If you look at this website:
you will see an example (with code) that attempts to up convert and down convert a sine wave. So at the bottom of the tutorial you will see 3 plots. The first shows the frequency spectrum of the sine wave, which has a frequency of 1 kHz. The second plot shows the frequency spectrum of the up converted sine wave, with a frequency of 50 kHz. The third plot shows a frequency spectrum of the down converted sine wave, which has the exact same frequency of the original sine wave( 1 kHz). So up and down converting appears to be working happily.
My question is, in the 4th plot, they show 3 different filters (or filter stages) within the ddc object (I believe the cic decimator, cic compensator, and halfband decimator). How can the ddc object be modified to use different filters? What if I want to use a low pass filter? Or a high pass filter? Or a butterworth filter? Am I not able to change this?