a pseudo random noise code generator has two code data generators which respectively supply first and second code data through respective first and second bit inverting circuits to respective input ports of a multi - stage shift register having an output . each bit inverting circuit is responsive to the state of a control signal for producing at its output either the code data at its inputs or an inverted version of the code data at its inputs . in a modified version , first and second code data generators each respectively supply first and second code data to respective input ports of a multi - stage first shift register , and a third code data generator supplies third code data to an input port of a multi - stage second shift register having a further input port to which is supplied constant code data , and a modulo 2 adder is provided to add the outputs of the first and second shift registers . a modified version of this latter arrangement includes a third shaft register having two input ports to which are respectively supplied the first and second code data , and a further modulo 2 adder is provided to add the outputs of the second and third shift registers .