The PPG PRK is a velocity sensitive 72 key master keyboard to be used only with the PPG synthesizers.
The PPG PRK was created for a number of reasons; firstly many PPG owners wanted to have velocity sensitivity when using their PPG synths - the PPG PRK offers this.
Also the 'closer-to-piano-feeling' was something that musicians wanted at this time - the PPG PRK offers this as well.
The PPG PRK also offered optional voice boards to be inserted into the unit.
This does not mean that the PPG PRK itself can create any sounds - but by inserting the optional sound cards you could send the information on these to the PPG Wave 2.2, PPG Wave 2.3 or PPG EVU.