A printhead for an inkjet printer, that has a printhead IC with an array of nozzles for ejecting ink and a support structure for mounting the printhead IC in the printer adjacent a paper path. A signal analyser is configured to perform dimension reduction or factorisation of said excitation signal in the time domain or in the frequency domain followed by a sparse transform, the resulting relatively clean output signal providing the input to an inverse transformation step to create the sparsified signal. At least one of the plurality of sensors includes a light emitting element configured to transmit light through the tube when the tube is disposed in the slot and a light receiving element configured to receive the light emitted by the light emitting element after the light passes trough the tube.