The continuous progress in fusion plasma technology and comprehension of the stability and confinement properties of Tokamak fusion plasmas offers encouraging support for the success of ITER. Plasma diagnostics are essential for real time plasma control and the reconstruction of important plasma radial profiles such as the ion/electron temperatures and densities, ion toroidal angular rotation, the plasma current density and the safety factor q(R). The latter two, in particular, are essential for the assessment of the discharge operational regime (ohmic, reversed shear or hybrid) and for the integrated plasma modelling. Probing directly inside the plasma, the Motional Stark Effect (MSE) diagnostic is one of the best techniques to evaluate the radial q-profile. In the MSE diagnostic installed at JET, the direct determination of the magnetic field pitch angle is made measuring the polarisation angle of the +p component of the Stark splitting of the Balmer Da emission from deuterium atoms injected by the heating neutral beams. Polarised light from all 25 spatial channels, covering the full outboard minor radius, is intensity modulated at characteristic frequencies, 20 and 23kHz, by two photo-elastic modulators (PEMs). Making use of a set of 25 Avalanche Photo Diodes (APDs) and associated fast tunable narrow band filters, one gets Amplitude Modulated (AM) (at harmonics of the PEM modulation frequencies) electrical signals which are then digitized at 250 KSample/s.