrations in the pass band of each drive unit while also adding more precision to the fit of the general models. The result is a response from the speaker which runs more closely to linear phase than before. Previous filters resulted in a phase response to within +/- 15 degrees of linear over the full range of the speaker. The extended models get closer still to a truly linear phase response.