What types of filters are there?

Early measuring instruments used analogue (electronic) filters. These types of filters are also known as 2CR filters. The 2CR stands for two capacitors and two resistors. These electronic filters, although still accepted and recognised by international standards, do suffer from phase distortion caused by the nature of their electronic components. To remove this effect, we have another type of filter called a 2CR PC filter. The PC in this case stands for phase corrected. This type of filter suffers from less distortion than the 2CR but is still an electronic filter and, as such, still suffers from some distortion. 

Modern instruments use phase correct filters, such as the Gaussian filter. These types of filters drastically reduce filter distortion, although they can only be implemented where filtering is done by mathematical algorithms; computer based processing. 

On most modern computer-based instruments, analogue filters are digitally simulated so that correlation between new and old instruments can be made.