Bandpass filter: This let�s frequencies in a certain range of the spectrum (the band) pass through while dampening frequencies both below and above this range. This accentuates the mid-range of a sound.
Amplitude Fc (Cutoff Frequency)
Frequency
Notch filter: This filter type (also known as Band Reject) can be seen as the opposite of a band pass filter. It cuts off frequencies in a �mid-range� band, letting the frequencies below and above through.
Amplitude Fc (Cutoff Frequency)
Frequency
In the Nord Lead the Notch filter is combined with a 12 dB Lowpass filter, for greater musical versatility (see page 44).
Roll-off
Filters of one and the same type (lowpass, highpass etc) can have different characteristics. One of the factors determining the exact filter curve is the roll-off, which is measured in dB/Octave (�decibels per octave�) or poles. The simplest possible filter has a roll-off of 6dB/octave, which is referred to as �1 pole�. The next step up is 12dB (2 poles), 18db (3 poles) etc. The most common synth filters are the 12dB and 24dB lowpass filters. The difference between the two can be studied in the graph below. The 12dB filter let�s more of the high frequency pass through which gives the sound a brighter and �buzzier� character than the 24dB filter does.
Amplitude Fc (Cutoff Frequency)
12dB (2-pole) 24dB (4-pole) Frequency
In the Nord Lead, the lowpass filter can be switched between 12 and 24dB modes. For sounds with high resonance (see below), similar to those in the Roland TB-303, we recommend the 12dB variation. For most other sounds we recommend 24dB.
