Moog Music Forum

Can somenone help me understand the differences and similarities between the Low PAss Filter and the MuRF in terms of the sounds they produce. Can the Murf do what the Low Pass does or are they they completely different effects? Thank you for your help.

Ok, let me try. I'll start with the basics. As you probably know, a low pass filter is a device that removes frequencies from a source signal above the cutoff frequency. A high pass filter is the exact opposite: it removes frequencies above the cutoff frequency. A band pass filter is the combination of this: frequencies above and below a point (or rather a band) are removed. There is one more concept that you need to understand before proceeding: resonance. This is a process whereby the frequencies at and around the cutoff frequency of the filter are emphasised. Important note: the way the filter removes frequencies is imperfect (and graduated) and this, together with resonance, determines its character.

The MuRF has 8 band pass filters with their cutoff frequencies set to (basically) cover the audible frequency spectrum (you can see the frequency in question printed beneath each slider). A signal is sent to all 8 at the same time and the setting of each determines whether that band is audible or not. At this point you should be thinking that a MuRF has quite a bit in common with a graphic equaliser. The difference with the MuRF is that each filter is designed to be resonant (an equaliser should be as flat as possible) and all 8 can be animated by the pattern, rate and envelope settings. This can lead to very complex and interesting filter effects: from phaser and chorus type effects to weird rhythmic popping, blooping and bleeping. A MuRF will not function very well as a low pass filter although I guess you could get it to operate (sort of) as one by switching off the pattern and using the silders. A low pass filter simply cannot do with the MuRF does.

Check out the manual for more...