BSS DPR 901 Multi band compressor.
The 901 is a multi-band compressor, also known as a dynamic equalizer. Think of it as four compressor/expanders, each with a sidechain EQ. Many live sound engineers use a 901 to smooth vocals.
The DPR-901 is a 4-band parametric equalizer which varies the degree of equalization dynamically, as the program level changes. The process can be thought of as frequency selective compression and/or expansion, but it is fundamentally different to simple ‘frequency-conscious’ dynamics processing as used for noise gating, de-essing and the like. It is also very different to the growing family of ‘sliding filter’ techniques used in single-ended noise reduction processors. A split facility allows the unit to be used as 2 separate 2-band units, and a side chain listen button allows monitoring of the filter setup.
Frequency selective compression is equivalent to applying CUT on a normal equalizer while frequency selective expansion is like applying
BOOST. However, the DPR-901ii can be more subtle as the process is dynamic: it will only occur above (or below) the threshold you have set.