King Tubbys high-pass filter was known as "the big knob". He often used it on the effect returns to sweep the reverbs and echos in his dub mixes. In the same filter series there where also a low-pass filter – both with specific stepped frequencies. This is where the inspiration for HighPass 1012 and LowPass 1024 filters came from.
HP1012 is a 2 pole high-pass filter and the LP1024 is 4 pole low-pass, both have the fixed frequency cut off points, peak control and input level indicator and transistor drive to boost the signal while giving it some analog heat. The CV jacks on the back panels lets you smoothly modulate the filter frequency with control voltage.
The HighPass LowPass Filter 1224 combines them both in the same unit for quick and easy usage.